Building the Superinsulated Dome
The original plan was to just pay a concrete contractor to build us a slab and then I'd
take over from there. Then I read some horror stories online about concrete guys --
who normally deal only with rectangles -- screwing up people's 15- or 20-sided dome foundations,
and decided maybe it should be an owner-built concrete form as well.
Spring is a busy season for excavators. By the time we got our site levelled,
a perimeter trench dug, and the under-slab plumbing in place
(all jobs we contracted out), it was the end of June, 2005.
Fortunately I had been using this time to pre-fab dome panels and struts.
This shows the first of several truckloads of 1" clean gravel that Marcia and I
spread out and compacted:
I built the concrete form from the outside in, starting with a 20-sided perimeter
of pressure-treated plywood:
There's 7-l/2" of extruded polystyrene foam inside the treated plywood perimeter, and 4" of the high-density type under the footing. Once the concrete is poured, the plywood stays in place to protect the foam:
Notice how the thickened perimeter trench is still not very deep. We're building a frost protected shallow foundation. (The link is a PDF that goes into great detail.) This kind of foundation is fairly new to the US, but they have been used successfully for many years in Canada and the Scandinavian countries. Instead of placing deep footers below the natural frostline, you use insulation to raise the frostline near the building.
There's 4-3/4" of high-density foam under the middle expanse of the slab:
A layer of 6 mil polyethelene is topped by rebar 18" on center. That's a lot of rebar, but I don't entirely trust this Iowa clay soil. A local builder told me that with that much rebar, we could park concrete trucks on our slab.
Most people skip the polyethelene if there's already foam under the concrete, but
we're in an area with potentially high radon levels, and the poly helps keep
the radon out of the living space.
The idea is that if any
radon seeps up under the slab, it will travel through the 4" thick layer of clean 1"
gravel and leave through the 4" PVC radon pipe. This pipe starts as a slotted T-shape
in the gravel layer, and runs vertically up through the foam, through the poly,
through the concrete, through a wall in the house, then through the roof to
vent to the outside air. If you still have worrisome radon levels, you can install a fan
in the pipe. Like a plumbing vent, the radon pipe needs to be
at least ten feet away from any openable window or skylight.
Even though it's going to be superinsulated, we put 500' of PEX tubing in the slab so we
would have the option of adding a little radiant floor heat. A house insulated only
to conventional levels would have its PEX tubing much closer together than this:
The slab was poured September 15th, 2005.
When the riser wall corners were bolted in place, it reminded me of Stonehenge:
Struts going up:
The trick to putting up a Natural Spaces dome when you're the only
person up on the scaffold is to keep a couple Phillips screwdrivers
within reach at all times. You lift the strut up next to its hub just
long enough to slip the screwdriver into the slot where eventually
the 4-1/2" pin will go. The screwdriver is thin enough to go in easily,
but strong enough to hold the strut in place while still allowing some
adjustability while you complete a triangle.
If it's a horizontal strut, hold one end in place with a screwdriver
then use a three or four foot 2x4 as a lever to persuade the sleeve at the
other end of the strut past the curve of its hub. When both ends of
the strut are where you want them, remove the screwdriver and pound in the pins.
Then the sheathing:
Next came a layer of Vaproshield breathable roofing underlayment.
We got a deal on some fun, unusually-shaped windows, but it took me months and months -- literally all winter and most of the spring -- to build seven custom dormers. I'm glad there's some overhang on these south-facing windows, though. Too much unshaded south glass can cause overheating in the summertime.
The big triangular skylight from Natural Spaces was expensive, but it only took two hours to install. If your time has any monetary value, it would probably make sense to have more triangle skylights and fewer windows requiring dormers.
On the other hand, besides the summer heat issue, skylights can be obscured for weeks following an ice storm, while vertical windows under overhangs remain clear.
I put 6" of foam around the base of the riser wall in an attempt to keep the cold air away from the concrete slab.
The foam was followed by plywood, the angle of which makes the riser wall look like a continuation of the curve of the dome.
The plywood got a layer of ice-and-water shield to resist leaks in case of melty snow drifts. This was late May, 2006.
Shingling the roof took all summer. One thing about the Reinke shakes is that you can't use a nail gun because it would dent the metal. I had to drive all 16,000 nails the old-fashioned way. But it's not really the hammering that takes the time, it's hanging sections of 18" wide 15# roofing felt between each row. Near the top, where the slope of the roof gets shallower, I backed each section of felt with Tyvek for extra protection.
Midway through the shingling we had a hailstorm during which the Vaproshield started letting water through. After that happened, I put a layer of Tyvek over everything. The Vaproshield had been exposed to the elements about eight months (thanks to the endless dormer-building phase), and that was just too long I guess.
Roofing the main dome surface was completed September 20th, 2006. The south dormers just got a layer of poly-backed ice-and-water shield at that time, and I shingled them in the Spring after it got warm enough to polyurethane their wooden undersides.
Once the roof was weatherproof, my priority became getting the interior finished enough that the foam crew could insulate. This involved inner framing for the doors, windows, and riser walls, then running electrical conduit for lights and outlets. I also had to build a little bump-out for the electric meter and a section of wall for the breaker box.
The spray-foam insulation we used was
On other below-zero nights I've left a 1500 watt space heater going overnight, and it's always warmer inside the next morning. This suggests that the dome loses under 5,000 BTUs per hour, which is what a space heater that size produces.
The triangular spruce panels went up fairly quickly, but fitting the spruce around the windows took me a long time -- almost every piece of wood had a compound angle on both ends.
Next I started framing the internal rooms.
Meanwhile, Marcia's garden area was taking shape.
