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Building The 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 leveled, 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:

Gravel


I built the concrete form from the outside in, starting with a 20-sided perimeter of pressure-treated plywood:

Concrete Form


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:

XPS perimeter

Notice how the thickened perimeter trench is still not very deep. We're building a frost protected shallow foundation. 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:

Foam


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.

Rebar

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.


PEX

I put 500' of PEX tubing in the slab for possible future radiant floor heat. My idea at the time was that whenever the sun was shining and the house needed heat, antifreeze would pump from the slab to a solar heat collector and then carry that heat back into the slab. This direct loop probably would have worked to some degree, but I found reasons it wasn't the best plan:

If I had put a lot more PEX in the slab, it would have been possible to transfer a whole day's worth of heat during the brief hours of winter sunlight. But would this result in a huge afternoon temperature spike? I don't know. Without sophisticated controls, even radiant slabs heated by conventional boilers are known to overshoot their mark.

The current thinking is that radiant floors aren't a great fit for superinsulated houses. For one thing, radiant's big selling point is the emotional appeal of warm floors, but in an extremely well insulated house the floors only need to be 2-3°F above the target indoor air temperature, so they don't feel particularly warm.

Commenters at Green Building Advisor often make the point that there's no reason to bother with the expense and complexity of a radiant slab because a superior solution exists: just heat your superinsulated house with a ductless mini-split heat pump, which also provides cooling in summer. It's a compelling argument. The Mitsubishi MSZ-FH09NA, their smallest, highest-SEER model, works down to -13°F and would be able to heat our whole dome. It would be too small for our cooling needs, though, so if I got a mini-split I'd actually get the largest one in that series, the MSZ-FH15NA.

Despite all this, I'd still like to try adding some solar heat. A heat pump can be three times more efficient than electric resistance heat, but solar can be many times more efficient than that.


The slab was poured September 15th, 2005.

Slab


When the riser wall corners were bolted in place, it reminded me of Stonehenge:

Riser Wall Corners


Struts going up:

First Row of Struts

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.

Struts done


Then the sheathing:

Riser Wall Sheathing

More Sheathing

Sheathed Dome at Twilight


Next came a layer of Vaproshield breathable roofing underlayment.

Vaproshield


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.

South Dormers


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.

North Dormers


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.

Perimeter Foam


The foam was followed by plywood, the angle of which makes the riser wall look like a continuation of the curve of the dome.

Angled Riser Wall Plywood


The plywood got a layer of ice-and-water shield to resist leaks in case of melty snow drifts. This was late May, 2006.

Dome in Green and Black


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.

Partly Shingled


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.

Tyvek


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.

Done Shingling!


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.

Electric Meter Box and Breaker Box


The spray-foam insulation we used was Earth Foam 1.75. The goal was a uniform 10" (about R-60), but the thickness actually varies between 8" and 12". But even 8" is a LOT of foam. We had a night that dipped below zero a while back and the dome only lost 6 degrees in 16 hours. That's with no heat source of any kind inside.

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.

Update now that we're living here: We dipped below zero last night and only lost 2-1/2 degrees inside overnight. At 11:00 p.m. it was 3 degrees outside, and 68.9 inside. At 5:30 a.m. it was -1 outside, and 66.4 inside. Both the heaters and the Energy Recovery Ventilator were turned off during that time. Normally the ERV would be running on low, but we turn it off when it drops below 5 degrees outside. Losing only 2 or 3 degrees over a winter night with no heat on is typical though, even with the ERV bringing in fresh air. The thermal mass of the slab is part of the reason it doesn't cool down faster, plus we're getting a little bit of heat from the refrigerator, the water heater, and our own bodies.

Foamed Interior


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.

Panels Up

Gothic Arch Dormer Interior


Next I started framing the internal rooms.

Room Framing Begins


The walls hold plumbing and ductwork.

Plumbing in Wall

Drain Pipes in Wall

Ductwork


What to do with leftover scrap drywall? Instead of hauling it to the landfill and paying them to take it, I put it between the studs in the wall shared by the bedroom and studios. There it will add thermal mass and reduce noise transmission.

Scrap Drywall


Meanwhile, Marcia's garden area was taking shape.

Garden

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