Expert: Daniel Humphrey - 3/1/2009

Question
Hello Daniel,

Since you work in colder climates I thought you may be able to help me with this.

I live in a house that has a crawl space that isn't insulated, and I can literally feel the cold air rising up through the carpet.  I have a heat pump, and last month my electric bill was $409!

What is the best type of insulation to use for this applicatiion?  It has about a 3 foot clearance and the regular floor joists with the metal support braces, so I thought maybe the bat type wouldn't be the best.  So that leaves the spray foam, right?  What would you do?  

Thanks,
Gary in WV

Answer
Dear Gary,

For insulating a floor with regular floor joists, fiberglass batts are usually the easiest and cheapest, and you can do it yourself. You can insulate with one layer of faced insulation. Or you can do it in two layers: begin the job with a 3-1/2" layer of  faced insulation with the vapor guard facing against the subfloor and stapled on the sides to hold it in place, then finish with another layer of faced insulation, stapled the normal way with the facing seams overlapped on the joists. This gives you a pretty good air seal, especially if you tape the joints with Tyvek tape or the equivalent. The layer against the subfloor can tuck under the metal braces; the edges of the fiberglass batt are snipped with scissors so that they fluff out around the bracing. The layer closest to the crawl space is snipped the same way, and the fiberglass fills around pretty well. For plumbing and electrical, the batts can be peeled in layers and fitted on either side.

Another way to insulate the floor is blow-in cellulose. It makes an airtight seal, but of course it must be applied with a machine. Use a panel light and sturdy, such as 1/4" plywood, beneath the floor joists to hold the insulation and create the cavity for the cellulose.

A third way to insulate the floor from below is to use panels of extruded polystyrene either as infill between joists or as full sheets attached with screws and large washers from below. A 2" panel gives you R10. I make the "washers" out of 4x4 squares of 1/2" plywood, with a 3/16" hole drilled in the center. The screws I use are Torx type deck screws that are long enough to screw into the joist at least an inch. Tape the seams with Tyvek tape.

I recommend insulating the inside of the crawl space walls with foam at the same time.

For you to feel cold air coming up from the floor, you obviously have some air leaks. Before you insulate, you might go looking for those cracks and fill them with spray foam, tape, or caulk. And you know that for air to come in, you have hot air leaving the room. In an airtight house, there is no great temperature differential between the ceiling and the floor. So look for places higher up than can be sealed--light fixtures, window and door openings, outlet boxes, etc.

I hope this helps you winterize and save some of that money. Heat pumps are interesting, and I'd like to explore that.

Daniel