Spray foam and rigid foam insulation can be used, but batts are less expensive and easier to handle in confined spaces. If there are any electrical outlets (use care to avoid shocks - and also take this opportunity to upgrade wiring in this area) or pipes in the knee wall, keep them on the warm side of the air and vapour barrier and insulation, and seal the air and vapour barrier around them.

• Treat the attic floor behind the knee wall and the attic space above the half storey's ceiling (see Figure 5-17) exactly as described previously for a standard, unfinished attic; in addition, add air sealing under the knee wall.
• Treat end walls as described in "Insulating walls."(End walls are the full-height walls that are exposed to the exterior.)
• The knee wall can be treated like an unfinished attic floor (see Figure 5-9), making allowances for the fact that it is vertical. Install a combination air and vapour barrier made from polyethylene strips sealed between each stud. Sealing all cracks and penetrations and painting a vapour barrier on the interior surface also can create the air barrier. Next, install the insulation and secure it in place with building paper, cardboard, olefin sheets or chicken wire. Better yet, nail 25 mm (1 in.) or more of rigid board insulation to the exterior side of the studs to hold the insulation. This will also increase the thermal resistance of the wall section and reduce thermal bridging. A wall in the attic that adjoins a heated space can also be insulated like a knee wall.
• The section between the rafters may be filled with insulation if all penetrations through the ceiling are sealed and if this is permitted by local building codes or standards (see hot roof in Concentrate on air sealing, subsection Spray foam under the roof). The hot roof could involve the slope ceiling and then the knee wall space. It is unlikely to be necessary in the small triangular top attic. Having an insulated roof slope allows the knee wall spaces to be used for storage (as they often are).
• Exposed foam in accessible areas must be fire protected.
• Due to limited space for insulation and venting on the sloping ceiling, consider adding rigid board directly to the ceiling, and then air seal and cover with drywall. This method will greatly increase comfort levels in summer and winter, though it may reduce headroom.
• Some contractors specialize in high density blown insulation that will help reduce airflow while improving thermal values.

Dormer windows

Many 1 ½ or 2 ½ storey houses have dormer windows. The walls of the dormer may be insulated with batts, as described for the knee wall. Dormers typically include a lot of framing members that limit the amount of insulation that can be added. Besides air sealing, consider dense-pack blown-in insulation and also the addition of rigid board insulation and drywall to reduce thermal bridging.

Floorboards

Some houses will have the attic floored over, even when it is not used as living space. You can insulate it by lifting the floorboards and treating it as you would an unfinished attic, though some or all of the boards may have to be replaced to maintain the ceiling stiffness.

Alternatively, a contractor can fill the subfloor space completely by blowing in dense-pack loose-fill insulation through access holes. Air leaks, such as around plumbing stacks and open eave spaces, should be sealed first. With the contractor, ensure that the space is filled completely and at the right density.

SAFETY WARNING:

Before insulating, check for wiring (replace damaged or frayed wiring), recessed light fixtures or other sources of heat that may be concealed beneath the floorboards. All heat sources must be protected from the insulation or removed entirely.

You will not likely be able to achieve the minimum manufacturer's recommended levels for attic insulation by filling this space alone. If possible, add some insulation on top of the floorboards to keep them warm and to reduce thermal bridging through the joists and floorboards.