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

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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!