Superinsulation For the Masses
(Page 2 of 5)
September/October 1986
By the Mother Earth News editors
Because they present a conductive path from the inside to the outside, chimneys are rarely installed in superinsulated houses. Besides, wood heat isn't well adapted to houses with low (and intermittent) auxiliary heating needs.
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Base plates and sills are a couple of difficult spots to insulate, but it's important to arrange those wooden members so they can be thoroughly covered. In a house with R-30 or R-40 walls, the roughly R-3 presented by 1-1/2 inches of wood is the equivalent of a hemorrhage. Similarly, highly conductive concrete areas — such as foundations, footings, and slabs (when used) — should also be heavily insulated.
Cooling is a problem that superinsulation techniques only partially address. Recent research has revealed that, though insulation is helpful in keeping heat out, radiant barriers are more cost-effective. A radiant barrier consists of a layer of foil (with an air space next to it) that reflects energy radiating toward it. These can consist of sheets of foil attached to the undersides of rafters, of foil-faced rigid foam insulation, or of several other possibilities. However, even radiation barriers may not be sufficient to passively cool a superinsulated house, because heat is only part of the problem; moisture may account for more than half of the cooling load in Southern states, and only ventilation or mechanical dehumidification can handle that.
Air leakage: Infiltration should be controlled to the greatest degree possible. A continuous air/vapor barrier should enclose the building, and all seams should be lapped and sealed with an appropriate caulk. Particular attention should be paid to sealing electrical outlets, plumbing passes, and window jambs with tape and caulk. If at all possible, avoid recessed lighting that would ofer a leakage path from the home's interior to the attic. Double doors or an air-lock entry are often used, though the latter is increasingly considered not to be cost-effective. Once the infiltration is controlled, use an air-to-air heat exchanger to maintain an air exchange rate of about one-half of the building's volume per hour.
Windows: A house that's designated strictly as superinsulated has a south-facing window area equal to only about 5% of the building's floor area, but blends of superinsulation and passive solar may have more south glass. Don't exceed about 7% south glass without adding thermal mass (beyond that found in standard 1/2" drywall construction with wood or covered concrete floors). Windows facing other directions should total about 5% of the floor area, bringing the aggregate glass area to about 10% of the total floor area. Most designers prefer casement windows because they seal well and enhance ventilation when they're open. Use fixed windows wherever practical; they are less expensive and offer the best seal.
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