World's best floor insulation

Q: How should I insulate a wood-framed floor? I’m building a 12-foot x 16-foot shed/workshop, and I’d like to heat it sometimes. A building store guy recommends fiberglass batts between the floor joists, supported by wood underneath. Is this the easiest, best option?

A: : That's a terrific question because it's so easy to make a mistake when insulating a 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 the underside of your floor frame. A far better approach is to forget the fiberglass altogether, and leave the underside of your floor frame completely open. Instead, install a 1 or 2-inch thick layer of rigid, extruded polystyrene foam on top of your subfloor, then cover this with another layer of 5/8 or 3/4-inch plywood 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 plywood. Just put it on the foam, then secure the whole thing with screws driven down into the underlying floor joists. Foam-and-ply insulation systems also provide toasty-warm performance in basements and older buildings with perennially cold floors.