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 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 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.  No conventional HVAC system

--  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 - First 18 months

 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 is enough data to suggest some trends.  The data comes 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 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.

An  earlier post described using several piezometers to monitor the behavior of the water table below the proposed house site that eventually lead 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, 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.  By contrast, convectional air movement in a typical -- leaky -- house results in temperatures that are 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.  And, with global warming, a thermal mass temperature that is not rising would be a plus for summers and a non-issue for warmer winters.

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 stood in one of the water lines running less than 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 did not appear until 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 chart 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 chart are accurate to within +/- 2 degrees at best.  The bold figures on the chart 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.  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 (click on  "Featured Post" in the left sidebar to access a series of posts describing their passive solar designs).  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 will morph in the right direction.

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!