Insulating a Crawl Space Using Rigid Foam

Crawl spaces typically are not insulated well enough, especially in older homes. Insulating and air-sealing the floor of an addition or heated porch with a combination of rigid foam and spray foam can make a big difference in your energy use and comfort.

The method below is the same as that used to seal the band joist, but you will install the rigid foam against the underside of the floor of the porch or addition in the bays between floor joists, then air-seal around the edges with spray foam.

You should also add a vapor barrier over the dirt floor so it doesn't emit moisture into the crawl space to possibly cause mold later on.  Install 6 millimeter plastic across the dirt floor.  Anywhere 2 pieces of plastic connect, overlap them by 6" and tape together with a good quality foil tape.  Run the plastic 6" up the wall and use the foil tape and perhaps staples through the foil to really connect it to the wall.

1) Spray Foam Safety. Because you will be applying spray foam above you, probably while on your back, you are more vulnerable to spray foam getting on you, especially in your eyes. Please read Spray Foam Safety and Application before proceding.

2) Measure the bays between the floor joists (they may not be the same widths). Cut the insulation with a saw or steak knife, so that you leave a small gap for the spray foam between the foam and the joists. If the insulation is too loose and wants to fall, you can then hammer a few nails at the sides to hold it in place until the spray foam can dry. Get every joist bay filled.

Again, watch out for the spray foam. It expands and can dribble down if you use too much. Never work right above your head and eyes.

3) Then fill the gap where the porch attaches to the house.  Make sure to get every crack or gap in the house wall so no air can infiltrate.

NOTE: If the spray foam is kept at 70 degrees beforehand, it should work fine, even if the day is cold.