Many homes, both old and new, either have had attics converted to third-floor living spaces or have original top floors with cathedral-type ceilings. Typically, fiberglass insulation is installed between the rafters from the eaves to the peak of the roof, and the knee walls are constructed in parallel with the roof line. The knee walls are usually about 2 to 4 feet in height and are sometimes insulated with fiberglass and other times are not insulated. There is almost always a small door in the knee wall that provides access behind it that can be used for storage.
A top-floor living area with knee walls can be a huge energy waster. This is in part due to the area being a poorly defined thermal boundary (or thermal envelope, which restricts or slows the flow of heat, meaning what keeps the cold air out and the warm air in), and it is also an energy waster because of the high probability that air flows around the knee-wall framing and through the fiberglass insulation. This air flow significantly reduces the insulation's thermal resistance (the manufacturer's listed R-value, which is how well this insulation keeps the warm air in). The floor of this living space (which is usually the second-floor ceiling) is typically not insulated, contributing further to this problem of a major thermal bypass. Basically, heated air is mixing with outside air in the areas behind the knee wall creating an intermediate zone that is neither fully inside nor outside the thermal boundary.
In order to address the above situation, one must view the knee wall and second-floor ceiling as a continuous thermal boundary and air barrier that should be in alignment. If the ceiling and knee wall are insulated with fiberglass, the fiberglass should also be fully enclosed to be effective. The back side of the knee wall needs to be covered with an air barrier, and the top of the knee-wall bays need to be sealed. This approach will keep the area behind the knee wall outside the thermal boundary, or conditioned space. The other approach is to insulate the sheathing under the roof all the way to the eaves, effectively bringing the area behind the knee wall into the conditioned space. To determine which approach is best, the area should be evaluated by a professional.
Adequate ventilation is necessary to control moisture and ensure that the ceiling cavities stay dry inside. Choosing the best ventilation equipment is the subject of another article, but in short, it is preferable to have a humidistat equipped with a timer so that control is automatic and runs only when necessary.
On a separate energy note, an energy conservation seminar, "Warm Your House — Cool Your Planet," will be held at the Amesbury City Hall at 7 p.m. on Dec. 11. It is an opportunity for area residents to get specific information on residential energy conservation measures.
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Tim Gould is director of Energy Egghead, an Amesbury-based company that can be found at www.energyegghead.com, and provides professional energy audit and conservation services.
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