Rim Joist - Spray Foam Project Guide

DETERMINE YOUR GOALS

The first step in any successful project is making sure you know what you are setting out to accomplish. The rim joists are the area where floor joists sit on top of a foundation, whether it’s a crawlspace or basement. This area of a home is notorious for air leakage because of the different materials connected to each other. To air seal the rim joist bays, a simple one inch layer of closed cell foam will do the job.

DO THE MATH

Your total required number of spray foam kits is determined by your total board-feet need. A board-foot is one squared foot at one inch thick. With a rim joist spray, the perimeter of the basement or crawlspace multiplied by the height of the bays is your square footage of surface area. Keep this measurement in feet to make it easy math. If you are spraying the top of the sill plate and back of the bay cavity, and it measures 18” total, use 1.5 feet. A perimeter of 300 linear feet times 1.5 feet is 450 squared feet. At one inch, it’s 450 board feet; at two inches it’s 900 board feet. Each Foam it Green 602 system has enough in it to cover 602 board feet under ideal conditions.

PREPARE FOR SUCCESS

It’s difficult if not impossible to get foam off of surfaces it hits. So, be sure to take the time to cover anything you do not want foam stuck to including floors, windows, equipment in the area, and even you once you start spraying! Along with the recommended PPE, the sprayer should have no exposed skin or hair during application. How do you get foam out of a beard? In a word, scissors. As always, make sure your target spray surfaces measure between 65F-85F, so heat the area if needed. The foam can be painted when done, and a latex based paint is usually the best way to go. Foam it Green is ASTM E84 Class 1 Fire-retardant insulation.

TECHNIQUE TIPS

Target a wet foam application thickness of 1/3” as that will expand to one inch cured. When spraying overhead, it’s recommended to leave off the fan spray tip and simply use the mixing nozzle alone. Get your hand a bit closer when pointing up, and use a lighter trigger pull so you don’t “bounce” the foam off the surface.