How to Spray Foam a Rim Joist

Measure the length and height of all the rim joists to be sprayed with foam. Multiply those numbers together to get the number of square feet of rim joist. To air seal a rim joist, figure 1.5 inches of foam and, for insulation purposes, 3 inches. Spray foam kits are sold in board feet (1 foot by 1 foot by 1 inch), so if you have 50 square feet to spray and you want to install 3 inches, it would require 150 board feet of spray foam.

Remove all items from the area to be sprayed. If there are large items like washing machines and dryers, simply cover with a drop cloth. Rule of thumb when using spray foam is if you do not want foam on it, cover it or move it. Spray foam sticks to everything and is difficult to remove.

Read the instructions and the Material Safety Data Sheets (MSDS) with the spray foam kit before turning on the tanks. Manufacturers' spray foam kits are different, so read information even if you have operated a spray foam kit before.

Put on disposable coveralls, gloves, safety glasses and respirator with organic vapor cartridges before pressurizing lines.

Holding the spray foam gun 12 inches away from surface to be sprayed, make slow, steady passes over the area. Apply between 1 1/2 to 2 inches per pass. Allow foam to set before re-coating to desired thickness.