Button Up: Attic doors and kneewalls

The Old House Web

Adapted from material developed by the U.S. Department of Energy

Don't leave a hole in the ceiling

A homes attic access, such as an attic hatch, pull-down stairs, or knee-wall door, often goes uninsulated, representing one of the biggest deficiencies in the thermal barrier between the attic and conditioned space.

This gap in the attic insulation increases heat loss in winter and heat gain in summer, and makes indoor living areas uncomfortable. Such accesses are often not sealed properly.

A -inch gap around the perimeter of an attic access can potentially leak the same amount of air supplied by a typical bedroom heating duct (~100 CFM). Unsealed, the attic access in a home leaks energy dollars and causes the house to be less comfortable.

Attic Hatch

One inexpensive and common type of attic access is referred to as a scuttle hole or attic hatch, which is simply a removable portion of the ceiling that allows entry to the attic above.

A scuttle hole is commonly located in a closet or main hallway. To create the hatch, the installer should cut a plywood piece or save the ceiling drywall piece that is cut out for the hole. To ensure a tight fit, securely fasten the trim around the hole to the ceiling and make sure that it is flat and level. An uneven base can cause air leakage.

Weather stripping can be installed either on the hatch itself or on the inside of the trim or base where the hatch rests. Adding a latch bolt will help ensure a tighter seal. After the trim or base is aligned to seal properly, insulation should be added to the attic side of the hatch. Rigid insulated sheathing is recommended.

Cut the insulated sheathing inch smaller than the hatch size to allow for clearance when moving the access panel. Apply 3 or 4 inches of insulation to the hatch with construction adhesive and screws. As an added measure, glue the kraftpaper side of batt insulation to the top of the last layer of rigid insulation.

Attic Stairs

Pull-down stairs are another common type of attic access. The frame for the stairs fits in a rough opening and leaves a gap, much like that for a door or window, which must be sealed.

If the gap is small (less than inch), caulk can be used as the sealant. If a larger opening exists, then non-expanding foam or a backing material (backer rod) is recommended in conjunction with caulk. Expanding foam can be used, but care must be taken because of its highly expansive nature; it could warp the frame and interfere with the ability of the stairs to open or close properly.

To ensure a tight fit between the stairs flat panel and the frame, weather stripping or gasket material should be added either to the frame or the panel. Latch bolts may be installed to help ensure a tighter seal. To insulate attic stairs access, a lightweight, moveable box can be constructed from rigid foam or fibrous duct board to fit over the stairs from the attic side. Insulating kits are also available through weatherization suppliers or from local hardware stores.

As with all home projects, follow the attic stairs manufacturers instructions for proper installation. Manufacturers guidelines often provide unique techniques and safety considerations for each particular unit. For safety and the extended life of the stairs, be sure to cut the stairs to the proper length. This action will prevent strain on the hinges and reduce wear on the pivotal joints of the stairs.

Attic access location

Location of an attic access is important.

If possible, locate the access in an unconditioned part of the house (e.g., garage, covered patio, or porch) that is also secure against potential break-ins. A garage location, where the vented attic is uninsulated, can eliminate the need for sealing and insulation. For a pull-down attic staircase, make sure that opening the stairs will not interfere with the placement of furniture.

The position of the staircase in relation to ceiling joists might also affect location -- placing the staircase between joists instead of across joists can speed installation because structural cross-member framing is not as complicated.

Attic decking

Attic decking is often used to provide additional storage space or a platform for an HVAC unit or hot water tank installed in the attic.

The decking is often installed directly on top of the ceiling joists; this limits the amount of space available for insulation and lowers the attics average R-value. To ensure proper insulation depth, the attic decking must be raised above the ceiling joists. To accomplish this edge-nail 2x4s or 2x6s to the tops of the ceiling joists where the decking is to be located or, as an option for trusses, lag 2x4 members to the truss webbing at the desired height. Install the decking securely to the top of the raised lumber after the insulation has been installed.

Knee walls

Another type of attic access is a knee-wall door. A knee wall is typically a partial height wall that is usually found in the upstairs level of finished-attic homes.

Knee walls are notoriously leaky and often poorly insulated.

Make sure that the knee-wall door is weatherstripped and has a latch that pulls it tightly against the frame and weatherstripping to achieve a solid seal. Use construction adhesive and screws to attach rigid insulation to the attic side of the door. Some attic doors are full-height interior doors which should be insulated, weatherstripped, and equipped with a tight threshold.

If you need to add insulation, first air seal the knee wall before insulating. Consider covering the back of the vertical knee wall with rigid insulation. Insulated sheathing, with the seams caulked or sealed with housewrap tape, reduces heat flow and minimizes the comfort problems commonly associated with drafty attic knee walls.


Building an attic stairs cover box

Use the template shown below to cut pieces from a single 4x8 foot sheet of -inch rigid insulation for a box 53 inches long (51 inches inside), 24 inches wide (22 inches inside), and 9 inches high inside.

This box will fit stairs with an outside framing dimension of 53 inches by 24 inches. Adjust dimensions to fit the specific stairs being installed.

Be sure to carefully measure both inside and outside dimensions to the appropriate length, width, and depth clearances for the cover box. Create the box as shown: Apply adhesive/mastic and use roofing nails to construct the two end and two side pieces.

Assemble the side and end pieces into a box using adhesive and longer nails. Add the center support brace and cover piece with glue and nails. A faced insulation batt may be glued to the cover piece with adhesive applied to the paper backing.

Materials you'll need:

  • -inch rigid insulation
  • Insulation batt (optional)
  • Duct sealing mastic or construction adhesive
  • 1-inch roofing nails (8d or 16d)
  • Tape measure, sharp utility knife and straight-edge, or table/circular saw
  • Weatherstripping/gasket material with adhesive

Source: U.S Department of Energy's Energy Efficiency and Renewable Energy Clearinghouse (EREC). Learn more about the department on the web at http://www.eren.doe.gov

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