Foil and Two Airspaces

1. Metal Foil Used As Insulation

2. Ineffective Foil and Effective Foil

3. A bright foil surface makes an excellent radiant heat barrier between rafters in a roof. Several products of this type are commercially available. The one shown here is a bright aluminum foil laminated to a cardboard backing, configured so that it forms an air passage just beneath the roof sheathing of a wood light frame building. This is particularly effective in keeping solar heat from being transmitted from the roof into the rooms below, blocking as much as 40% of summer heat gain. The airspace between the foil and the sheathing serves both as a clear space into which one side of the foil can reflect and as a ventilated airspace to carrv awav solar heat. In winter, the

foil acts as additional insulation to retard the heat llow out of the building into the cold outside air. It does this, not only because foil is a poor absorber that is, a good reflector) of radiant energy but also because it is a poor emitter, meaning that it docs not radiate heat effectively into space. In fact, its rate of absorbance and its rate of emittancc are exactly the same, meaning that it doesn't make any difference which way heat is trying to llow through a foil that faces an airspace; the foil will be equally effective in blocking heat flow in either direction.

4. The reflective qualities of a bright metal surface diminish rapidly as the surface becomes dusty or tarnished. A

m foil in a dead, dry airspace will generally stav cleaner and retain its ther-

mal effectiveness better than one in a circulating stream of air.

5. Metal foils are excellent vapor rc-tarders and should be installed on the warm side of a wall or roof assembly (see Warm-side Vapor Retarder, page 60). If a foil must be used on the cold side, it should be perforated to allow water vapor to pass freely. ■

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