How do I insulate my crawlspace?

April 14, 2009 11:59 AM

How do I insulate my crawlspace? I'm thinking about next winter, now that I've barely survived this past one in the US Northeast. My house doesn't have a basement: it's built on concrete piers with a 1-2.5 foot crawlspace between the ground and the first floor. What would be the best way to insulate in this situation?

I suspect the best thing to do would be to super-insulate the ground floor and then build a skirt around the house to trap the air. However, insulating the ground floor is really difficult due to limited space and obstructions. The ground floor is currently minimally insulated: it would be a lot easier to build an well-insulated skirt around the base of the house.

Also, because of topography and soil type (sand, practically beach) the ground under the house is really dry so I'm not too worried about moisture build-up....

posted by geos to home & garden (9 comments total)

We did this at our last house with a crawl space. We got rolls of insulation and a box of metal wires that my husband referred to as "tiger teeth." The wires were a bit bowed and fit snugly between the joists. While on his back in the crawl space, he would hold the insulation up between the joists with one hand and push up the wire with the other so that it held the insulation firmly in place against the underside of the sub-flooring. He was able to do the entire house in one weekend and it made a HUGE difference.
posted by onhazier at 12:13 PM on April 14

Icynene foam. That's what we did with an 1880s house in our crawlspaces, though we now wish we had put radiant heat in the kitchen from below, before blowing the foam. Also, take cable and wiring needs into account since you can't get through it very easily again. A conduit to handle any future needs is a good idea.
posted by cocoagirl at 12:15 PM on April 14

If you cannot find the wires, you can make your own by getting a bunch of metal hangers and cutting them to be a bit longer than the space between the joists.
posted by onhazier at 12:16 PM on April 14

An insulated skirt would not work very well. You don't want to be heating the crawlspace and the soil underneath it (soil is not a particularly good insulator). In order for the skirt to be reasonably effective, you would have to dig the insulation a significant depth into the ground.

Spray foam under the floor is a good option with limited space and obstructions.
posted by ssg at 12:39 PM on April 14

This is what you want to do. The foundation vent isn't strictly necessary though.

Also, floor insulation isn't quite as important as sealing against air infiltration since warm air tends to rise. Spray foam insulation seals against air infiltration and insulates, but it tends to be pretty expensive and isn't really a DIY job.
posted by electroboy at 12:42 PM on April 14 [1 favorite]

You should do a vapor barrier as shown in electroboy's link. The problem isn't condensation, it's the energy that goes into vaporizing the moisture that works it's way up from the ground.
posted by Carmody'sPrize at 4:26 PM on April 14

make sure that, if you have plumbing running under the floor, it is on the warm side of the insulation..
posted by HuronBob at 5:03 PM on April 14

My mum hired someone to do what onhazier's husband did. She's happy with the result.
posted by deborah at 5:36 PM on April 14

I think a closed-cell spray-foam insulation is the best way of insulating this (by getting it between the joists). If you use an open cell material (fiberglass or rockwool fall into this category, and apparently Icynene), unless you have a vapor barrier on the warm side of the insulation, you will get condensation occurring, most likely within the insulation. electroboy's diagram shows the location of the vapor barrier. Condensation occurs when warm moisture laden air meets cold air (the exact location depends on several factors). When this happens within the insulation, the insulative properties drop down significantly, and heat can leak out of your house. To put the vapor barrier on the warm side, you could attach it to the underside of the floor, before putting the insulation up but with such little space it sounds tricky.
posted by a womble is an active kind of sloth at 6:04 PM on April 14