Where you have access to the attic space, follow the directions for the control of air and vapour flows as outlined in Easily accessible attics.

Houses with half storeys have one major source of air leakage that must not be overlooked: through the ceiling joists immediately beneath the knee walls (see Figure 5-17). This critical air leakage area must be sealed flush with the back of the knee wall finish. This can be done by cutting and fitting foam board that will fill that ceiling joist opening and seal it into place with caulking or spray foam.

Another option may be to dense-pack loose-fill insulation between the floorboards and the ceiling below while ensuring that an air barrier is installed.