For Your Home
Attic Insulation

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.

attic.jpg (8032 bytes) Increasing the Roof Height at the Eave
One problem area in many roof designs occurs at the eave, where there is often insufficient space for full insulation without blocking air flow from the soffit vents. Often the insulation is compressed to fit the space, diminishing its R-value.

For a truss roof, consider raised heel or oversized (cantilevered) trusses that form elevated overhangs in combination with rafter baffles and soffit dams. These should provide clearance for both ventilation and full-height insulation. Use of 2- to 2½-foot overhangs also provides more room for insulation at the wall junction and additional window shading.

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
Loose-fill or batt insulation is typically installed in an attic. Although installation costs may vary, blowing loose-fill attic insulation—fiberglass, rock wool, or cellulose—is usually less expensive than installing batts and provides better coverage. Steps for installing loose-fill and batt insulation:

  1. Seal all attic-to-home air leaks, especially chases, dropped ceilings, wiring and plumbing penetrations, light fixtures, and bathroom fans. Most insulation does not stop airflow.
  2. Install blocking (metal flashing) to maintain clearance requirements (usually 3 inches) for heat-producing equipment found in an attic, such as flues, chimneys, and exhaust fans.
  3. Use only IC-rated recessed lights because they are airtight and can be covered with insulation.
  4. Select insulation levels in accordance with the 1995 MEC or the DOE Insulation Fact Sheet. The Insulation Fact Sheet (DOE/CE-0180) can be ordered from the Energy Efficiency and Renewable Energy Clearinghouse.
  5. Locate the attic access in an unconditioned part of the house if possible. Otherwise, weatherstrip the attic access and insulate it by attaching insulation to the cover or by installing an insulated cover box.
  6. If mechanical equipment or storage areas are located in the attic, elevate the attic decking to allow full-height insulation to be installed.

Knee Walls
Knee walls are vertical walls with attic space directly behind them. They are often found in houses with finished attics and dormer windows, such as in story-and-a-half designs.

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 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 air 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: US DOE, Office of Building Technology, State and Community Programs, Technology Fact Sheet


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