The rim joist on top of the foundation is an area of heat loss. The fiberglass batt insulation blocks conductive heat loss, and the foam caulk reduces air leakage.
Question: My house is fairly new, and the wall and blown-in attic insulation are at recommended levels, but my utility bills are still high. What else should I check for inefficiency?
Answer: Walls and ceilings are the areas of the greatest heat loss from a house—proper insulation there is of utmost importance. But other areas in the exterior thermal envelope of your house can have insulation voids or air leakage that contribute to high utility bills.
First, check the depth of your blown-in attic insulation. It may have settled, no longer reaching the required depth and R-value for your climate. (The U.S. Department of Energy has a website that calculates the amount of insulation you need for your climate zone.) Using a rake, make sure it’s level across the attic floor. Wind from attic vents can blow it around, creating high and low spots.
Any break in the thermal envelope of your home creates potential for energy loss. One common spot is electrical wall outlets and switches on outside walls. Often, they are completely uninsulated.
Switch off the circuit breaker to these outlets and switches. Remove the faceplate. If you can fit the tube from a urethane foam spray can into the wall around the conduit box, shoot some expanding foam in there. This should fill insulation voids and seal it.
You can also install foam draft sealers behind the faceplates. They add only a slight amount of insulation, but they will improve the seal.
Recessed ceiling lights are another typical area of energy loss, because they get hot, creating a natural upward draft. The most efficient option is to replace old canister lights with newer sealed models.
Don’t just pour or pack insulation against recessed lights in the attic. This can cause older styles, which were not designed to be insulated, to overheat. You can caulk around the hole in the attic floor and the canister, but some room air will still leak out through the canister itself.
Ceiling fans are another place to check. If they were installed after the house was built, the insulation level may be less around the support blocking or where the hole was cut to run wiring. From the attic side, push the insulation away and caulk the hole around the wire, then cover it with additional insulation.
Next time you paint the trim around doors and windows, pry off the decorative molding. You may find an uninsulated gap between the rough opening and the door or window frame. Apply low-expansion foam in the gap—but use it sparingly, because it can deform the frame as it expands.
Another area that wastes a lot of energy is where the sill plate meets the rim joist. The sill plate is the piece of lumber that rests on the top of the foundation. The rim joist rests on top of the sill plate, and your house walls rest on the rim joist. The rim joist typically is not insulated.
Buy fiberglass wall insulation batts and cut them into short lengths to fit between the rim joist and the floor joist. With their short length, they should stay in place without stapling.
While you are looking at the rim joist and sill plate, you may see gaps between the top of the foundation and the sill plate. The top of a concrete foundation wall is seldom perfectly level. Apply urethane foam insulation from a can between the sill plate and foundation wall. This will block outdoor air leakage and add some insulation value to that area.
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Check these spots for air leakage:
Around outlet and switch boxes on outside walls
On the attic floor around holes drilled for wiring fans and ceiling light fixtures
Beneath the molding of door and window frames
Where lumber meets the foundation