What Do I Do About Window Seat Mildew?
Q: "Why is there mildew on the underside of my window seat cushions?"
The underside of the cushions in our bay window sometimes gets covered with mildew. Why does that happen when there are 2 inches of rigid foam insulating the underside of the seat?
—Daniel Heckman, Orefield, PA.
Tom Silva replies: Mildew needs moisture to thrive, so you have to figure out where that moisture is coming from and reduce or eliminate it. Perhaps warm, humid indoor air is condensing on the cold window seat and keeping the underside of your cushions damp. The fix in that case would be to add even more foam insulation under the seat, reduce the indoor humidity, or both.
Or it could be that condensation on the glass is dripping down and seeping under the pillow. The solution then would be to reduce humidity and add storm windows.
But if condensation is not the problem, maybe there's a small air or water leak somewhere. Check around the windows the next time it rains or during the next cold snap. Caulk the gap, and insulate if necessary.
Here's another thought: Elevate the cushions slightly to create an air gap that will help them stay dry. I use a product called Cedar Breather to ventilate the underside of wood-shingled roofs. A few square feet of the same stuff stapled to your window seat might do the trick for your damp cushions.