RADIANT BARRIERS
Heimhenge uses three lines of defense to reject the heat of the desert Sun ...
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BARRIER 1: The roof surface is white -- the only sensible color in this climate. The immediate result of choosing this color is the reflection of up to 80% of the Sun's radiant heat energy. I would have used mirrors, but they tend to blind airplane pilots. On a hot summer day I can put my hand on the roof and feel the coolness. Try that on a roof with red clay tile or asphalt shingles and you will burn your hand. | |
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BARRIER 2: This is a cross section of the roof. There's about 2 inches of hard polyurethane foam between the white surface coating and the plywood roof base. You can walk on it without causing damage. This is the same type of foam insulation used in picnic coolers. The 2-inch thickness is rated R-15 against heat conduction. It was sprayed directly onto the plywood base, and then coated with the white elastomeric surface layer. This roof treatment was done by Sprayfoam Southwest, and cost about $2500. They recommend reapplying the white surface coating every five years, or as needed due to wear. Ours is going on 6 years and may need a recoat next year. | |
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BARRIER
3: Underneath the plywood base, and
inside the "attic," is a layer of
foil-coated cardboard (InsulFoil radiant barrier
by RFAX Technologies, Inc.). It also acts as a
reflector of radiant energy, and turns back
whatever amount of heat penetrates the first two
barriers. The total insulation value of these three barriers is about R-35. The FHA recommends R-26 for our climate zone, so we're well above average already. But when you add the effect of the solar-powered attic vent fans, and the 6" layer of fiberglass insulation above the interior ceilings, the vertical total works out to around R-75! This, along with the earth sheltering, is why Heimhenge is so easy to cool. |