How do I insulate beneath a bay window?

Q: How should I insulate a cold floor underneath a bay window that juts over my open front porch? I've removed all the old wood covering the underside of the floor frame, and I have a clean slate. What's the best way to preserve and seal this space so the bay window up above is warm?

A: It's not unusual to require insulation upgrades in floor cavities that sit over unheated spaces, and there are two things you must consider when settling on an approach. The first is thermal-insulation properties of the product, and the second is air-sealing action. If this were my project, I'd go with professionally applied, spray-on foam. This could be either polyurethane (which I have lots of good personal experience with) or icynene (which I'm still learning about). Either way, filling the floor cavity with spray foam delivers excellent thermal insulation while also keeping drafts out of your bay window area. No vapour barrier is required and the foam blocks a considerable amount of sound transmission from the outdoors, too.

The only hitch in all this is availability. Although the number of professional spray foam contractors is growing, there are still lots of rural areas where local service isn't available. An alternative that also costs less money involves cutting and applying pieces of rigid foam to floor frame cavities. Cut 2-inch-thick pieces of extruded polystyrene foam an inch shorter and narrower than the space involved, use hot melt glue or construction adhesive to hold the foam to the underside of the subfloor, then seal the perimeter gap around the rigid foam using a can of expanding polyurethane foam. Two layers of 2-inch foam delivers R-20, which is a good minimum level for a floor like yours.