Figure 5-16 The wall and floor sections of a half storey should be insulated
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.
Figure 5-17 Rigid insulation can be nailed over the studs of the knee-wall section
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.
Figure 5-18 Each section has its own cross ventilation
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.