Monday, December 3, 2012

This and That

So how does our solar-heated house perform today?

Try this if you can. Go into a neighborhood where fancy, three-story, million-dollar McMansions are being constructed. Find one that has been fully enclosed but not yet heated. Go sit in it around 6 a.m. and imagine eating breakfast with no heat. Are you wearing gloves? Teeth chattering? See your breath?

Now it's 6 a.m. here and I'm eating breakfast next to one of our large south-facing windows with our R-7 insulated drapes drawn closed. It's cold outside, but our house has had no supplemental heat from our masonry stove since late February, 2012. In our uniform 68°F environment, I'm more comfortable than I thought I'd be, since my body prefers 72°F or so. My body temp always runs low, and my hands and feet too often get too cold. But my stocking feet are resting comfortably on the bare tile floor. Our house has all tile floors, which are easy to maintain and require no repairs.

So what was the theory behind my designs? Unfortunately Hait's book was not as helpful as I would have liked. And, as everyone knows, there is scant useful PAHS information on the Internet. My theory was mostly "A Shot In The Dark." I would like to have used elmer, but the learning curve was too high. So I wet my thumb, held it just so, and sighted. The most difficult parts were sizing and locating the tubes, and choosing between expanded and extruded polystyrene insulation boards. I sized the tubes to obtain an air-change rate for about 20 people and purchased a semi-truck load of 10 psi expanded polystyrene insulation. I purchased schedule 35 tube at wholesale and worried that it wouldn't withstand the rigors of construction. But it came through amazingly unscathed, except that the back-filler's dirt pushed some of the vertical tubes into leaning tubes that I had to work around. A number of researchers in Canada had tested 10 psi expanded polystyrene insulation underground and subject to moisture and claimed that it would hold up and retain reasonable insulation value. Time will tell.

When the air tubes are run over the structure, the backfill has to be deep enough so they can be at least two feet or so above the concrete and below the insulation layer, and the insulation layer must be at least two feet below the ground surface. Thus at least five feet of ground cover must be planned for above the structure, and this will add to the reinforcement requirements. The reinforced concrete structure was designed by a professional architect, and it was approved by the county inspectors.

A significant length of the two sets of air tubes are run parallel to each other and enclosed in insulation to act as one long heat exchanger. Thus, incoming air is always tempered by outgoing air to help reduce heating and cooling loads. Ambient ground temperature in this area is around 52°F, but the incoming air from the lower tubes this time of year is around 66°F, which I just verified.

The other issue that Hait emphasizes is the peril of water percolating through the soil surrounding the structure. I took his advice very seriously and was relieved that the excavators found only pure clay, a single six-inch rock, and no gravel. Pure clay meant that no water should find its way into or out of the surrounding area. I also buried a 4 inch perforated drain tube four feet below floor-level around the perimeter and brought it out to daylight. I ran another tube down the center of the house to insure that no moisture would reach the underside of the floors. And I placed a 4 inch perforated drain tube all around the footings to carry away water that might work its way down along the walls. Davis Caves sealed the outer walls, but I also placed a layer of dimpled covering over the walls before backfilling with clay to allow water to drain out and avoid hydrostatic pressure leakage through the walls.

Struggles? Yes. Try to find a contractor to build an earth-sheltered house. They don't exist. So I was my own contractor. Now you can be a smart contractor or a dumb contractor. I was somewhere in between; I read a couple books on how to be a contractor, but forgot most of what I read. So you have to line up all those professional and not-so-professional people to do the work, and you have to expect that some of them won't have a clue of what you want them to do, because they have never worked on an earth-sheltered house, and they have never heard of PAHS. So, mistakes were made, fortunately none that couldn't be remedied or worked around. OK, so the conduit for the incoming power cables came up through the ground a foot from the wall that they were to run up to the circuit panel in, and this young electrician's helper and I were out at night under a pathetic light in a three-foot deep muddy hole in a wet driving snow with an electric heating element melting the conduit so that it could be bent over a foot to line up with where the wall would eventually be. And I forgot to tell the contractor that a footing needed to be poured to support the masonry stove, so they had to cut a hole in the freshly poured floor, pour the footings, and fill in the hole.

And the outside walls were another issue. The structure has twenty or so one-foot square reinforced concrete columns supporting the roof. About half of them run along the outside wall. Most people place these columns in line with the wall, but I knew that columns in the wall would allow heat to easily pass through. So I put them inside the house adjacent to the walls and then framed around them. For the exterior walls, I had investigated using Structural Insulated Panels, but I didn't make the effort to figure out how they would be interfaced with, and secured to the concrete structure. So I went with the builder's suggestion of installing double walls using 2 X 4's. Using 2 X 4 construction was a mistake, because these walls ran right next to the concrete support columns, and it made spraying a uniform layer of sealing insulation into some areas behind the columns very difficult, especially in the corners. Fortunately, the Latino fellows who worked for Home Comfort Insulation did an excellent job of getting to every crack and crevice under my watchful eye. They were a pleasure to work with and certainly earned their money that day. SIPs would have been so much easier and cheaper in the longrun, considering their ease of fabrication and installation.

Did I mention when they poured the roof the first week in March, 2010, just one day after the frost had gone out? The concrete pump truck arrived around 5:30 a.m. and was set up about half an hour before the first cement truck arrived around 7 a.m. It was cloudy and drizzly that day as seventeen cement trucks backed up our 1/5 mile lane and up a steep hill, cutting deeper and deeper ruts until it was impossible for us to drive on it. Fortunately we had parked our car at the road. The truckers had to back in because there was no solid place for them to turn around. When the concrete was finished and the last cement truck had left, it was dark and time for the pump truck to back out to the road, but that was easier said than done, because that behemoth wound up stuck in a giant mud-hole left by the other trucks. Now I won't bore you with details of how he made it back to the road.


  1. Roger,
    Glad to hear your house is starting to perform. I am curious about the conduction surrounding your home during the summer. I am starting construction in the spring in Northern Mn using Don Stephens Annualized Geo Solar idea, similar to Hait's PAHS. I think heating the soil with non occupied spaces is probably more summer friendly. I will use an entry/solarium area for year round collection. Also the metal roof for spring, summer, fall collection (170 sq meters). The heating collection tubes will be for soil heating only. I will have separate tubes drawing fresh air in through the heated soil for space heating. The insulated umbrella abuts the house, as in the PAHS. My concern is in having the tubes the best distance from the house. Not to overheat at the end of summer but also not to underheat in the winter. Any info you have collected on how fast the heat is conducting would be useful. Like you, I am using intuition for lack of hard evidence. Any and all of your design descriptions are most useful. Thanks for sharing. Andy

  2. Andy,

    Since you are building in northern Minnesota, the first bit of information is your average annual temperature. In Duluth it is 38°F. It determines the steady-state ground temperature at about 20 feet below the surface and on down. In an uninsulated surface, there will be a rising temperature gradient from 38°F to the surface in the warmer months and a falling gradient in the cooler months. So the important observation, as Hait mentioned, is that heat energy takes about six months to travel 20 feet. That averages to less than 1.5 inches per day.

    Do I understand that you will be pumping heated air from below the metal roof into the ground surrounding and under the house? What will be the R-value between the living space and the air-space below the metal roof? It should be high. And the dynamic R-value can be made higher by placing a layer of thermal mass adjacent to the insulation on the metal roof side and placing the air collection tubes close to the thermal mass.

    Now assuming that the hot metal roof and superhot entry/solarium area is adequately isolated from the living quarters (the temperature in our east-facing sunroom can go above 100°F on sunny summer days), then the living space should not overheat at any time during the year. In winter months, it is more likely to be too cold and will require supplemental heat.

    What percentage of the walls will be underground? If it is small, then more heat must flow in and out through the floors. Carpeting can slow that process. If it is high, then heat will also flow through the walls and will be affected by wall coverings.

    How far out does the insulation umbrella go? 20 feet? Heat will always flow toward colder temperatures. The soil way down below and around the house is icy cold, 38°F or so, year-round, and it is sucking down some of the precious heat. Placing the tubes too far from the floor and walls will not allow it to be recovered in the cold months. Placing them too close will allow excess heat to enter in the warmer months.

    What would I do? Place the tubes about the same depth waterlines are buried. Five feet? Run tubes about five feet below the floors, five feet out from the walls, and five feet below the insulated umbrella. I would error on overheating in summer, because one can always open windows.

    My experience is that perceived heat conduction depends the most on the amount of thermal mass. Our house has so much that heat conduction rate seems imperceptible. The highest temp inside this summer was 74°F and the lowest without any supplemental heat this fall has been 66°F. Our heat comes primarily through many east and south facing windows, and it is blocked from escaping by insulated R-7 drapes.

    I recommend contacting for further support.

  3. Roger,

    Thanks for responding. I only saw this when you posted again. The link looks interesting. Digging into their energy program.

    Yes, would like to pump heated air from below the roof to below grade. R80 roof insulation, R40 walls. The building is entirely masonry. Combination of concrete columns/beams and CMU. I have been playing with ideas about that. There will be massive heat loss just pulling air from under a standard metal roof. Gary from BuilditSolar suggested putting SunTuf glazing on the metal roof. Have also thought of developing a collector similar to the downspout one and just using the metal roof and insulation as it's backdrop.

    Will berm the back three sides. Two story building, built into a small 5' high hill. Will berm to within 5' of the roof. So the roof will not be that far from grade. The north side of the roof is the high side of a shed style roof. Figure I will make a plenum that runs the length, east to west to capture the air from the roof. I end up with 40% of the walls below grade.

    Developed a nice insulated sliding panel for all windows complete with a pan to collect condensation. In fact, my windows will be fixed. I will have insulated hinged panels that will allow direct fresh air when wanted. I can't stand manufactured windows-a lot of money and very low R values-unless a lot of money is invested. My sliding insulated panel will have 2" of foam and be something like an R11 assembly. I have also been working on a system to capture heat from all south facing windows when the panel is closed in the summer-to further enhance heating the ground. When I build I will duct this but not install fans until I have a chance to see how the house performs. May prove to be overkill.

    The pipe placement and insulation umbrella's size and configuration are my biggest concerns. The soil-that 38 degree business is a huge heat suck. I have a bobcat on site and considered doing some serious excavation and creating a huge insulated area that extends 10'-12' on the back three sides and as deep as the footings (insulating not only the top but sides and bottom too). Just 2" of foam would go a very long way to mitigate the losses. It's extreme though. 5' away from the house seems pretty close. 60" with that heat moving, as you say, 1.33" a day gives me a 45 day lag. Don Stephens suggests 10'. That would be 90 days. I should have a good 6 months of heating from the roof and glass. Guess I need to settle on the roof configuration, figure out roughly how many btu's one could harvest and judge pipe location on that. Then hope for the best.

  4. My brother-in-law, who lived in central North Dakota at the time, built a large, solar-heated machine shed. The south-facing side of the roof and the south-facing wall were covered by corrugated fiberglass panels, perhaps similar to . An 8-12 inch air space separated the panels from a black surface that absorbed the sun's energy. He filled a six-foot deep hole below the floor with 3-12 inch diameter rocks and poured reinforced concrete floor over them. He used a large fan to circulate the heated air from behind the panels into the rock-filled cavity under the floor. Even when the outside temperatures were -20°F and lower, the inside temperatures remained well above freezing without supplemental heat.

    Using solar PV panels to drive DC air-blower motors could save energy. This method works because the sun is generally shining when the air must be moved. A relay could switch to grid power if air must be moved when the PV panels aren't producing sufficient energy.

    Converting the sun's energy to heat in the metal roof material itself allows much of the heat energy to be lost to IR radiation and conduction to the cold outside air. Our 56 square foot solar water heater panel with a glass face and a black absorption surface about 2 inches below the glass illustrates how well an enclosed cavity works. On a cold, sunny December day this heater is capable of putting out 150°F water into our 80 gallon water heater's heat exchanger.

    We are happy with our Marvin windows. They have high solar heat gain so don't qualify for energy rebates. But they also have low U value and an E-coat to block IR energy flow. All through last winter and into this winter, condensation has been insufficient to run onto the wood frames, partly because our winter indoor humidity rarely gets above 40%.

    When locating tubes in the soil, one must consider that heat flows in all directions, and the flow rate in any direction will be proportional to the temperature difference and inversely proportional to the flow resistance. If the sandwiched insulation’s thickness or R-value is insufficient, too much heat will flow through it. Below the heated volume, the ground temperature is approaching 38°F, so some of the stored heat will happily travel in that direction and may never return. Where the heat is desired in the living space, the temperature will already be relatively high compared to the thermal mass's ambient temperature, so the heat-flow rate in that direction will be much slower. Some people have run tubes between the thermal mass volume and under the floor. That way air can be used to move heat from the thermal mass to the living space a little faster. We do something like that with our masonry stove by placing a plenum chamber in the wall next to, and above the stove. A fan draws heated air into the plenum and distributes it through ductwork to overhead registers in all rooms of the house. Running tubes under the concrete floor and blowing the heated air through them into floor registers might have been better.

    Getting back to placing tubes in the surrounding soil, the amount of heat energy that flows in any direction may be more important than the 1.33" per day heat flow rate. If the thermal mass’s enclosing insulation R-value is too low and the tubes are too close to it, then more energy may be lost to the outside. If the tubes are too deep, then more energy may be lost to the subsoil. But temperature differences are usually the smallest between the tubes and house interior, so lesser amounts of energy will travel in that direction. Thus it may happen that placing the tubes 10' from the walls will allow the soil adjacent to the walls to become too cold. In essence, the heated interior of the house may end up heating the surrounding thermal mass in the winter months when it should be in equilibrium or somewhat cooler than the surrounding thermal mass.

  5. Hello Roger,

    Thanks for your very interesting blog.

    I wonder about the wisdom of: "I would err on overheating in summer, because one can always open windows."

    Surely, during summer heatwaves (which are more and more frequent) windows are closed to prevent filling the house with even hotter air, and there's no escape... Whereas in winter one can always add a pullover, or a coat or two, or go chop wood.


  6. Roger,
    First I want to thank you for the blog, it is very informative. I am looking at building and would like to know after all your work on the PAHS system for your home, if you feel that the return on your investment for the system is cost effective for the average earth home?

    Thanks Dennis