Insulating a Floor

Q: How should I insulate a floor? I’m building a 12-foot x 16-foot tool shed/workshop, and I’d like to build the floor so it’s warm. One guy at a building supply store recommended fiberglass batts between the floor joists, supported by wood underneath. Is this the best option?

A: This is a terrific question because it's so easy to make a mistake when insulating a wood frame floor. Although many people do install fiberglass batts stuffed between the joists, this is a poor approach for three reasons. First, the batts usually sag down in time, eventually falling right out unless they’re completely supported by lots of wood or expensive screen. Also, small animals love to steal the fiberglass and use it in their own building projects. And finally, if either gravity or critters don’t get you, there's always the problem of moisture buildup within the fiberglass. Being close to the ground, the material can easily pick up water vapour, encouraging the growth of mould and mildew on your floor frame. A far better approach is to forget the fiberglass altogether, and leave the underside of your floor frame completely open and ventilated. Install a 1 or 2-inch thick layer of extruded polystyrene foam on top of your sub floor, then cover this with another layer of cement board on top. You’ll get a fantastically warm, durable and moisture-proof installation. Extruded poly foam is dense enough that no strapping is needed under the top layer of cement board. Just put it on the foam, then secure the whole thing with screws driven down into the underlying floor joists. Foam insulation systems also provide warm-toes performance in basements and older buildings with perennially cold floors.