Design - Earth Sheltering For East and West Slopes

Traditionally, Annualized GeoSolar (AGS) houses*, or for that matter, nearly all earth sheltered or partially earth sheltered houses, are nestled into south facing hills.  Like most of Illinois, our topography is glaciated flat land that was once tall-grass prairie.  But after a long search for a suitable location for passive solar, we were lucky to find a south facing slope adjacent to the prairie in the hilly Mississippi River "bluffs" across the river from downtown St Louis.  

Probably most AGS enthusiasts are not lucky enough to find a south facing hill so the paradigm needs to change.  And our experience with AGS makes me confident that it can be modified for east and west facing slopes when, for other earth sheltered designs that have less control over as much thermal mass, it would be a stretch.

Designs for energy neutral or near-neutral housing that do not require earth sheltering continue to evolve.  However, it seems that AGS is still the best alternative for cost-conscience DIYers and, for that reason, we need to find ways to utilize east and west facing hills as well as flat land (the subject of my next blog post).

I now believe that the type of solar collector we have in front of our house is unnecessary for AGS.  If it were replaced with a inflow chimney and the conduits ended at an outflow chimney with a fan in it, the system would be more efficient at a considerable reduction in construction costs and would lend itself to east and west facing houses.

The following discussion assumes that the reader already understands the basics of AGS passive solar, especially its advantages over 1970's earth sheltering designs.**

East and West Facing Slopes

The following discussion attempts to take what we have learned from our house and adapt it to houses identical to ours but on slopes not facing south.  It is only an example and not intended as a ridged blueprint for other creative DIYers.  Here are some parameters to guide the discussion:

--   the hillside faces east and the house faces south

--   the layout of the living quarters is identical to our house

--   a "vertical basement' wraps around the end of the house (instead of being restricted to the north wall as in our case)

--  the insulation/watershed umbrella replicates what we have now, viz., extending below grade outward 20' from the living quarters on all sides irrespective of grade changes and retaining walls 

--  all non-earth-sheltered exterior walls and ceilings are super-insulated to an R-50 or more

--  the house has no soil on the roof

A drawing of our south facing house that has been modified for an east facing hill.  Since the second floor remains unchanged, it is not included.  Concrete walls are dotted, stick-built exterior walls are black.

The drawing represents our house built into an east-facing hill and is a mirror image of a design for a west facing hill.  Note that the vertical basement wraps around the west (uphill) end of the house instead of being confined to the north side like previous designs for AGS.

The structures running at angles from the north and south sides are retaining walls that preserve full-depth earth sheltering even as the hillside slopes more rapidly downward from west to east.  The red line identifies the outer edge of the insulation/watershed umbrella, including that portion under the garage.  The green lines depict north-south conduits and the dotted blue lines identify alternative east-west conduits.  

Two Options for Conduit Configuration

The only way the conduits shown here in green ink differ from our build is the lack of a solar collector in front of the house and, to simplify the drawing, fewer in number.  The inflow chimney captures hot summer air.  Running under the floor of the house are conduits connecting it to another chimney located just north of the umbrella.  The latter houses a small furnace fan that pulls hot air through the system.

A second option for the layout of an AGS system -- the dotted blue lines -- are running east and west for which the inflow chimney is located just east of the garage and the fan-outfitted outflow chimney penetrates the umbrella next to the west concrete house wall.  The chimney could also be positioned westward beyond the umbrella but at the expense of making the long conduits even longer.

North-South Conduits

Our original passive solar design has north-south conduits running from a solar collector in front of the house to daylight behind the house. The collector was supposed to generate heat that would passively rise through the conduits but it didn't work that way.  The air falling out of the cold thermal mass overwhelmed the hot air from the collector, necessitating the addition of a solar chimney containing a fan to pull air through the conduits.

I now believe that it would be smart to replace the collector with a simple inflow chimney.  The question then is where to locate it?  One option would be in the area vacated by the collector whereby a large insulated pipe would connect it to the conduits under the house as shown in the drawing. 

A better option would be to eliminate the pipe by siting the chimney next to the foundation.  Care would have to be taken for an accurate fit of the umbrella against the chimney in order not to compromise the water seal or the integrity of the insulation. And it would be best to insulate the walls of the chimney down to its connection with the conduits, at least 6 feet below floor level.  The north outflow chimney should be left at the outer edge of the umbrella so that any residual heat in the airflow warms the soil under the umbrella.  And it remains uninsulated.

A considerable advantage to the north-south configuration is that the distance between the two chimneys would be much shorter than for east and west chimneys -- about 58' in the N-S direction (with the intake chimney next to the foundation) versus 93' in the E-W direction.  When air exits our outflow chimney, it is cool to the touch, even in the hottest part of the summer, which means it has done its job heating the soil.  However, there is no way to know when it gave up its last bit of heat.  Was it early on or near the north side or somewhere in between?  Since E-W conduits would be about two-thirds longer, I think the extra length would cause the heat to be distributed to the mass long before the airflow reached the outflow chimney at the west end of the house, taking an indeterminate amount of the thermal mass out of play for heat storage.

Arguably, a conduit with a direct path to the outflow chimney moves more air than a conduit with a more convoluted pathway.  If so it is also likely to distribute heat throughout its travel, including nearer the outflow chimney.  The corollary is that a more roundabout and branching conduit likely distributes heat in the early part of its journey and leaves thermal mass cooler further on.  Presently, our conduits do not branch until they are within a few feet of the outflow chimney.  Their minor looping to reach the ends of the house does not seem to impair a uniform comfort level throughout.  And the vertical basement with its tall north concrete wall, backed up by earth covered by the insulation/waterproof umbrella and running the full length of the house, provides thermal mass directly in the pathway of all of the conduits.  Unfortunately, the long vertical basement on the north would not work for an east or west facing hill.  

Since reasonably straight conduits are probably best, it makes sense to move the first pair of chimneys and their conduits eastward and add another pair of chimneys with dedicated conduits westward, say, under bedroom #2 in the drawing.  Then, with minor looping of the conduits, it would be easy to heat the vertical basement from below.  One or two conduits could even be swung behind the tall earth sheltered west wall, perhaps in parallel at two heights, to mainline heat into the thermal mass adjacent to the largest area of the vertical basement.

East-West Conduits

Another way to warm the thermal mass would be with conduits running east to west.  But fewer could be squeezed into the narrower dimension of the house and their longer pathways almost certainly would cause them to exhaust their heat long before reaching the vertical basement and the thermal mass beyond it.  And, since this arrangement is less likely to support pure passive solar, conventional heating in some form would be necessary.

Southeast or Southwest Slopes

 AGS being based on heat from the summer sun with less dependency on solar gain in winter provides more latitude for siting the house.  While a south facing house built into a northeast or northwest facing slope would leave too much of the back of the house sticking out of the hill, a house in a southeast or southwest facing hill would work just fine -- even better than strictly east or west facing hills because it would increase the amount of earth sheltered north wall.

Also, without compromising its thermal performance, a south facing house built into a east or west facing hill could be turned slightly eastward (east facing hill) or westward (west facing hill) to bring more of the back of the house into earth contact.  We rotated our "south facing" house slightly westward for a different reason -- to pick up more winter sunlight during the afternoon.  

French Drains

If French drains are necessary to prevent the water table from encroaching on the conduits, it might be well for the designer to review my posts on planning, fabricating and laying the French drains which detail methods and materials that are unique.  Otherwise, expect the excavation contractor to ruin most of the undisturbed soil that would support the house with the wide trenches that are necessary to accommodate the trench shoring cages that keep workers safe in deep trenches.

Respect the Umbrella

The amount of time and expense that goes into the insulation/watershed umbrella might tempt the designer to compromise on its size but, in my view, at great peril. The umbrella should extend outward from the house at least 20' on all sides 

Extending the umbrella (orange line) 20' minimizes heat loss
from the thermal mass under the house because it takes
  6 months for heat to travel 20 feet through dry soil.
Spring arrives before significant heat loss occurs.

to make sure that the temperature of the thermal mass under the house reaches a steady state.  The large umbrella requires an oversized excavation and running it behind retaining walls is an added hassle.  The cost of multiple layers of 6 mm plastic and several of foam board insulation is not inconsequential.  And backfilling over the umbrella without crushing the foam or puncturing the plastic takes ingenuity.  But the payback is the time not spent running furnace ducts and writing checks for HVAC installation and, best of all, to the utility company ad nauseum.

Fewer Windows

One minor disadvantage when our floor plan is adapted to a east or west facing hill is that the wrap-around vertical basement obliterates the south facing windows for the master bedroom.  But it would still have the second story windows adjacent to the catwalk,  And an energy efficient skylight could be added to the north-sloping ceiling without compromising the overall thermal performance of the house.

Underperformance?

I am not privy to AGS for slopes other than south facing which makes this post mostly informed guessing.  If it misses the mark, it is probably with regard to its optimism about winter, rather than summer, comfort, particularly in more northern climates. If so, before rushing to the nearest HVAC contractor or dismissing AGS altogether, consider the green solution we employ.

There are times in the early spring*** when our house, in a south facing slope in a southern-ish climate, is a little chilly but it is something we easily handle with a couple of portable infrared space heaters during the daytime.  Even with the heaters, our minimalist PV array produces more electricity than we use 8 months of the year.  And there is the possibility that the thermal performance of the house may still improve over the next few years as the umbrella-protected thermal mass absorbs and retains more heat.  Therefore, early adopters utilizing an east or west slope for AGS might consider living in the house, using infrared space heaters -- either fixed or portable -- and tracking energy consumption for a year or so then adding just enough PV to zero out consumption.  If we had done so, rather than trying to predict our needs ahead of time when buying the PV array, we would be energy neutral today.

The total amount of earth contact wall of the vertical basement in the above drawing, is about the same as for our house but it is not as well distributed relative to the living space, being bunched to one end instead of running all the way across the back the house.  And, where we have an abundance of through-and-through ventilators at the top and bottom of the wall separating the living quarters from the basement, skewing the basement towards the west end of the house potentially limits the the number of ventilators.

But for the house under discussion here, the problem might be resolvable.  The cathedral ceiling in bedroom #2 could become a flat ceiling, creating space above it for air passage from the basement to the dining room.  Ventilators across the bottom of the bedroom wall and an open bedroom door would suffice for bottom ventilation.  A ventilator in the bottom of the door would do the job when the door was closed.  Similarly, the door from the hall to the vertical basement could house a ventilator to pair with ventilators high in the wall above the door.  And, with the basement wrapping around the master bedroom, ventilators at the top and bottom of its north wall would duplicate what we have.  

If the hill sheltering the house ran southeast instead of due east, the back concrete wall could be extended eastward quite a bit and, with it, the vertical basement enough to allow ventilators at the top and bottom of the living room wall, pretty much duplicating what we have now.

_____________

* For a refresher on AGS, check out the "Featured Post in the left column labeled "Timeline -- Annualized GeoSolar".

**  The best source on early earth sheltering is Rob Rob's book, Earth-Sheltered Houses, How to Build An Affordable Underground House.

***  Our low temperatures in the living space and thermal mass occur during the late winter and early spring rather than in the dead of winter.  By then, the heat stored in the thermal mass during the summer has been depleted enough to compromise comfort somewhat, at least for those of us who look for sweaters when the thermometer drops below the mid-seventies. For details on the thermal performance of our house, consult the recent post.

Design - A Journey Taken

This post is a brief summary of the evolution of earth sheltering since the oil embargo days of the mid-70s and how our project fits in.  It was originally written to supplement sit-down discussions during group visits to our home, especially when the level of interest in the details of passive solar is likely to be average or less than average.  I post the document here thinking that bloggers with only a casual interest in sustainability might find it interesting enough to pause.  Spoiler alert:  A meaningful telling of the story still requires some technical details, so hang with me.

(Click on any photo to enlarge it for better viewing.)

Early Research

Our early research, beginning several years before breaking ground (including visits to several Midwest earth sheltered homes and one of special significance located in eastern Washington state), revealed the following typical attributes of early earth sheltering:

--  South-facing slope in a rural location built in the 70s and early 80s; probably more prevalent in northern US and western mountain regions than in the hot/humid lower midwestern and southern climates

--  Concrete roof covered with earth; concrete north, east and west walls buried in earth with insulation and waterproofing on the outside then backfilled with earth; concrete floor with insulation below it.  The amount of useful thermal mass for moderating inside temperatures limited to concrete in the walls, ceiling and floor to the total exclusion of any adjacent earth

--  Most, if not all, of the south wall conventionally constructed but with more than average insulation and lots of windows to maximize solar gain during cool months

--  Commonplace reports of water infiltrating living spaces through both roof and walls; a problem that was hard to manage with complete earth sheltering

--  Heated with winter solar gain through south-facing windows but invariably supplemented by wood-, cob-, sawdust-, or corn-burning stoves

--  No air conditioning, at least in cool climate locations; most of the houses we visited in our lower Midwest area had AC if not actually full blown HVAC

--  Limited floor space -- usually one story and often one room deep

Further Research

More research radically influenced our understanding of earth sheltering:

--  Considerably less earth contact suffices but still sited on a south-facing hill, no dirt on the roof!!

--  No insulation behind earth contact walls or under the floor; soil becomes principal thermal mass

--  Extra-thick conventional insulation for the exposed non-earth-contact exterior walls and roof

--  Winter heat provided mostly by summer sun and stored in the thermal mass supplemented by winter sunshine through south windows; dubbed "Annualized GeoSolar" by an early advocate (see "Featured Post on the left sidebar); main features of AGS...........                    

Early concept drawing.  Black line below floor level depicts the conduit linking
the solar collector in front of the house with the solar chimney behind the house.
  The orange lines in front and in back of the house depict the insulation/watershed
 umbrella.

1.  Solar collector for harvesting the heat during the long days of summer

2. Conduits under house to distribute the heat to the soil, exiting in a "solar chimney" behind the house

3.  Insulation/watershed umbrella to increase the amount of dry and insulated thermal mass

--  With no insulation between living quarters and earth contact walls and floor, heat flows freely in and out of the thermal mass – out during cold weather, in during warm weather

Our Iteration 

--  Our design utilizes everything listed above under "Further Research" with these additional features:  
     1.  Larger house -- almost 3,000 square feet -- and multi-story

     2.  Town location rather than rural; readily available utility hookups

     3.  Complete absence of conventional HVAC heating (or air conditioning)

--  Strict adherence to sustainability practices -- from groundbreaking to present -- regarding location, design and construction  (in fact,
over-qualified for Energy Star or HERS certification (see the recent post on blower test results))

Two of five rain gardens after a spring downpour, located behind a
berm running the breath of the property that directs runoff to the gardens

--  Blower door test (that measures the rate of air leakage through the building envelope) recorded 1.1 air turnovers per hour, a score, according to the consultant administering the test, much lower than any he had seen in 20 busy years of testing

One of several native gardens; notice in the background
the southern extent of the eastern red cedar shelter belt 

--  Surrounding grounds utilize berms and rain gardens to hold surface runoff until it soaks in and leaves underground and purified                      

--   Landscaping largely limited to plants native to the Midwest 

-- Cold west and north winter winds slowed by a red cedar shelter belt rimming the property on the west and north sides

Thermal Performance

 We have been gathering data on the passive solar performance of our house for a year and a half.  Although we plan to continue reporting on over time, there are enough data to suggest some trends.  The data come from several thermometers within and below the house and one outside the house as follows:

Thermometer on first floor wall 
(the black speck to the right of the picture)

 1.  Living space thermometers at eye level in three rooms and one, read with binoculars, just under the highest point of the second floor vaulted ceiling.  These are labeled "Living Quarters" on the first graph.

2.  Thermal mass thermometers, one heavily insulated from room air at the junction of the vertical basement floor and the north concrete wall and at three depths below the concrete floor of the living room.   These are labeled "Thermal Mass" on the graph.

3.  Outdoor thermometer outside the back door labeled "Outdoor Temperatures" on the graph.

Temperatures have been recorded twice monthly -- on or near the first day and on or near the 15th of each month.  Analysis of the data for this writing revealed that using only one set of data per month was sufficient for painting an accurate picture so, with one exception, the first-of-the-month data were used.

Several piezometers were employed to monitor the behavior of the water table below the proposed house site, leading eventually to a system of French drains that lower the table.  Now we use one of the piezometers still protruding from the living room floor to take the temperature of the soil at three levels under the house down to 15' and to monitor the water table.

Overview

The data for the following graphs span 18 months, from January 2023 through June 2024.

The obvious take-away from the first graph is that, most of the time, the temperatures in the living spaces and in the thermal mass fluctuate in tandem with the thermal mass temperatures running slightly cooler than those in the living spaces.  What is apparent from the raw data in the charts below, more so than from the graph, is that the seasonal temperature changes of the thermal mass lag a month or two behind room temperatures, which is to be expected since air gains or loses heat faster than dry soil.

The second graph shows the benefits of a tight house (see the previous post regarding our blower door test results).  The temperature at eye level on the first floor, compared to 20 feet up at the intersection of the wall and the 

Thermometer at the peak of the
second floor vaulted ceiling

vaulted ceiling, diverged only slightly, marginally more in cold weather than in warm.  By contrast, convectional air movement in a typical -- leaky -- house results in temperatures that are distinctly higher at the ceiling than nearer the floor. 

The rationale for the insulation/watershed umbrella was to expand the size of the dry and insulated thermal mass beyond that directly under the house in order to protect more of the latter from outdoor temperatures.  Over time the mass would cool the house by absorbing heat during warm months and warm the house by re-radiating it into the house during cool months.  And, in so doing, the thermal mass would gradually warm from its lower Midwest legacy temperature of about 60 degrees with the effect being greater at shallow depths and less so deeper down.  But that is not quite what came to be.

The third graph shows temperatures in the soil beneath the living room floor at depths of 5', 10' and 15', taken with a thermometer on a string (pictured below) lowered into the piezometer from which some preliminary assumptions can be made.  First, the "legacy temperature" had already risen to 64 degrees, presumably due to the soil having been exposed to ambient temperatures during construction.  Second, a year-to-year comparison

The thermometer for measuring subfloor temperatures;
weight for measuring ground water levels

of the average temperature for the first 6 months of '23 versus the first six months of '24 differed by only one degree for each of the three depths.  It will be interesting to see if the close tracking holds for the rest of '24 and beyond.  If so, it will be good news because it suggests that the thermal mass may not be warming as fast as anticipated, if at all, in the face of global warming.

The last graph shows temperatures above the floor and at two shallow depths below the floor. The room temperatures were read on the wall thermometer shown in the first photo above.  The second temperature was recorded from water that had been standing for hours in one of the water lines running about a foot below floor level.  The third temperature was for 5 feet below floor level taken through the piezometer.  As might be expected, the room temperatures fluctuate more than those in the thermal mass and, as might also be expected, the water temperature fluctuates more than the soil does 5 feet down.  But as discussed below, all of the differences recorded for 2023 vs. 2024 between the three thermometers are largely insignificant, averaging only a few degrees.

We went to great lengths to control the water table with French drains at the time we excavated the house site.  The purpose of the drains was to syphon off ground water before it could saturate the soil and carry away the valuable heat stored 

Piezometer protruding through living room floor

in the thermal mass.  We monitored the water table by dropping through the piezometer to the 15' depth a spike nail attached to a cord (pictured above).  If water existed, the nail and the cord would be moistened in a way that could be measured. In 2023, water first appeared in mid-March, peaked at 32" in May and was gone by the first of July.  In 2024, it appeared in early May, peaked at 30" in early June and stood at 18" by mid-June, when this was written.  It would appear that the drains are holding the water to a level slightly deeper than 12' below the floor of the house and the highest levels exist for only 4 - 6 weeks.  These phenomena do not pose a serious risk to the thermal performance of that part of the thermal mass that influences indoor temperatures.

Discussion

While the graphs provide an overview, the data in table flesh out the story.  However, it needs to be said that taking accurate temperatures with a thermometer small enough to fit inside of the piezometer was frustrating due to its small graduations. So the subfloor figures in the table are accurate to within +/- 2 degrees at best.  The bold figures on the table are the high and low temperatures over the 18 month period.

It is easy to see a couple of things with regard to high temperatures.  First, based on the one full year (2023) for which data is available, the highest temperatures in the living space, in the thermal mass and in the shallower depths in the thermal mass, were recorded in late summer and early fall instead of in the middle of the summer.  Second, the high readings for the 15 foot depth were so erratic that I was uncomfortable settling on one high figure.  Suffice it to say, though, that the numbers overall for the first couple of years do show a slight warming of the deeper thermal mass.

The low temperatures in the living space and thermal mass occurred during the late winter and early spring for both years, rather than in the dead of winter, but the ones for 2024 are higher than in 2023 by, in some cases, 4-6 degrees.  However, at the 15 foot level, readings year-over-year haven't changed much, maybe a degree or two.  

The outdoor temperatures are useful only in general terms because they vary so much and  cannot be captured with one or two readings a month.  A good example is the low of 35 in February 2023 and a balmy 56 in February of 2024.   However, they do show how the temperatures in our passive solar structure remain comfortably temperate without conventional HVAC, irrespective of the fluctuations of outside temperatures. 

SUMMARY

Seasonal temperatures in the living space and in the thermal mass move up and down slightly but do not deviate much from each other.  The thermal mass stays a little cooler than the room temperatures and changes slower.  There is only a weak correlation between temperatures inside and outside the house, which could be expected given the thickness of the wall and ceiling insulation and lack of air infiltration.  Temperatures within the house do not vary much between floor level and ceiling level, also due to lack of air infiltration.  Contrary to expectations, the temperatures in the thermal mass during the first half of 2024 appear to be almost identical to those in the first half of 2023.  The water table rises to within 12' of floor level but only for 4 - 6 weeks in the late spring or early summer and as such it does not threaten the efficacy of the passive solar design.

These are very preliminary observations based on 18 months data.  We will continue to monitor and report as more data become available.  

Our lower Midwest hot and humid climate is different than the high-plains / mountain west climate of the two pioneers whose designs informed ours.  So it is understandable that, with a warming globe, the summer temperatures in our house could reach the tipping point for future owners who will choose to add conventional air conditioning.  Our current photovoltaic array might even then continue to generate more electricity than consumed (meter running backwards) since its solar gain peaks during the air conditioning season.  If not, it could be minimally enlarged to handle the small amount of air conditioning that would be necessary for comfort.  A nice trade-off, though, is that, with global warming, inside winter temperatures might morph in the right direction.

*               *               *               *               *

UPDATE:  December 31, 2024

The numbers are in for the latter half of 2024 with interesting implications for the entire data set.

The following table is different from the chart above (for the twelve months of 2023 and the first six months of 2024) in that the last six months of 2024 are included and it is reconfigured into cold vs. warm months.

(The bold figures in the chart are the high and low temperatures for each season.)

2023 versus 2024

The best way to look at the figures is to compare the cold months of 2023 to the cold months of 2024 and the same for the warm months.  As was pointed out in the original posting, the coldest temperatures did not occur during the middle of the winter.  They were recorded during March and April after the heat stored in the thermal mass was diminished to the extent that it warmed the house less efficiently.  Similarly, the warmest temperatures did not occur during the early months of summer but transpired during the late months of August and September when the uptake of heat by the thermal mass leveled off.

The figures for last six months of 2024 round out the picture so that additional observations are possible.  For example, I had predicted during the construction phase that it would take several years for the temperatures in the thermal mass under and behind the house and under the insulation/watershed umbrella to reach equilibrium, i.e., remain constant year-to-year.  As soon as we moved into the house in the late spring of 2022, I began taking the temperature of the house but in a ways that were not as encompassing as those that are reported here for 2023 and 2024.  But the less sophisticated figures did show temperatures that were colder in winter and hotter in summer than in the later years.  

So by 2023 I began to think that it might indeed take several years for temperatures to equalize and was surprised when the 2023-2024 temperatures seemed to indicate otherwise.  As a test, I averaged the figures for all nine columns for indoor temperatures and then combined them to give one temperature reading for cold months and the same for the warm months of 2023 as well as for the cold months and warm months of 2024.  The figures for the cold months differed by only one degree -- 69 or 2023 and 70 for 2024.  The figures for the warm months were identical at 74 degrees.  In a nutshell, it seems that the thermal mass temperatures have stabilized quicker than expected.

Interesting Correlation

The last graph in the original 18-month post labeled "Room and Shallow Thermal Mass" shows an interesting correlation between the room and the subfloor temperatures.  The room temperatures were taken from a thermometer on the wall in mid-house and the subfloor temperatures were recorded via water drawn from a water supply line running in the thermal mass about a foot below floor level.  The first take-away from the graph is that the temperatures at the subfloor level ("water temperature") and at five feet below floor level ran in parallel and separated from each other by only a few degrees, validating the use of the subfloor temperatures as representative of the thermal mass.  The second take-away is that the room and the subfloor (thermal mass) temperatures ran in parallel and close together except for a spike in room temperatures during the warm months.

This phenomena can be better appreciated in table form.

The striking thing is that air and thermal mass temperatures are clearly bunched by seasons --  "balanced" for cold months and "absorbing" for warm months.  This means that, during cold months, the temperature of the thermal mass and the living quarters were within at least three degrees of each other but not at the same level throughout the cold month season.  The heat in the thermal mass was indeed gradually depleted but not so fast that the room temperatures fell faster.  And the room temperatures never reached a level that could not be handled with a few mobile space heaters -- a situation that would be impossible in houses without HVAC that were not as tight, as well insulated and as earth-sheltered.

Unlike the matched cold month temperatures, there was a noticeable deviation, except for early (May) and late (October) months, between warm month temperatures in the living space and those in the thermal mass, especially as the warm season progressed to August. This means that the thermal mass is unable to absorb heat as fast as the house accumulates it.

Typically, the conversation around passive solar is about staying warm in the winter, which is undoubtedly warranted in latitudes north of our St Louis area   However, the problem with passive solar this far south will continue to be staying cool in summer.  While global warming may improve our winter comfort, it will worsen our summer comfort to the extent that "passive solar" may include beefed up photovoltaic (solar) panels that support a minimum of air conditioning. 

Human Comfort and Mean Radiant Temperature

Another way of looking at the relationship between the air temperatures and the subfloor (thermal mass) temperatures is through the lens of mean radiant temperature.  The following passage is taken from an incredibly important blogpost for understanding how our house works:

Mean radiant temperature (MRT) is simply the average temperature of solid matter in the surrounding environment and it is more important for comfort than the air temperature in the same environment.  In fact, a 1 degree change in MRT has a 40% greater effect on body heat gain or loss than a 1 degree change in air temperature.  Therefore, when designing living space, it is far more efficient to control MRT than it is to control ambient air temperature.  And the higher the MRT, the lower the air temperature can be.  For example, if we can maintain the MRT at say, 76 degrees, the ambient air temperature could be as low as 62 degrees but our comfort level would be the same as if the air temperature were 70 degrees. Although MRT applies to matter such as wall studs, drywall, wood floors and furniture, it takes something much more massive to provide comfortable environments.

In a nutshell, the temperature of the thermal mass of a structure is more important for human comfort than the interior air temperature.  For us this phenomenon held true in summer more than in winter.  Take for example  August 2024:  if the thermal mass had not stayed 12 degrees cooler than the room air, the comfort level would have been dictated by the 86 degree room temperature.  Instead, with the thermal mass at 74 degrees, the comfort level would have been around 78 degrees.  The fact that thermal mass and air temperatures were balanced during cold months meant that the heat stored in the thermal mass during the warm months was sufficient in quantity and availability to moderate air temperatures during cold months -- not enough to keep them from falling as low as the mid-60s eventually but enough, with the help of portable space heaters, to be comfortable.

Humidity

Another interesting phenomenon has been the amount of humidity in the house.  For the first year, humidity was unpleasantly high due to moisture trapped in the soil under the floor and under the insulation/waterproof umbrella, particularly the soil behind the concrete back wall, the only escape for which was through the floor and back wall into the living quarters.  By early 2023, the humidity stabilized to the 50 - 60% range for most of the year so that humidifiers are no longer necessary and portable dehumidifiers are utilized sparingly to get rid of humidity that originates within the house.  But the Energy Recovery Ventilator and exterior doors do admit moist air that has to be monitored and occasionally handled with dehumidifiers.         

Construction - The Blower Door Test

To enlarge the image for better viewing, click on it.

A blower door test is the gold standard for rating the tightness of a structure or, said another way, the amount of air leaking through its envelope.  Originally, we expected the test to be done in conjunction with certification by        LEED,  Energy Star, or HERS.  However, we learned early on that our our project is too small to interest the local LEED certifiers and that it was too unique to fit the narrow Energy Star qualifications.  Our certifier remained somewhat confident, though, that our project might suit the HERS (Home Energy Rating System).  But in the final analysis, our build was even too unique for HERS -- mainly because it has no conventional HVAC system.   Moreover, none of the rating systems could account for the myriad other unique features  (summarized in outline form in a previous post) that make our build more sustainable than the rating systems are designed to handle.

Following is the slightly edited certifier's report on the blower door test summitted after our mutual decision to forgo any further effort towards HERS certification.

     "I felt compelled to write this summary of the project that I have been involved with since Jerry started almost 10 years.  He came to me wanting to certify his project so people had some idea of how energy efficient the project would be.  We started out as if we could give this project a HERS score and eventually did a blower door test on May 24, 2024.  I had already made several trips to witness the processes and insulation that were installed before drywalling so I could verify what was completed before being covered up.  

     The final (blower door) test results were 1.105 ach (air changes per hour).  Code in the state of Illinois is 3 ach.  It is quite a feat to get to 1.105.  On top of that, with the 16" of rice hull insulation and heating and cooling generated with tempered, radiant ground effect, the homes costs will be extremely minimal.

     There were other factors involved in the design of the house including built into a sloping hill, windows facing in the correct direction, natural ventilation and several other items I cannot attest to.

    It was my pleasure and a very interesting project to be involved with.  I have to say this the tightest home I have ever tested since I received my training in 1996."            (bold face and italics added).

Stan Clark

Advance Green Consulting, LLC, Maryville, IL

*          *          *          *          *          *

Over the 10 years that I have known Stan, we have bantered over the eventual blower door test.  Me:  "Stan, I hope you realize that our house will be so tight than your blower door fan will stall and burn out."  Stan:  "Ha, ha, hope you have the ego strength to handle a different outcome."  Well, the fan didn't stall but it did pull the least amount of air he had ever seen.  Both fan and ego still intact!

Construction -- Interior Casework

It was the fall of 2023 when this post was posted -- a little over 9 years since we broke ground.  That it has taken this long to be within a few months of finishing shouldn't be a surprise considering our unwavering insistence on time as the variable and quality the constant, that we would spend whatever time necessary to do things as well as we were capable.  And, of course, much of what has gone into the project is entirely without precedent and therefore much more inefficient and time consuming than standardized construction.  Then add DIYing.  I lacked experience in the "real" construction world and, when assisted, it was mostly by other DIYers.  

The interior casework has followed the same pattern but even more so.  Doing it with raw sawmill lumber has been extraordinarily time-consuming, not even counting the time it took to sticker and dry the green lumber.  The advantage of using sawmill material, though, is that it lowers costs sufficiently that a committed worker with the proper shop equipment can produce high grade casework at reasonable cost. 

(Reminder: click on any picture for an enlarged view.)

Acclimating the Sawmill Lumber

Sawmill oak for interior trim

Using the saw table as a "paint station" when the
dedicated area for staining and painting was not enough.

The rough red oak for the interior trim was purchased from a local sawmill.  Since its lengths, widths and thicknesses had to vary to meet our needs, its sawing was done while I watched/helped.  It was "stickered", meaning carefully stacked in level layers separated by thin strips of wood (the stickers) and covered with corrugated metal roofing.  Thus it was protected from direct rain, snow and sunlight but was open on all sides to circulating air so that the sap in the wood gradually dried over time without warping.  The recommended drying time is at least a year but longer is better with the knowledge, however, that stacking too long risks "sticker burn" which occurs when the stickers cause discoloration in the milled lumber.  Fortunately, we saw sticker burn on only a few pieces despite our extended timetable,   And our extra effort in stacking and stickering paid off when even the longer pieces were straight and easy to mill.  When crookedness did occur, the boards were usually okay flatwise but somewhat bent, mostly around knots close to the edges of the boards.

Milled lumber in vertical basement

When it was finally time to begin work on the interior trim, the sawmill lumber was de-stacked and stored for awhile in the vertical basement of the house so as to acclimate to the interior temperature and humidity levels.  Thus, its moisture content was matched to that of the house so that the fit of the installed woodwork would stay tighter over time.

Milling and Sizing

In the garage workshop, a piece of sawmill lumber went through at least four processes to become a useable board.  A jointer was used to plane one side perfectly flat -- no concavities, no convexities, no twisting.  With one side flat,

The window sill is extra wide to hold potted plants.  Notice
in the background the raised metal vegetable gardens
and the fenced solar collector for the AGS system.

the thickness planner shaved the other (rough) side flat, smooth and to the desired thickness of the finished board, say 3/4" for most applications.  Next the jointer was used to true up and smooth one of the rough edges of the board.   And finally, the table saw with a long and stable rip fence was used to cut the board to the desired width, running it through the saw with the true edge against the fence.  The sharp blades in the jointer and thickness planer and a fine-tooth blade in the saw minimized the amount of sanding needed to produce a smooth surface ready for staining and "varnishing".

I milled most of the raw sawmill pieces before starting the trim work so as to know what was available to work with.  Then I segregated the pieces; first the widest for baseboards then the narrower pieces for door and window trim -- head casings, side casings, sills, aprons, plinths, jam extenders, etc. --  in order to be sure there was an adequate supply in each category to finish the job.  To my pleasant surprise, the sawmill operator and I had communicated pretty accurately on the width and thickness of lumber needed -- with one hitch -- I forgot to include jam extenders in the order but was able to use oak veneer plywood for them that blended in well enough.

Fitting, Staining, Poly-coating and Installation

We used pre-hung unpainted solid core oak veneer doors rather than making doors from scratch.  After hanging, the doors were moved one-by-one to the shop for staining and clear finishing.  And the casework was dry fitted then stained and poly-coated before installing.  Only the door frames had to be stained and poly-ed in place.

The doors and windows had been set into the 15" thick exterior walls such that there was a 5" space between the window frame and the back surface of the side and head casings that had to be filled with "vertical jam extenders" on the sides and "head jam extenders" on the tops.  Since I was short sawmill lumber for them, I used oak veneer plywood for an acceptable result.  The windows are set back from the plane of the exterior wall at least 8".  Not having them flush on the outside ruled out windows with nailing flanges, making it necessary to eliminate air leakage by traditional means.  Minimally expanding foam filled the gap between the windows and the rough openings supplemented by generous caulking when the exterior casework was installed then more caulking as the jam extenders and sills went in.

Unique Staircase

When I approached the sawmill operator about "showy" wood species for an open riser staircase, he recommended an option new to me -- hackberry.  We stickered and dried it in the roughed-in walk-in closet lined by plastic sheeting.  Aided by a dehumidifier, it was dry by the time we needed it.

The rough hackberry was drab and uninteresting but, once milled, "showy" did not do it justice.  Pieces wide enough for 12" steps were not available but, just as well, my jointer was only 8" wide.  So the steps had to be assembled from narrower hackberry boards with a filler of black walnut interposed.  The stringers were conventional 2-bye lumber veneered with hackberry.  Clear poly-coating made the unique character of the hackberry pop and the dark walnut added a nice touch.  The store-bought balusters and handrails, stained to match the other woodwork in the house, provide a pleasing contrast to the light colored stair treads and stringers.

Door and Window Trim

In order to mimic the country style of a bygone era, the head casings and the splinth blocks (below the side casings next to the floor) are wider and thicker than the 3/4 inch thick door side casings and the baseboards.  The thickness of the head and side casings of the windows are dissimilar in the same way.  And the baseboards are 6" tall, definitely a throw-back. 

Filling the nail holes in the woodwork was no small task.  It was done with stainable wood filler and a long learning curve to be able to know how soon to remove the excess stain in order to reach an optimal result. The hole filling and wood filler staining nevertheless caused enough blemishing of the poly-ed surfaces that a final (third) coat of poly was required throughout.                                                                                             

Early Spring, 2024

The interior casework is finally finished leaving only three major projects before the house is complete.  The build-ins for the second floor office still have to be done, the porch needs screening and the temporary solar chimney needs to be replaced with a proper chimney with the conduits running to it buried rather than exposed as they are now, mainly for esthetic reasons but also to make mowing easier.

The office project will be the most time-consuming in that the cabinetry will be custom-built from the sawmill walnut that I stickered many years ago then stored under cover until there was a good use for it.  A walnut dining room table is also planned.

A Note of Appreciation

Despite being 90, I continue to work on the house at least six hours daily and often seven days a week but getting up from the floor is harder now.  I completed the more accessible door and window trim but was only too happy to delegate the baseboards to son-in-law, David, a retired machinist who made hundreds of ups and downs from his knees to the saws in the workshop in order to stick to the machinist-like tolerances that made him happy.  He also took on the fastidious job of installing the balusters for the stairs and the second floor catwalk -- about 140 total -- which also required dozens, if not hundreds, of trips to the workshop in order to meet his workmanship standards.  If I had attempted the baseboards or the balusters, there's no doubt the quality would have suffered.

Odds "N Ends - Estimated vs. Actual Costs - A Preliminary Analysis

It is safe to say at this stage that the cost overrun for our project will be formidable, even as much as twice my original estimate but still less than half the cost of contractor-built comparables.  (Reminder: click on any picture to enlarge it for better viewing.)

                                                 UNDER-ESTIMATING COST

A page from the original estimate

For a DIYer to think s/he could come even close to estimating the cost of such an atypical custom-built home is the definition of naivety if not insanity.  When the project is done, I will, as best I can, spend the time to research invoices, credit card statements, cash payments, etc. and compare the findings with my original estimate in order to quantify my naivety.  
In the meantime, it's interesting to look at our project in view of online cost estimates for home construction in the Midwest.  What follows is a theoretical analysis (read: wild guess) based upon industry averages for traditional stick-built homes having some degree of customization.  

While we avoid the cost of conventional HVAC, we add costs for French drains, the various components of the AGS system*, greater volume of concrete, insanely higher R-values for the building envelope, a free-standing energy recovery ventilating system, top-of-the-line windows and doors, ventilated (double-layered) roof and compulsive air sealing for the building envelope to name some of the unique attributes of the home.  Except for the AGS system, all of the features enumerated would improve the energy performance (and raise the cost) of a conventionally built house as well.  So it seems fair to say that not having to buy and support a HVAC system (initial cost, long-term maintenance and energy consumption) more than pays for the AGS system, leaving the cost of the rest of the building fair game for comparing with conventional construction but, at the same time, realizing that "conventional construction" is merely a poor stand-in for the lack of comparables for our advanced and enhanced project.

(Spoiler alert.  If your interest level is waning already, you might want to forego the details in the next three paragraphs by skipping to the last, summarizing, paragraph in this section.)

A quick online search suggests that the per-foot cost in the Midwest for conventional construction ran $121 in 2017, a year that fell about midway through our construction period.  A figure of $121 / sq ft would bring our not quite 3,000 sq ft build in at $363K, half of which, according to the online source, would be the cost of labor with another 10% going towards contractor profit, leaving 40% for materials.  For our project, not all labor was DIY or volunteered.  Some of it was done by professionals, including a journeyman carpenter and a plumber friend.  However, all non-volunteers, whether professional or amateur, received at least $15/hr and, more often, $20, which is what I consider to be a fair range for minimum wage.  And, since we paid separately for the materials involved with all phases of construction, including the dirt and concrete work that preceded the actual build, the excavation and concrete contractors' fees were for labor only, mirroring the wages of the others that helped with later construction.  I would guess then that the ratio of hired and contracted labor as opposed to my (sweat) labor and that of volunteers would be about 

1.5 : 8.5 or 15% for paid labor and 85% for volunteer and my labor.  How that plays out in terms of dollars is the rest of the story.

First, the generic labor cost has to be reconciled to fit our situation in order to be able to consider the other costs of construction -- materials and contractor profit.  If 50% is the generic cost for labor, the labor costs for a $363K house would be $182K.  In terms of dollars, the 85%:15% ratio for labor amounts to $155K for sweat/volunteer labor and $27K for hired labor.  Subtracting $27K from the $363K leaves $336K for everything else -- contractor profit, sweat/volunteer labor and materials.  Since there is no contractor profit to be paid, all of the $336K can be allocated to materials and to the value of sweat/volunteer labor.  Since the time has not come for a tally of the cost of materials, let's assume that their cost is what is left after valuing the sweat/volunteer labor.

So where are we now?  The generic cost of labor is 50% of the cost of construction or $182K from which we have subtracted $27K for contracted labor leaving $155K for sweat/volunteer labor.  Not having to pay the 10% contractor's profit saves another $36K which, when added to $155 savings on labor means that $191K represents that part of the cost of construction for which no cash is needed.  Or, to say it another way, $191K subtracted from the total cost of $336K leaves $145K for materials and miscellaneous costs requiring cash.

So, remembering that the generic square-foot cost of construction in the Midwest is $121, how do our figures compare?   When $145K is divided by 3,000 sq ft, our per-foot cost is $48 or about 60% less than the generic cost.  My original estimate was $34 sq ft for a smaller house (2,100 instead of near-3,000 sq ft), so, on a square-foot basis, I underestimated the cost by at least $30%.  What that translates into in terms of real world expenditures will have to wait until I do the post-build analysis but I suspect that it will prove to be somewhat on the low side.  Even so, it's nice to think that my major bucket list item -- to DIY a house -- has not only kept me enthused and in better health during my golden years but will have provided a nice return-on-investment for our heirs.

UNDER-ESTIMATING TIME

                                                    

My ability to estimate cost proved to be better than estimating the time it would take to build the house and to reconfigure the surrounding grounds, largely because I grossly over-estimated how fast I could get things done working mostly alone.  Having even one other pair of helping hands throughout construction would have sped things up, not just twice as fast but three or four times faster than working alone.

Does DIYing pay dividends?  Maybe, Maybe Not.

I really did think early on that it would take only a couple of years, maybe three, to build the house.  If it could have been done that quickly, the sweat equity figure that I used above would mean that my wages would have been $57K to $85K per year.  All well and good except that construction by the time it is completed will have taken at least eight years.  Divide the $170K sweat equity by eight and $21K per year might be okay for us retirees but nothing to write home about otherwise.  So I guess the lesson here, based upon these wild-guess figures is that a person should think twice about quitting his/her day job to build a house this unique, complicated and time-consuming.  Either s/he should keep the job and hire a contractor or stick to a more conventional design such as using structural insulated panels (SIPs) that could in fact be DIYed in 2 - 3 years.  The problem with having our house professionally built is that it would be hard to find a contractor who would be willing to undertake the risk involved with a ground-breaking, non-standard project unless the owner was willing to contract on a time-and-materials basis which would probably open a bag of worms for the owner.

Dirt Work

Having never sat in a track loader, I was ill-equipped to appreciate the amount of time it would take to move dirt around for our passive solar design.  Digging into the hillside to site the house was time-consuming because of the volume of dirt that had to be moved with a 6' wide bucket and the fact that each bucketful had to be carted to the storage area behind the excavation at least 100 yards away rather than swung to the side as is typically done for basement excavations.  Lots of dirt had to be rearranged in connection with earth sheltering and for the insulation-watershed umbrella of the AGS system.  Contouring the surrounding grounds to funnel run-off to numerous rain gardens also was time-consuming. 

Salvaging lumber from one of several tear-downs

According to the hour-meter on the track loader, I spent 415 hours operating it or about 52 eight-hour days.  Within the context of my original estimate of two years and using 2,000 hours per year as working time, 415 hours would have consumed +/-10% of the total construction time.  As it turns out, 3% over six years is still not inconsequential.

Pre-Assembled Wall Trusses 
I began tearing down old houses and

Jig with truss in process.

out-buildigs  and de-nailing the salvaged lumber a couple of years in advance of starting construction.  Later, when the weather interfered with the dirt work, I worked under shelter using some of the lumber to pre-assemble 30+ wall trusses for the 15" thick exterior walls.  These pre-construction tasks took hundreds of hours, mostly for processing the salvaged lumber once it was on our property.

 Altogether there were seven French drains as long as those
on the left; all were covered with filter cloth as is being done 
to the one in the middle; notice off to the left the edge the 
huge dirt storage area behind the building site.  (Click on 
 the picture for an enlarged view.)

French Drains

Knowing that the thermal mass under the house would have to remain dry for proper performance of the AGS system we enlisted the help of a soil engineer.  On his recommendation, a contractor drilled and sank four peizometers quite a few feet below the anticipated location of the AGS conduits.  Two years later during an especially wet spring and early summer, ground water rose sufficiently high in the peizometers to threaten the future AGS conduits.  The seven French drains that

Friend, Pat, laying a conduit that starts at the white
PVC pipe angling up from the trench behind the footprint
of the building and ending at the location of the future
 front foundation of the house and later to be connected
to the yet-to-be-built solar collector. 

were necessary to mitigate the problem were installed before final excavation for the house.  They took only one day with a large crew of family and friends assisting the excavation contractor but assembly of the drains from plastic culverts ahead of time took me considerable time.

AGS Conduits

Collector shell ready for backfilling the gap between it
and where the front foundation for the house would be
 situated.  By the time the picture was taken, solid PVC
pipes had been laid and buried, joining the black  AGS
 conduits with the solar collector.  Notice the terminal
ends of the conduits in the distance.

Trenching and burying the nine AGS conduits also was easily done in a single day by the contractor with several of us helping.  The time consuming part was connecting them to the solar collector on the south and managing, late in construction, the terminal ends on the north which stuck out of the ground several feet initially.  

Solar Collector Shell   

Construction of the solar collector shell out of dry-stacked concrete blocks that were fiber-bonded-cement-parged and connecting it to the AGS conduits took several weeks mainly because of heavy spring and early summer rains, and was way more physically taxing in the summer heat than I anticipated.  Once it was completed, the conduits run to it and the open pit between it and the footprint of the house backfilled, construction on the house could finally begin.

UNANTICIPATED COST OVERRUNS

The amount spent on contracted dirt and concrete work prior to the carpenter phase of construction was indeed a major surprise.  The cost of concrete itself was somewhat anticipated but the amount paid to contractors for the final excavation and pouring was beyond pale or so it seemed to me at the time being unaware of the size of their capital investment in equipment and the seasonal nature of their work in our climate.

Once that expensive phase was behind us and we started the carpenter phase, most of the overrun was for unanticipated labor and for inflation.  Otherwise, we were doing familiar things that hued reasonably close to the original budget.  Labor costs started rising when I found that I needed a second pair of experienced hands for such things as setting trusses, sheathing walls and roofs and installing metal roofing, siding and soffits as well as for some of the drywalling and blowing insulation.  The 8 years after groundbreaking in late summer 2016 was a long enough span to see inflation of the cost of materials.  For example,  the price of 1/2" drywall rose by 33% between the time I did the estimate in 2014 and actually purchased it in 2019.  The rice hulls for insulation went from $1,400 per truckload to $5,000 due partially to inflation but also other factors (as explained in one of the posts on insulating). 

And there was an exuberance factor.  I found it too easy to have the mindset that, "Oh well, we are already overbudget, so why not spend a little more for upgrades on X,Y,Z.".  This tendency manifested primarily while wrapping up the interior for such things as flooring, bathroom fixtures, kitchen and laundry appliances and lighting.

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*For those who have not followed the blog enough to know about the Annualized GeoSolar system that will provide year-round comfort in the absence of conventional HVAC, click on the title under "Featured Post" near the top of the left column.