Because of their position on the roofs of houses, dormer windows can often be susceptible to leaking and dry rot. Consequently, it is crucial to seal dormer windows properly. Normal trim wrapping can be insufficient to protect from the elements, so buying a waterproof membrane to place over your existing trim wrapping is recommended. In addition, you can apply flashing - an impervious material such as metal or plastic - to the outside of your dormer window for further protection.
Apply the trim wrapping to the outer edges of your dormer window. Make sure there are no cracks in the wrapping that could allow leakage.
Put a layer of waterproof membrane over the trim wrapping. In addition, put a layer of the waterproof membrane on the dormer window's nail fins in order to prevent corrosion.
Apply a thick bead of exterior caulking with the caulk gun around the outside of the dormer window.
Place the plastic or metal flashing along the dormer window's edges.
Caulk any gaps between the window frame and the window from inside the house with polyurethane foam. You may also seal the inside frame with fibreglass insulation held in place with aluminium tape.