Insulating the Rim Joist
When you frame a floor, you have structural members called floor joists. When a joist ends on the outside sill plate another board is used to cap off the end; thus creating a box with floor joists in the middle. The boards that cap the ends are called header joists. When I am talking about the rim joist, I am referring to all of the boards that go around the outside of the floor system.

Usually, when a person uses a TJI floor system, the header joist would be a 1" thick piece of oriented strand board (OSB), the same depth as your floor system. Instead of ordering this board for the header joist, I use another TJI, so that the perimeter of my floor is made completely of TJI's.

This is a deviation from the standard installation of the floor system. By using TJI's for the rim joists I can insulate the entire perimeter. With the outside rim joists completely insulated with 2" of extruded polystyrene on each side, I have built in an insulation value of R-10 all the way around the perimeter of the floor system. The extruded polystyrene is easily cut on a table saw and placed on both sides between the flanges.

To attach the insulation to your rim joists, use construction mastic or other adhesives formulated to use with foam. Make sure that the adhesive you use, will stick the insulation to the joist without melting it. Simply put a dab every two (2) feet or so to insure that the insulation will stay in place. By cutting the insulation on a table saw, you can get a tight friction fit with little glue needed.

Note: Multilevels and concentrated loads may require squash blocks
to transfer the load to the foundation.

To increase the floor perimeter insulation value to R-20, simply cut pieces of 2" extruded polystyrene and place them inside the insulated rim joist. On the ends you have to notch and fit the insulation which takes a little time, but not much. This second step can be done at a later time if you are trying to beat the weather.

	Ripping insulation is quick and accurate on the table saw.
	Use several spots of construction adhesive (for foam application) to hold insulation to TJI being used for the rim joists.
	Installation of extruded styrene to TJIs.
	With 1" of extruded sytrene on both sides of the rim joist, an R10 insulation factor is built into the floor system during the framing process.
	Mock up of floor system showing insulated rim joist and floor joists.
	Mock up of the rim joist area shows the installation of 2" of extruded styrene to the inside of the joists. This gives an R-20 insulation value in your floor system.

Insulating at this point with extruded polystyrene is an efficient way to start the framing of your house for energy efficiency. Generally, when framing the floor, the thinking has been about structural issues, rather than combining structural/energy efficient components. The standard practice is to come back after the house is framed and stick batt insulation between the floor joists and around the exterior joists to insulate the house. I found that when I used this technique, this rim area was hard to seal and insulate properly. Even though we tried to do it correctly, the batt insulation often came loose after a few years and became less and less effective.