Window air leakage can be reduced by applying a continuous bead of caulk around the window trim where it meets the wall, at the mitred joints of the trim, and between the trim and the frame (see Figure 8-3). Make sure the caulk is intended for indoor use (do not use exterior caulking indoors), can be painted and is of good quality. For more information about caulking products, see Air barrier materials.

If a window is particularly leaky and the trim can be easily removed and re-installed, remove the trim, add insulation and seal the gap before reapplying the trim. If the gap is small, 6 mm (¼ in.) or less, insulating the gap followed by caulking may suffice. Larger gaps may require either a backer rod with caulking or low-expansion foam (see Figure 8-4).

To further reduce air leakage, apply a layer of red technical tape to cover the joint between the wall and window frame. Ensure that the tape will be hidden by the trim as it cannot be painted and red adhesive may remain after excess tape is removed.