Green Building Library
Air Sealing

Air Sealing Options for Rim Joists

The "super highway" of air leaks runs through just about every house with a basement or more than one story. It collects traffic from every interior wall that holds a pipe or wire. If interior wall cavities are used as ducts, it's rush hour whenever the furnace kicks in. This highway is the space between floor joists. Air travels back and forth to the outside wall where it leaks in and out around the rim joist.

Blocking the access routes—every penetration for receptacles, pipes and ducts—may be the only option for existing homes, but it's tedious and not very effective. In new construction and major remodels, you have several options for sealing rim joists (also called a band joist). By sealing the rim, you avoid the need to plug all those little penetrations that lead into joist cavities.

A question you often hear about sealing goes like this: "once the framing is nailed together, how can air get through there?" Consider this: Lumber comes off the truck with damage and defects that mar the edges and prevent an air tight fit. Houses tend to move over time and gaps open up. Lumber dries and shrinks over the first year. Later, the wood shrinks and swells with changing moisture conditions.

All houses benefit from sealing rim leaks. And, if you're shooting for a "continuous air barrier," an effective seal at the rim is essential. Unfortunately, this may be the most difficult leak in a house to seal. Some techniques require framers to seal as the rim goes in. People that carry 24 oz. waffle-faced hammers and dance on rafters don't always appreciate the distraction of caulking and gasketing. It's also possible to get a continous air seal after all the framing is in place. Take your pick of these methods.

Connector Strip

Here's an idea that orignated with the polyethylene air/vapor barrier that's still used in Canada. This variation uses vapor permeable housewrap instead of polyethylene sheets. That way you don't have to insulate the outside of the rim to keep it warm and prevent condensation. The 2 to 3 ft. strip drapes over the top plate of the first floor wall. It hangs there getting in the way until the upper floor is framed and sheathed. Then the framers flip the strip onto the second floor deck, tack it in place temporarily. They lift the second floor wall on top of the strip. Later, the strip is caulked to the air barrier on the walls above and below. Gusty winds and general abuse on the construction site shred some housewraps. Housewrap material can be slippery, which might cause an otherwise sure ffooted worker to fall when walking on the plate. Placing the strip between the top plates of the first floor wall keeps it out of the way and reduces wear.

Outside Gaskets

Here's a promising idea from a Canadian builder. Gaskets four-to-six-inches wide span the gaps on the outside. He used polyethylene sill sealer that was about 1/4-inch thick and reported no problems with bulging sheathing or siding. Wide gaskets made of EPDM, closed-cell foam or saturated open-cell foam should work, too. Because the gasket doesn't prevent the rim from sliding on the plate, it's no trouble to position the rim and square the floor framing. Outside gaskets will be easy if you tip up your wall framing and then nail up the sheathing. If you install sheathing on the deck, you'll have to tack the strip of gasket to the top plate, then lay the sheathing on top.

Spray Foam

Foam sprayed into the rim cavity easily seals all the leaks. You probably won't call a spray foam crew just to do the rim. At the least, you'll want them to fully insulate the walls and seal the pipe and wire holes to the attic. This option wins the easy award. You simply hire out the job. The weakness is considerably higher cost. Since the current formulations of urethane foam use CFC foaming agents, you might consider icynene foam, the ozone-friendly alternative.

Offset Plates

Keeping the rim joist warm is critical in cold climates, where condensation can collect on the cold lumber. The well-known trick of kicking out the bottom plate of a 2x6 wall allows room for rigid insulation on the outside. You may need to add fiberglass insulation on the inside to match the R-value of the typical above grade wall. For air sealing, this detail creates a nice corner between the top plate of the first floor wall and the rim. That corner can hold a continuous bead of high-quality elastomeric sealant. Then a continuous bead of construction adhesive seals the rim to the subfloor above. This method uses inexpensive materials, and doesn't interfere with the work flow during framing.

Gasket and Adhesive

Before setting the rim, you could tack a resilient gasket onto the plate. The resilient rubber gasket will fill up the gaps due to movement and flaws in the wood. The gasket forms an excellent seal, because it's not interrupted by each floor joist like some other methods. Even when compressed, the gasket will hold the rim up by 1/16 in. to 1/8 in. You may want to use scraps of gasket as shims to bring the floor joists up to the same level. Interior bearing walls should get a full length gasket so the building loads are carried evenly. Setting the rim will be harder, because the gasket prevents the rim from sliding on the plate. You'll have to lift it into place. This may also make it harder if you find yourself trying to square the floor framing. In your battle against floor squeaks and in the interest of structural strength, you probably glue the subfloor to the joists. Running a continuous bead of adhesive on top of the rim creates a long lasting seal with very little extra effort.

Foam Squares Caulked in Place

Here's a method that you would probably find too tedious for new construction. But in remodels where the plaster or drywall have been removed, it may be a good option. You could simply caulk the framing from the inside, although it's difficult to reach into the cavity with an awkward caulking gun. And it may be impossible to get into tight spaces.

Here's a refinement. Cut a piece of rigid foam insulation to fit in the cavity. Slide fiberglass insulation up next to the rim, then cover it with the foam. The combined thickness of the fiberglass and foam should bring the foam flush with the inside face of the plate. Caulk all four sides of the foam, plus the crack under each floor joist. This will give you a continuous seal. With this method, sealing doesn't interfere with framing at all. This will take more time, but you can use less expensive labor. The foam insulation adds a bit of cost, but it's easy to cut. (Actually any rigid material that qualifies as a vapor retarder will work.) If carefully done, the seal will be just as good as with other methods, but there's more room for error.

What About Housewrap?

About now you may be saying: "Why all the fuss? Isn't this what housewrap is supposed to fix?" Housewrap helps. But to be more than high-tech building paper, it must be meticulously sealed to the building and to itself. To make housewrap a continuous air barrier takes many hours, a case of caulk and half a dozen rolls of that expensive tape. The cost of these extras make the options on these pages look cheap and easy.

Housewrap makes a great weather barrier but there's no definitive proof that housewrap alone achieves the kind of air tightness many builders strive for. Better information may soon be available when engineers at duPont (the makers of market-dominating Tyvek) finish their field study of actual homes.

This article appeared in Energy Source Builder #26 April 1993,
©Copyright 2008 Iris Communications, Inc.