How to Install Windows Over Foam Insulation Sheathing

Cut a piece of adhesive-backed weather stripping that is approximately one-third the depth of the sill of the window rough opening with scissors to a length that is equal to the total width of the sill. Peel the backing from the weather stripping and apply it to the sill so that it is flush with the inside (rear) edge. Note that this will create a back-dam that will force any water that may leak around the window to flow to the outside of the building instead of the inside.

Cut a length of adhesive-backed window flashing to a length that is equal to the total width of the window opening plus 12 inches with scissors. Peel the backing from the flashing.

Position the flashing so that its rear edge is even with the rear edge of the weather stripping on the sill and that 6 inches of flashing overhangs each edge of the sill. Press the flashing down with your hands. Cut slits in the front corners of the flashing with scissors. Fold down the front edges of the flashing and press it down so that it sticks to the foam sheathing.

Peel the backing from two corner flashing patches. Adhere the patches to each lower corner of the window opening to seal the corners in the previously installed flashing.

Cut two pieces of adhesive-backed flashing material to lengths that equal the height of the window opening. Apply the flashing to each vertical side of the window opening. Fold over the front edges of the flashing so that it sticks to the sheathing.

Apply a bead of silicone sealant to the top and vertical sides of the backside of the window nailing fin. Do not apply sealant to the bottom edge of the window, as this will create a water dam that may rot out the window sill prematurely.

Lift and insert the window into the rough opening from the outside. Press the window into the sheathing so that the sealant compresses. Place a level on the vertical and horizontal surfaces of the window. Add wood shims to the gap between the window and the rough opening to bring the window level and plumb and to ensure the bottom of the window is resting on the sill and not pressing down on the edge of the foam sheathing.

Drive 2 ½-inch galvanized roofing nails through the holes in the exterior of the window nailing fin to secure the window to the building. Cut strips of adhesive-backed flashing and press onto the nailing fins on the bottom, sides and top of the window.