Foam insulation is an excellent material to insulate the wall underneath the window sill and prevent drafts. Although you’ll want to finish up installing the foam insulation by sealing any cracks with silicone caulk, the foam insulation should make a difference if you are attempting to avoid heat loss.
When installing foam insulation, start on the exterior of the house and look for cracks concealed by caulk. If you cannot find any, but still want to add foam insulation to your wall, you will have to drill holes in the interior wall and insert it there. Also, if you suspect the cavity is not being completely filled, you can also drill holes in the interior walls.
As with many insulation projects, it’s easiest if you add foam insulation without the window sill in place, trim it, and then install the sill to create a tight fit. If you have the opportunity to do this, take it rather than working around the window sill.
Step 1: Remove any old caulk that may be concealing small crevices between the window sill and the wall using an utility knife.
Step 2: Place a drop cloth underneath the window to catch any foam insulation that may bubble out.
Step 3: Insert the applicator tip into a can of foam insulation. Use the smallest applicator tip you can find, so you can be sure to get into all of the crevices you can find.
Step 4: Start inserting the applicator tip into any crevices you find, starting right to left so you don’t miss any. When you find a crevice, insert the applicator tip and depress the trigger to release the insulation. Fill a bit at a time, allowing the insulation to expand to fill the area. Wait 5 minutes to allow all of the foam insulation to dry completely.
TIP: If you can’t find any crevices, you can drill a few holes through the drywall and into any suspected cavities. Fill these areas, then patch and paint the holes you created.
Step 5: Trim the foam insulation so it is flush with the wall using a utility knife.
Step 6: Caulk around the entire window sill and window using silicone caulk to completely seal any crevices and prevent water damage.