Design - Update On the Thermal Performance of the GeoSolar System

The fenced area in front of the house is the solar collector,
 the downstream air intake for the GeoSolar system

The analysis of the of the house temperatures during 2022, the first partial-year of occupancy, was incomplete and inconsistent as we positioned and repositioned thermometers in an attempt to capture the best data.  Consequently, this report uses data only from the two full years of occupancy --  2023 and 2024 -- that were tracked with identical protocols.   2025 will not be included until the end of the year.

The chart below is a distillation of a more detailed comparison of 2023 and 2024 but it sufficiently summarizes the data in a user-friendly way.  The "feel good" temperatures were calculated as follows:

               Room temperature - Ground temperature x  0.40  =  X

                Ground temperature  +  X   =  Feel Good Temperature

As described in a previous post on the thermal performance of the house, the room temperatures were taken from a thermometer on the wall in the middle of the house.  Ground temperatures were registered by lowering a thermometer to a depth of 5 ft through a pipe protruding from the living room floor.  0.40 is a multiplier that reflects the fact that the temperature of the thermal mass (the soil surrounding the pipe under the house) has a greater effect on the comfort level in the house than does the air temperature -- actually slightly more than twice the effect of air temperature.  As an example, the hottest recording for 2024 was taken on August 1 when the air temperature was 86 and the mass temperature was 74, rendering a tolerable feel good temperature of 76.

The striking feature of the year-to-year comparison is the similarity between them.  In the table, deviations of more than three degrees are highlighted with colored font -- blue when the temperature for 2024 is lower than in 2023 and red when the opposite occurred.  Over half of the year-to-year values deviated less than one degree; five less than two degrees.  If the figures in future years are similarly consistent, it would signal that the GeoSolar system has reached equilibrium.  

The conduits for the system are widely separated under the house and therefore widely separated when they emerged to daylight behind the house.  My thinking was that, by leaving their terminals individually accessible, some could be deactivated if the thermal mass began to overheat.  But, since the passive functioning of the conduits didn't work as intended, they had to be connected above ground and run to a temporary solar chimney so that a fan could be used to pull air through the pipes from the solar collector end.  The temporary system worked so well that it was recently replaced by a submerged system that allowed successive "Y-ing" of the pipes so that only one large pipe entered the solar chimney from each side.  And the ugliness of the exposed pipes was eliminated.  But burying the pipes precludes future changes which, based on bimonthly thermometer readings for two and a half years, seems now to be a non-issue.

The summer feel good temperatures in the upper 70s might be too extreme for some folks who are  accustomed to air conditioning.  As the chart shows, the hottest temperatures occur in the late summer and early fall and last only four to six weeks.  The high temperatures are manageable like they were in the days before air conditioning -- by having high ceilings, keeping the air moving with fans and opening the windows at night.  And it occurs at that time of year when nighttime temperatures are dropping enough to make window opening as effective as it is earlier in the summer.  

Winter feel good temperatures dipping below 70 degrees last longer, actually from the first of the year until late spring.  But hovering in the upper 60s, as they do, is a healthy range and would probably be comfortable for most folks accustomed to conventional HVAC systems.  In our view, any temporary inconvenience, in either winter or summer, is a welcomed trade-off for the savings -- thousands of dollars per year -- in electricity costs. (The details regarding the cold spring temperatures and the warm late summer temperatures, are thoroughly parsed in a previous post.)

 

The photo at right shows the temporary solar chimney and the conduits that originally came to daylight individually.

The photo below shows the permanent solar chimney with the conduits running to it underground.  Like the temporary chimney, it contains a small furnace blower that pulls air from the solar collector in front of the house through the nine conduits under the house.  Despite the PV array in the background being the smallest the vendor would sell, it provides all of the power needed by the house, including for the 240v tools in the garage-workshop.  The electrical requirement for the new chimney blower therefore becomes an inconsequential load.  

(Update, late July, 2026:  Consistent with global warming, our summers in the St Louis area are hotter and wetter and our winters are milder causing to us rethink the way we use the AGS system (solar collector, underground conduits and the solar chimney) for warming the thermal mass during warm months.  The data we have for the first few years seems to validate the design of the system but will its efficiency prove to be counterproductive in a warming world?  With a focus on making our winters comfortable, might we be making future summers uncomfortable?  Shouldn't we be less focused on winter heating and more focused on summer cooling?  In an attempt to answer these questions, we are not using the AGS system for heat collection this year as we have done in prior years and intend to keep it mothballed for the rest of the warm season in order to see how doing so impacts our data -- both for cool weather and warm weather.  Accordingly, the data reported here for the first of 2026 reflects the new protocol.)

Electric bills divide costs between power consumption and various administrative costs.  Our power consumption for the entire year of 2024 amounted to $160 -- not absolutely passive solar but close.  "At 19 cents per kilowatt-hour, the current price of electricity is the highest on record going back more than forty-five years" according to a July 31, 2025 article in the St Louis Post Dispatch, further validating the wisdom of our project.  What is especially satisfying is that, with reverse metering, the array has already paid for itself.  But the diminutive size of the array is possible only because of the passive solar design of the house.  If its nearly 3,000 sq ft were conventionally-built, it would require a solar array that was significantly larger, more expensive, require a longer pay-off period and, even then, probably never supply the amount of electricity the house would need. 

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

Design - Sustainability Inventory - Part II: Living Quarters

 By now, our 8-year journey to energy independence has been documented with nearly 150 individual blog posts.  For those who may be curious about our project but not enough to wade through myriad posts, perhaps the previous Part I and this Part II will suffice.

Click on picture to enlarge for better viewing

Part I covered the non-living spaces of our passive solar home -- the grounds, the exterior shell of the house, the screened porch, the garage, the airlock and the vertical basement. This post outlines how the interior living spaces contribute to the advanced  passive solar design.

This report posts during early winter 2022 after having taken occupancy in late March.  

Common Features

            The windows are double-thickness with two kinds of glass – transparent on                  the first floor and translucent on the second

The transparent first story windows are backed up by thermal mass – the concrete floor and the soil beneath it. The solar gain through the windows warms whatever it shines on -- the floor primarily but also the furnishings which have minimal thermal mass but still contribute to warming the air.  And, the medium shade for the floor hits the sweet spot -- a dark color would overheat and a light one would reflect, rather than absorb, solar energy.

                         The fact that the windows on the
                      second floor are not backed up by thermal mass and are more                                    numerous than on the first floor requires a different strategy for                                    maximizing solar gain during the winter.  The glass in most of the                                second story windows is translucent (frosted).  Unlike transparent                                 glass through which sunlight enters uninterruptedly and heats                                    whatever it shines on, translucent glass diffuses or scatters sunlight                            so that, in the absence of thermal mass, it heats the interior contents                            randomly and without overheating. 

The most prevalent wall color in the house is rated at 65% reflectance which is the sweet spot between reflectance and glare.  The reflective paint works in tandem with the translucent glass to scatter winter solar energy. 

Our windows, in addition to transparency vs. translucency, have three additional energy-saving features – low-e coating, fiberglass frames and swing design.

Low-e coating for a double-glazed window blocks up to 90% of the heat loss on winter nights and cloudy days.

Fiberglass frames are “greener” than the more ubiquitous vinyl frames

             Less embodied energy (made from sand rather than                         petroleum).

Co-efficient of thermal expansion matches window glass for better seal of double-glazing as well as minimizing air leakage between frame and glass; contrastingly, vinyl expands (summer) and contracts (winter) much more than window glass.  

All windows are casement or awning (both swing style) that, on closure, create a compressible weather-proof seal.  Sliding styles (double- or single-hung or side-by-side), need greater tolerances for sashes to slide freely within frames and, as such, have more potential for leakage.

Polished and stained concrete floors, in the absence of tile and carpeting, maximize the flow of heat from the mass in winter and into the mass in summer; in a few years, after the mass temperature reaches equilibrium, tile would probably work without compromising the thermal performance of the house. 

Handicap accessibility, e.g., 36” doors with lever-type door “knobs",                           elevated wall  receptacles, no grade changes between drive and house.  

Combination fire and CO alarms for sleeping areas hardwired (with battery backup).

Numerous through-the-wall ventilators that allow stale air accumulating at the high point of the sloped ceiling of the first floor to escape to the second story. 

         Continuous ventilators at the top and bottom of the tall north wall, i.e., the one           separating the living space and vertical basement, allow air to move back              and forth through the wall and either deposit or extract heat from the thermal              mass (concrete vertical basement wall and the earth behind it) depending on              the season

Entryway / Kitchen / Laundry / Dining Room

Entryway wall veneered with handmade bricks salvaged from 19^th century farmhouse.

Salvaged kitchen and laundry cabinets are Facebook Marketplace finds; laminate (Formica) countertops instead of natural stone with its embodied energy (long distance importation or energy-intensive manufacturing).

            All appliances, range hood and ceiling fans are Energy Star rated.

Lowered countertop area that doubles as lunch counter and ADA-compliant work surface.

Supply and return vents for ERV system.

Living Room / TV Area

Continuous ventilators at the top and bottom of the tall north wall (mentioned above).

Wall switches for all TV and computer receptacles to control phantom loads.

Guest Bathroom

            Salvaged wainscoting for one wall

            Porcelain tile for tub surround gifted by contractor (left overs from a job)

            Recycled ceramic (rather than fiberglass) bathtub (Craigslist)

            Wash basin mounted on treadle-type sewing machine

            Low-flow faucets and shower

            1.2 GPF handicap height toilet

            Exhaust vent for ERV system

South Bedroom

            Energy Star ceiling fan

            Supply vent for ERV system

Master Bedroom / Walk-in Closet

            Energy Star ceiling fan

            Supply vent for ERV

            Repurposed closet organizer (Facebook Marketplace)

Master Bath

1.2 GPF handicap height toilet

Low flow faucets and shower

Exhaust vent for ERV

Recycled vanity with manufactured stone countertop rescued from dealer’s “bone yard” (laminate is no longer recommended for bathroom vanities)

Handicap accessibility

                        Wheelchair accessible shower with controls reachable before                                       entering spray

                        25 sq ft  of floor space in middle for wheelchair turning

                        One sink is handicap (and child) accessible

                        Numerous grab bars

Second Floor Balcony (Catwalk)

            Vents from first floor to syphon off warm air accumulating at high side of                       cathedral ceiling.

Click-together composite floating floor; composite is more sustainable than vinyl and, by being manufactured from pulp wood, is even more sustainable than conventional hardwood flooring; medium color for a balance between absorption and reflection of solar energy.

“Green” windows

Translucent window glass vs. transparent glass for better thermal performance in the absence of any significant thermal mass on the second story

Windows inset 11” from exterior wall plane to reduce heat loss due to “wind washing”

Only enough operable windows for adequate ventilation (fixed windows have a much higher Energy Star rating than identical windows that are operable)

Second Floor Bedroom / Bathroom

            ERV supply vent (bedroom) and return vent (bathroom)

Corrugated roofing for bathroom wall cladding (long-lasting and recyclable end life)

            Recycled wall cabinet (Habistore)

            1.2 GPF toilet

Unfinished projects

Custom stairway and railing system from rough sawmill lumber:  stair steps from hackberry; balusters from hackberry and red oak, railings from red oak

Interior casework (baseboards, trim around doors and windows, etc.) from rough sawmill red oak

Once the interior is finished, a certified consultant using a blower door tester will ascertain the tightness of the house – a major criterion for either a HERS rating (Home Energy Rating System) or NAHB rating (National Association of Home Builders) that we will seek.

ACKNOWLEDGMENT

We will be forever indebted to the government of Collinsville, IL, particularly the Building Director and his staff for their forbearance during nearly a decade of construction.  If the folks at city hall had not taken genuine “ownership” for the town's first "green" project, it would never have been built.

RECOGNITION 

At Collinsville's 2022 Awards Night, the local chapter of the Cool Cities Committee recognized our project with its "Cool Cities Sustainability Award".  (The nationwide network of Cool Cities Committees was birthed by the Sierra Club.  As ambassadors for sustainability, the club's members serve as advisors to the City Council as well as to civic committees, schools, organizations and citizens.)

Design - Sustainability Inventory - Part I: Non-Living Spaces

 

Click on picture to enlarge for better viewing

By now, our 8-year journey to energy independence has been documented with nearly 150 individual blog posts.  For those who may be curious about our project but not enough to wade through myriad posts, perhaps a "30,000 foot view" will suffice even if it takes this post and one more to do justice to the story.

This report posts during the first summer after taking mid-winter occupancy.

UNIQUE FEATURES - OVERVIEW

 Project located in Collinsville, IL on a south-facing slope nestled           in the Mississippi River bluffs opposite St Louis, MO             

         Grid-tied and nearly breaking even with the utility company

        Entirely passive solar; no conventional HVAC

        Sustainability-centric from preliminary sketches to occupancy

        ADA compliant

(The following outline sometimes dwells on these “unique features” but, for most part, they are woven into the larger narrative.

EXTERIOR FEATURES

House

Rectangular-shaped house with its long profile facing “solar” south but canted slightly westward to catch more afternoon winter sunshine

Square and rectangular configurations are the most energy efficient because they minimize exterior wall surface,

They also require fewer building materials and therefore minimize embodied energy for manufacturing and shipping of materials; fewer materials also lowers construction costs

Of note:   the amount of earth sheltering, the conduits for the AGS system and the
PV array  (click on picture to enlarge it)

          

            Earth sheltering on three sides of the house; the grade in back

            of the house is 12' higher than in front, creating the thermal mass described                 below under "Annualized GeoSolar System"

Exterior walls supported by custom trusses built from lumber salvaged from tear-downs; roof/ceiling supported by "store-bought" trusses

House insulation:  15" thick (R-45) in exterior walls, 18" thick (R-54) in cathedral ceilings         

The joints between individual roof and wall sheathing panels and joints between roof and walls are taped to eliminate air infiltration; all sheathing is plywood rather than OSB (both products may utilize resins containing formaldehyde but plywood has less)

All windows (except one small east window) face south which maximizes winter solar gain; having none facing north or west precludes penetration of north and west winds in winter and unwanted morning and afternoon sunshine in summer

Windows all have overhangs sized to minimize solar gain during warm months while allowing maximum gain during cool months; mid-summer shading by the overhangs for both stories is apparent in the picture above, so much so in fact that the exterior wall looks bluish instead of white

Front entrance and garage doors are shielded by the house from winter winds

Screened porch (yet to be finished) is extra tall for better ventilation; Energy Star ceiling fan; 3' wide soffits (eves) for more shade and better rain diversion

Light-colored steel siding, roofing and soffits

Steel has at least a 60-year life expectancy under normal circumstances; more hail resistant than petroleum-based siding and roofing

          100% recyclable end-life

Smooth, white siding is highly reflective, thus minimizing heat gain during warm months; light gray roof only slightly less reflective than the white walls        

Cool roof design:  A 3½” space exists between the first layer of sheathing (that which lies above the insulation) and a second layer of sheathing (to which the metal roofing attaches) that creates a “mini-attic” through which air passively flows from the soffit vents upward to ridge vents, keeping the roof cooler in summer

Rain Handler system replaces conventional gutters except over traffic areas; converts sheet water from the roof back into raindrops that nourish nearby native vegetation without causing soil erosion

Grounds

Four rain gardens slow runoff and keep it on site until it can soak down to the water table and leave in a purified state as opposed to running on grade and carrying noxious contaminants (from air pollution and from herbicides, pesticides or fertilizers) to neighbors’ or public property and, in our case, to the lake across the street

Rain gardens as well as other landscaping are or will be populated by native plants which require no energy-dense artificial fertilizers or soil amendments and, after the first growing season, require no watering

Shelter belt of native eastern red cedars shielding house from winter winds planted years ago as bare-rooted seedlings; cedars also provide excellent habitat for wildlife – four-legged, flying and creepy-crawling

Photovoltaic array sized to break even with the utility company (“reverse metering”); energy cost limited to natural gas for water heating, clothes drying and cooking

Annualized Geo-Solar System (AGS)

AGS utilizes the energy from the summer sun, supplemented by sunshine through the windows in winter to condition the house year-round in the absence of conventional HVAC

AGS comprises three components

Solar collector (fenced-in depressed area in front of the house) to harvest heat from the sun during warm months

Conduits (4” pipes) slanting upward from the collector to a solar chimney behind the house distributing heat from the solar collector to the earth under and behind the house; design still in flux at the time of this writing -- (see second thoughts on solar collection design)

Insulation/watershed umbrella that extends outward from the house below grade +/-20’ in all directions, hence enlarging the amount of dry and insulated thermal mass available for heat storage beyond just that below the floor

Umbrella lays 12" - 30" below grade and consists of several layers of 6 mil plastic sheeting and a layer of foam board insulation overlaid by a physical barrier of recycled carpet

Heat during cool months comes from that stored in the thermal mass during the summer moving passively into the living space (heat seeks cold); additional heat comes from sunshine through the south-facing windows during winter; conventional heating is unnecessary

During warm months, the heat lost from the thermal mass during the winter is replenished by (a) heat from the AGS solar collector and (b) by heat absorbed into the thermal mass from the living space; conventional AC is unnecessary

Target temperatures for the living space toggle between lower-to-mid-70s (winter) to high 70s - lower 80s (summer), although a good guess is that it will take the heat from at least two summer seasons to overcome the ancient cold exposed by excavation into the hillside (at our latitude, the legacy year-round soil temperature at the depth of 10 ft is 60 degrees)

The house floor (concrete) temperature on our early March move-in day was 60 degrees measured at the base of the concrete north wall; it had risen slowly to 79 degrees by the time of this writing in mid-summer

(The comfort level in an earth sheltered dwelling is a function of a phenomenon called Mean Radiant Temperature.  For an understanding of this important concept, click on the first post at the top of the left column under the heading “Understanding Our Project")

UTILITARIAN SPACES

Garage and Airlock

Garage has super-insulated walls and ceiling; the floor is insulated by the underlying insulation/watershed umbrella; the temperature in the garage was 80 degrees at the time of this writing, up from the upper 60s during the winter

Small area of garage wall is intentionally left uninsulated to create a storage area for fruits and vegetables during cold months (a mini-iteration of the old fashion root cellar)

Wall cabinets in the garage are a Facebook Marketplace finds

The main door of the house leads to a circumscribed space called the “airlock”, essentially a semi-conditioned breezeway that modulates outside air rushing in when the front door is opened – warming it in winter and cooling it in summer – before the outside air can follow into the living space through a secondary door

All four doors leading in and out of the airlock are self-closing so as not to be left open unintentionally 

Vertical Basement

The back (north) concrete wall, 60’ long and 12’ tall, by conducting heat in and out of the soil behind it, is critical for conditioning the house

The distance between the concrete wall and the stick-built wall separating the basement from the living quarters is 5' which provides space for storage shelves 2' deep leaving 3' for wheelchair accessibility

The insulation/watershed umbrella at the top of the wall shields the soil under it from surface water penetration while the insulation traps and holds heat

All but the top foot or so of the wall is backed by the soil under the insulation/watershed umbrella that will gradually become drier and, instead of its legacy temperature of 60 degrees, will maintain temperatures in the 70s 

Lack of insulation on either side of the wall under the umbrella allows heat to pass freely through the wall – from the thermal mass inward during cool months and outward into the thermal mass during warm months

The continuous row of vents near the top and bottom of the tall stick-built wall (that divides the vertical basement from the living space) are critical for allowing room air to reach the wall 

During warm months, three box fans equidistantly suspended from the ceiling  and directed downward can be used to transfer heat to the concrete wall by pulling (warm) air through the top vents and pushing it downward past the concrete wall and out through the bottom vents

Combination GFCI / AFCI circuit breakers in the service panel eliminate the need for individual GFCI / AFCI receptacles at the point of use in the living space

Free-standing (not attached to a conventional HVAC unit) Energy Recovery Ventilator supplies fresh, temperature-and-humidity-moderated outside air without which our airtight house would soon fill with stale and unhealthy air

Home run (as opposed to branching) water supply system using PEX (polyethylene) tubing and a central manifold; the PEX lies below the floor and was shielded from damage while pouring the concrete by running it inside of 1 ½” PVC piping; the PVC also makes it possible to use an original PEX line to pull a replacement line in the unlikely case of a water leak; PEX for supply lines is "greener" than copper lines initially and because it has an indefinite life span

Energy Star tankless (“on-demand”) water heater that heats water once as it is used; more energy efficient than tank types that heat the same water over and over, especially during periods of low or no usage

“Water Cop” system to prevent flooding in the living spaces due to plumbing failures 

___________________                        

The next post covers the living areas of the house.

Design - Annualized GeoSolar System - Second Thoughts On Its Configuration

Annualized GeoSolar (AGS) is explained in a string of posts early in my blog that can be accessed by clicking on "Timeline - Annualized GeoSolar" near the top of the column at the left under the heading "Featured Post".  This post now questions whether our interpretation of AGS was ideal, at least for our build, and, if not, what might have been better.

Our AGS system utilizes the insulation/watershed umbrella and collector system first advocated by Stephens who, in turn,

Solar collector in front of house

borrowed the umbrella concept from Hiatt.  Our system followed Stephens' prescription to the extent that the conduits run parallel from the collector to the front of the house then fan out under the floor of the house.  But, instead of converging behind the house in a "solar chimney" as Stephens recommended, our conduits continue on straight paths and emerge

Conduits widely separated behind the house

to daylight individually.  My reasoning was that, the conduits were slanted upwards sufficiently from the collector to daylight behind the house that the heat generated by the collector would rise through them unassisted.  I gambled that ending the conduits in a solar chimney would not significantly improve convection while anticipating that backfilling with the track loader around divergent pipes would be much easier than backfilling around convergent pipes (which largely would have been an arduous shovel and wheelbarrow job).

However, as explained in an earlier post, spontaneous convection did not occur due, I suspect, to the earth under the house being so cold as to reverse the airflow -- suck air through the upstream end of the conduits and expel it at the collector end -- to extent that the heated air generated by the collector was unable to reverse the flow.  I also suspect that, in a few years after the thermal mass under and behind the house warms to a constant year-round temperature in the '70s, a reverse flow would be much less likely to be a problem.  The basis for this assumption is thoroughly examined in the three posts at the top of the column at the left under the heading "Understanding Our Project".

Temporary Solution

Obviously, it would take a fan to change the direction of the airflow.  For it to work, the conduits would have to converge and empty into

The temporary configuration

a solar chimney just like Stephens recommended.  So we dry-fitted 4" drain pipes to connect the conduits with a temporary solar chimney made with dry-stacked concrete blocks with a stucco coating to seal any air leaks.  For a fan we repurposed a squirrel cage furnace fan and enclosed it in an airtight box setting on top of the blocks. 

It was soon obvious that the fan moved more air than necessary but we could live with that since the whole ugly-beyond-words setup would be replaced within at least a couple of years with a less complicated underground piping system and a permanent solar chimney outfitted with a proper-sized fan, either solar or hardwired.  The conduits have both  smooth and corrugated sections with the corrugated sections located under the house floor to caused turbulence in the air stream so that it would transfer more heat to the soil.  Interestingly, the air flow with the over-sized fan was strong enough to cause a gurgle/rumble sound that was detectable at the solar collector due to the air passing, probably too fast actually for greatest efficiency, through the corrugated sections.

 

Heat Transfer 

It took several weeks for the exhaust air to warm perceptively (holding a hand in front of the blower) because the soil surrounding the conduits was so cool that it absorbed all of the heat from the airstream before it made its way through the conduits even at the accelerated air speed.  The persistent coolness of the airstream was a positive in that it meant that heat was being lost to the soil as planned.

This scenario was playing out during July when temperatures -- day and night -- are as high as they get in the St Louis area.  So I suspect warming of the airstream was due to a combination of hot ambient air pulled through the solar collector by the fan and heat radiated by the solar collector.  I further suspect that the former contributes more heat than the latter.

Is the Solar Collector Really Necessary?

It is not a stretch to think that maybe the solar collector is overkill.  Maybe the AGS system could run on ambient heat without radiated heat -- at least here with our long and hot lower Midwest summers.  It is not hard to imagine a system without the collector that would comprise a "reverse solar chimney" or  "intake chimney" at the downhill side similar to the solar chimney at the uphill side.  The conduits would start out together at the intake chimney, spread out under the house like ours then rejoin at an uphill solar chimney that is outfitted with a fan to pull air through the conduits.

A simple store-bought thermometer inside our collector topped out at 120 degrees on sunny days so we do not know the maximum temperatures produced by the collector.  But it is safe to say that the temperature of the radiated heat is higher than the ambient heat passing through the collector from the outside.  But the solar collector is incapable of producing the volume of heat that can be realized from a constant stream of ambient air, especially on cloudy days when temperatures still reach the eighties and nineties and during hot summer nights.  However, harvesting warm air on cloudy days or at night would require a timer-controlled hardwired fan rather than a solar-powered fan.

Hiat demonstrated that just the heat gain through a few south-facing windows during Montana winters was enough to create comfortable year-round environment in a modest earth sheltered home that did not have an underground network of conduits and a solar chimney but did have a insulation/watershed umbrella.

In Passive Solar Energy Book, Edward Mazria

describes a school in Wallasey, England that runs entirely on passive solar energy.  It is a masonry building (lots of thermal mass) that is insulated on the exterior with 5" of foam board and has a glass south-facing wall comprising mostly translucent glass rather than transparent.  Its thermal performance in a climate thought to be marginal in terms solar energy harvesting was surprising.  Half of the energy required to keep the building comfortable year-round (fluctuations averaging only 7 degrees throughout the year) was provided by the sun with the other 50% supplied by heat from lights and waste body heat from the students.  The conventional HVAC system went completely unused.

What I Would Do Differently

I am beginning to feel that a full-blown Stephens-like AGS system is unnecessary, at least in our climate.  Here are the reasons why.

The thermal performance of our partially-insulated house last winter -- insulation in the walls and ceiling of the first floor and some of the walls on the second story (but not the ceiling) -- and how subsequently the fully-insulated house has remained cool well into this summer even with all operable windows open fulltime and a large industrial fan pulling warm outside air through them 24 hours a day (which we are doing in order to drive more heat into the thermal mass, making the house warmer for the first winter), leads me to believe that, if...........

-- A house is at least partially earth sheltered and has an insulation/watershed umbrella extending outward from the house 16 - 20' in order to increase the amount of dry thermal mass beyond just the floor,

-- There is an abundance of south-facing windows with as much translucent (vs.transparent) glass as possible and there is minimal glazing on the north and west sides,

-- The house envelope is super-insulated and equipped with high R-value windows and doors,

-- The colors of the exterior of the house are highly reflective,

the amount of heat gain needed to warm the thermal mass and maintain comfortable year-round temperatures becomes so minimal that it can be accomplished without a labor-intensive and rather expensive solar collector.

Knowing what I do now, I would be inclined to replace the solar collector with an intake chimney and add a solar fan to the solar chimney to pull warm-hot air through the conduits from late spring until early fall then welcome passive solar heat through the south-facing windows from late fall until early spring.  Then, with the addition of a third heat source --heat generated by merely living in the house such as cooking, baking, dishwashing, showering and waste heat from the bodies of its occupants -- I am confident that the three heat sources would be enough to maintain a comfortable environment during the cool/cold months and the insulated and reflective house and insulated thermal mass would extend the comfort through the warm/hot months.

Cost Considerations

So, is our AGS system with its solar collector, conduits and solar chimney considerably more expensive than a system without the collector would have been?  The collector indeed was the most expensive component in the system -- cost of excavation, cost of concrete blocks and parging cement, cost of corrugated roofing, tempered glass and fencing to finish it off.  My original intent was to hold the cost of the entire AGS system to something less than conventional HVAC but, if it did cost less, it was only marginally.  Without the collector, it definitely would have cost less and would have chopped several weeks off of construction time.

Future Reporting

We will be moving into the house in a couple of months which will give us a chance to begin monitoring and reporting on its thermal performance over time.  Popping up through the floor in the middle of the house is a PVC pipe that was installed a few years before ground was broken.  It was one of four piezometers used to monitor the water table so as to be sure that it would not rise during wet years and syphon heat from the thermal mass under the house.  I plan to repurpose it as a way of dropping a thermometer into the thermal mass to measure its temperature at various depths and intervals.  Eventually we will have a better data with which to judge the efficacy of eliminating the solar collector from the design of the AGS system.

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UPDATE, Spring 2025

We finally buried the ugly temporary above-ground conduits last fall with our plumber-friend, Bob Morgan, spearheading the effort.  The final design called for a branching system of 8"  PVC pipes running below grade between the 4" conduits to the short section of 12" corrugated culvert standing on end and hidden from view by a new solar chimney containing a small furnace blower.

Marking paint was used to layout the design of the system between the nine conduit stub-outs and the new solar chimney.  All connections would be 45 degree instead of 90 to facilitate airflow.  We began by removing the unglued (dry-fitted) temporary horizontal pipes and the temporary solar chimney.  Then we dug a lot.  Each conduit had to be exposed enough for 45 degree connection with a branching series of 8" pipes laid in trenches running towards the collector.  When done, the 12" culvert received only two 8" pipes -- one on its east side and one on its west side.  At the risk of redundancy but to be sure of clarity, the system comprised 9 stub-out conduits connected to a branching system of 8" pipes all below grade and connected to a 12" culvert standing on end inside the housing for the fan-outfitted solar chimney.

All that remained was to run wiring underground between the fan and a timer located beneath the soffit of the house.

Design -- Sustainable Flooring Choices

The time is drawing near when decisions about flooring have to be made.  The substrate for the first story floor is concrete while that of the second is tongue and groove OSB.  When it comes to winter heating for our passive solar house, the difference in substrates dictates flooring choices, irrespective of the usual factors such as esthetics, durability, ease of installation, ease of maintenance, life expectancy and price.  Following is the reasoning behind our final choices.

First Story

Deciding on the type of flooring for the first floor is a no-brainer -- either concrete as the finished floor or concrete overlaid with porcelain tile, either of which fits with our AGS design*.  Porcelain tile is reasonably easy (but time-consuming) to lay, quite durable and there is an abundance of patterns nowadays mimicking wood grain in all price ranges.

Direct gain on an insulated concrete floor (the thick
 black border outlining the house represents insulation).

The concrete floor and the soil beneath it are part of the thermal mass that makes our AGS system work.  The nearby picture illustrates the classic passive solar design -- sunshine on an insulated concrete floor.  In our case, the floor is not insulated so that the earth below the floor (not just the concrete) is warmed by the sun shining through the windows (direct gain) during the cold months to supplement the indirect gain it gets from the solar collector during the warm months.  Concrete, and presumably porcelain tile, are good choices because they conduct heat in and out of the mass, although I was unable to find any definitive information about whether thermal conduction for tile and thin-set match that of concrete.

At first blush it might seem that concrete or tile would be cold underfoot during cold months as would be expected with tile over concrete in a conventional HVAC environment whereby the air is heated rather than solid matter and the soil under the slab is poorly insulated.  In our situation, the heat is in the mass, not the air, so that the floor temperature tracks with the temperature of the underlying mass, varying only a few degrees year-round -- maybe a little warmer in summer and a little cooler in winter but remaining within a narrow comfort range.

Final Choice for the First Story

Having said all of this, our final choice was to bypass the tile altogether and return to our original inclination -- polished and stained concrete.  We had four reasons for favoring bare concrete.  First, tiling 1,700 square feet would have been a job too difficult and time-consuming for DIYing despite owning a commercial-grade tile saw and having considerable experience with tile work.  Second, buying the relatively inexpensive woodgrain porcelain tile of our choice at $1.79/sq ft plus $8/sq ft for professional tile setting would make the total cost just under $10/sq ft.  The cost of polishing and staining the concrete ran $6/sq ft, saving us a little over $7,000.  Third, we were not willing to commit to a forever responsibility for cleaning and maintaining a houseful of grout joints when polished concrete is about as low-maintenance as floor surfaces get.  Finally, in order for our AGS system to function properly, heat must pass freely through the floor to and from the heat storage mass below and the living space above.  By dispensing with tile, except for the airlock inside the main entrance, the issue about its thermal conductivity relative to concrete becomes moot.

Second Story

Flooring choices for the second story are less stringent because the amount of thermal mass is so limited.  Having said that, there might be one instance in which it might be marginally important.  If we were to overlay the OSB subfloor with a layer of cementitious board then install thinset-bonded porcelain tile, the cementitious board, thinset, tile and subfloor together would provide a thin veneer of thermal mass capable of absorbing a limited amount of heat on sunny days and re-radiating it at night.  But, since the glass of all but two of the second story windows is translucent rather transparent, most of the winter sunlight is diffused rather than concentrated on hard surfaces like the floor.  The diffused radiation, while it will not heat the inside air, will randomly warm the contents of the room, in which case, the floor would receive some of the energy and contribute slightly to a comfortable environment.

If we wish to overlook the slight thermal advantage of the tile floor, there are three other flooring materials that we might consider and one other that, for a green build like ours, should be avoided.  First, the one to be avoided -- the popular solid laminate flooring.  With a nod to its beautiful wood patterns and its DIY-friendly installation, it is a petroleum product that does not fit our commitment to sustainability.

That leaves three other choices -- bamboo, real hardwood and a new product, composite laminate flooring.  Bamboo at first glance would seem to be a good choice from a sustainability standpoint since it comes from a rapid growing ubiquitous grass instead of mature trees.  However, its embodied energy, mainly from manufacturing then transportation from the orient, makes it less appealing and its durability and life-expectancy is less than tile and real wood.  Hardwood flooring would be a good choice if limited to native species (excluding old-growth stock) rather than exotics from distant lands.  Wood composite flooring is the new kid on the block.  It looks and installs like solid vinyl laminate but is a certified green product eligible for LEED points made mostly from post-industrial recycled wood chips.  It also has all of the qualities of the best of vinyl products such as hardwood realism, stain and scratch resistance and 100% waterproof protection.  Needless to say, composite flooring was easy choice for the second story.

Prices

Of the choices we would consider, hardwood is the most expensive at +/-$8 per sq ft DIY-installed and +/-$19 per sq ft contractor installed.  Tile, even with the added expense of cementitious board, is least expensive at a little more than $2/sq ft if DIY installed (but +/-$10 sq ft, if professionally installed)  Bamboo is intermediate.  The DIY-installed composite laminate that we selected ran $2.75/sq ft.

What About Carpeting?

Until I read Mazria's book, I assumed that carpeting of any sort would have no place in a passive solar build where the floor is part of the thermal mass.  However, Mazria makes sense when he says "Do not cover a masonry floor with wall-to-wall carpet.  Carpet insulates the heat storage mass from the room.  Scatter or area rugs, covering a small area of the floor, make little difference".  We plan to use an area rug in the master bedroom and a few scatter rugs around the rest of the house especially after a year or two when the year-round temperature of the thermal mass has reached equilibrium.

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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.