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Increasing the Roof Height at the Eave|
Attic Insulation Techniques Knee Walls
Buildings with attics should be inspected to insure that adequate insulation has been installed and that it is in good condition. Some kinds of insulation are easier and less expensive to install in an already sealed roof. Insulation can be installed by rolling out batt insulation or by blowing in loose-fill insulation. If additional insulation is added, to prevent moisture build-up, do not add an additional vapor barrier. When installing insulation yourself, avoid fire hazards by keeping it away from recessed light fixtures and other equipment that emits heat. (Because materials increase the weight of the roof deck, we strongly recommend having a qualified contractor evaluate the load bearing capacity of the roof) Installing batt insulation is relatively easy. However, blowing loose-fill insulation is best left to a professional installer.
In stick-built roofs, where rafters and ceiling joists are cut and installed on the construction site, laying an additional top plate across the top of the ceiling joists at the eave will raise the roof height, prevent compression of the attic insulation, and permit ventilation. When installing a raised top plate, place a band joist at the open joist cavities of the roof framing. The band joist helps prevent windwashing of the attic insulation—where air entering the soffit vents flows through the attic insulation—which can reduce attic insulation R-values on extremely cold days or add moisture to the insulation. The band joist also serves as a soffit dam for the insulation.
Attic Insulation Techniques
One approach to constructing an energy-efficient knee wall is first to seal the knee wall using conventional techniques (i.e., seal the bottom plate, seal penetrations through the drywall, etc.). The open joist ends below the knee wall should be plugged with squares of cardboard, metal flashing, or rigid insulation; cellulose insulation blown at a high density; or batt insulation stuffed into plastic bags. The plugs should be sealed to the joists using caulk or spray foam.
The knee wall and the attic floor in the attic space behind it should be insulated to recommended levels. The same techniques for achieving higher insulation levels in cathedral ceilings can be applied to knee walls. Twine is often used to hold the batt insulation in place. The technique of adding rigid foam insulation over the framing is particularly effective. Rigid insulation can be notched to fit over the floor joists. Sealing rigid insulation to floor joists effectively blocks open floor joists.
A better approach is to insulate and seal the rafter space along the sloping ceiling of the knee wall attic space. The rafters should receive recommended insulation levels. They should be covered with a sealed air barrier, such as drywall or foil-faced hardboard. The barrier must be caulked to the top plate of the exterior wall below the attic space and to the top plate of the knee wall itself. All other cracks and holes must be sealed as well. One advantage of this technique is that any ductwork located in this space is now inside the conditioned space.
Source: U.S. DOE, Office of Building Technology, State and Community Programs, Technology Fact Sheet