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 http://sourceforge.net/projects/elmerfem/, 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.