Westerlo House part 2: Foundation

May 2, 2007 by Jeff.

Welcome back.  It’s time to discuss energy-efficient foundations, and why this one works.

Our design called for storing heat within an insulated, 14″ thick basement floor.  Heat exchange would be accomplished by blowing air though a 1 by 3 foot main plenum running down the middle of the slab.  This plenum fed 24 equally-spaced 6” round ducts buried in the slab and emerging around the perimeter of the room.  My calculations showed that with 4 inches of Thermax foam beneath and around the slab, this 3000 cubic foot concrete block would bank enough heat to keep the house from freezing even with 90 days of no sun.  Under typical winter sun conditions, calculations said the slab would keep the house at a constant 65 degrees.  Concrete is cheap, dense, and a good way to store heat for active solar hot air systems.

Building inspectors want footings, not slabs.  It took some time and persuasion to convince our town building department that a 14 inch thick slab was better than a 12 inch footing.  We kept ductwork away from the outer 16 inches of the slab and the inspector reluctantly signed off.

Like many building sites, we were constrained by the locations of well, septic and existing drainage patterns, not to mention views and road access.  The excavation wound up on top of a hill.  Soil consisted of 3 feet of heavy clay loam on top of a loose shale and clay mix.  Some small seasonal springs surfaced as we dug the excavation, adding to the mess.  Luckily, we had planned for the worst.

The bottom of the hole received 6 inches of gravel, carefully leveled and compacted.  Into this we set perimeter PVC drainage tile leading to daylight down the hill.  This kept the springs at bay while we continued working. 

I wanted the basement to be living space - dry and warm - and all the more important since the floor was our heat storage.   Although the performance of Thermax should not be degraded by water (it is closed-cell foam), I wanted to keep everything as dry as possible.  I found a 45 mil reinforced geotextile fabric which was waterproof and able to stand up to being sandwiched between gravel and foam.  Even better was its availability in large, custom-seamed sheets.  I bought a 60 by 80 foot piece, more than enough to wrap our 40 by 60 foundation, and laid it on the gravel base.

On top of this went 2 layers of 2-inch Thermax, its seams offset and sealed with aluminum foil tape.  Thermax is hard to crush and is rated for slab and footing underlayment.  Around the edges went an 18 inch vertical Thermax wall (again, 2 layers offset and taped) to act as a perimeter form and insulation for the slab.  We backed this with 2×4 reinforcement and stakes to withstand the pressure of the wet concrete.  We completed the foam formwork on the hottest day in July, baked from above by the sun and from below by the foil surface.  Then we called the concrete company.

Since I needed the 6-inch ducts centered in the slab, we first poured a 4-inch slab, leveled it and waited overnight until it to set up.  On top of that went the main air plenum and the branch ducts.  We strapped the ductwork to the slab with metal banding and power-driven nails to keep it from floating up when the main slab was poured.  The end of each duct was turned 90 degrees upward, carefully leveled and propped, and sealed with scrap foam.  Once the 8-inch basement walls were poured, these ducts would see daylight just inside the solid perimeter required by the building inspector.

Pouring the main slab was one long, hot and exhausting day.  The Thermax walls held the pour, and we embedded 2×4’s along the edge to create eventual keyways for the wall pour.  We gave it 2 days to set up and then used a masonry blade to saw relief cuts into it on a 4-foot grid.  With the combination of foam underlayment, embedded ductwork and relief cuts, the slab has never cracked and remains dry and warm 12 years later.

Once the slab was completed, we had a foundation contractor pour 8-foot high perimeter walls.  These were again wrapped on the outside with 2 layers of 2-inch Thermax.  Then we unrolled the border of the geotextile fabric, cleaned it and brought it up and over the walls.  18 inches of gravel backfill completed the excavation.

I’ve been questioned about the attention we gave to insulating the slab.  Many builders feel that the ground provides sufficient insulation, and it’s true that below the frost line we could count on a constant 50 degree ground temperature.  On the other hand, that’s a 50 degree heatsink, damp and infinitely large.  Then there is the 4 feet of frozen ground above the frost line, not to mention exposed wall above the ground.  Our Thermax wrap prevents heat loss to the ground, keeps the slab and walls dry and warm, and definitely contributes to the overall success of the house.  Using it as formwork for the slab clinched the deal, and I would do it again the same way.  It’s worth it – the house is proof of that.

This photo shows the basement in 2007.  Just to the right of the vacuum canister is an air duct coming out of the slab and heading to the first floor.  Air ducts were arranged to route warmed air from the slab to the basement, first and second floors and run inside finished walls upstairs.  The exposed poured concrete walls are warm and dry, thanks to their insulated and waterproof exterior wrap. Most of the basement has been finished into guest quarters and a recreation room.

Next time: the shell.

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