Over the years, I've developed a simple system to control random air leaks in the energy-efficient houses I build in Massachusetts. Of all the possible energy-saving upgrades, air sealing is the most cost-effective, since about 30% of the heat loss in a typical home is due to uncontrolled air leaks. Using the techniques described here, we build houses with consistently low infiltration rates — 0.6 square inches (or less) of leakage per 100 square feet of shell area, well under the Energy Crafted Homes standard of 2.0 square inches.
Creating a Tight Air Barrier
If you are committed to minimizing air leaks in the houses you build, you need to be sure that everyone on the job site, including the framing crew and the subcontractors, understands the basics of air sealing, and understands your expectations for maintaining a tight air barrier.
In the colder parts of the U.S., builders typically install polyethylene under the drywall, calling it the "vapor barrier." But the most important function of polyethylene is as an air barrier. Moisture follows air leaks, moving through holes in a house, at much greater rates than it passes through solid surfaces as a vapor. Most of the problem-causing moisture that condenses in attics and building cavities is transported by interior air leaking through holes in the air barrier.
An air barrier should be continuous with the thermal insulation. In most cases, the air barrier will be on the warm side of the thermal insulation, but it doesn't have to be. In some cases, it makes sense to establish the air barrier on the outside of the thermal insulation.
Assemble your materials. For a careful air-sealing job, you'll need to be sure you have a few important materials on hand: reinforced polyethylene (for example, Tu-Tuf, Good News Reused, or Tenoarm); Tremco acoustical sealant (a multi-purpose air-sealing caulk that sticks to polyethylene); 3M Builders' Sealing Tape (also called contractors' tape, it's used for polyethylene and housewrap); airtight electrical boxes (from Ryeco or LESSCO); and a good urethane foam gun (see "Sources of Supply").
Keeping the Basement Warm
We always install 6-mil polyethylene and at least 1 inch of rigid foam insulation under all of our basement slabs. Besides saving energy, the foam keeps the slab warm, greatly increasing comfort and helping to minimize condensation. After installing crushed stone to the depth of the footing, we lay down the poly and then the rigid foam. We also install a strip of 1-inch foam at the perimeter of the slab, between the slab and the foundation wall, as a thermal break. If the basement floor is getting radiant heat, we'll increase the thickness of the under-slab insulation to 2 inches.
Insulating basement walls. We no longer insulate our basement walls from the exterior, since exterior foam is vulnerable to insect damage, and above-grade foam is difficult to protect. Instead, we frame up 2x4 walls inside the basement, leaving a 2-inch gap between the back of the studs and the basement wall. This allows enough room for the installation of R-19 fiberglass batts, which we cover with flame-retardant poly from Poly-America. One advantage of interior insulation: With the 2x4 perimeter walls installed, all it takes is wiring and drywall for a customer to finish the basement.
Since most basement walls have few penetrations, they are relatively simple to air seal. But be careful of bulkhead doors, which are often leaky. The area between the door and the band joist, especially, needs to be sealed and insulated.
Preventing Drafty Floors
Most houses leak a lot of air through gaps at the perimeter of the floor system. To keep this area tight, four critical areas need to be addressed: under the mudsill; along the band joist; between the band joist and the subfloor; and between the subfloor and the wall plate.
Tight sills. Between the foundation wall and the mudsill, we use regular polyethylene foam sill seal, folded in half lengthwise. Doubled sill seal does a better job of air sealing than a single layer. Any gaps that are too big for the sill seal to handle are filled later, using our urethane foam gun.
Warm band joists. On many houses, the band joists are poorly insulated, so condensation forms on the cold interior surface of the lumber. Keeping the band joist warm with exterior foam insulation prevents condensation that can lead to rot.
Since we frame our walls with 2x6s, we can recess our band joists 2 inches. We attach 2-inch-thick rigid foam to the band joist with continuous beads of Tremco acoustical sealant. The rigid foam, once it is caulked in place, becomes the air barrier. Although Tremco sealant, which is also known as "black death," can be messy to apply, it's the best caulk to use for air sealing a wide variety of materials, including most types of plastic.
From inside the basement, we stuff a piece of R-19 kraft-faced fiberglass batt into each joist bay, behind the band joist. Finally, when we put the subfloor down, we put a continuous bead of construction adhesive along the band joist to prevent any air leaks at the perimeter of the subfloor.
Gasketed bottom plate. We install regular foam sill seal under the bottom plates of our exterior walls. To make the sill seal go twice as far, we usually cut it in half lengthwise. We roll out the foam and hammer-tack it to the subfloor just before we stand up our walls. Instead of foam sill seal, you can also use sticky-backed foam weatherstripping in this location.
Cantilevers. Cantilevered floors are particularly difficult to air seal, especially if wires or pipes create an air path through the floor and up the exterior wall. At the point where the cantilever begins, we install solid blocking between each cantilevered joist. The edges of the blocking, as well as any penetrations through the blocking, get carefully caulked. Because the plywood subfloor over a cantilever is the air barrier, be sure the subfloor is installed with construction adhesive. Finally, the bottom of the cantilevered joists need to be wrapped with housewrap.
Keeping Walls Tight
Most of our walls are framed with 2x6s, 16 inches on-center. If the studs are spaced wider than 16 inches, the dense-pack cellulose insulation pillows out, interfering with drywall installation. If the customer is willing to pay for an upgrade, we space the 2x6 studs at 24 inches on-center and then install horizontal interior 1x3 strapping at 16 inches on-center. The strapping restrains the cellulose and also provides a thermal break between the drywall and the studs.
When it comes to sheathing, we prefer to use plywood or OSB, unless the homeowner insists on foam. Foam sheathing causes several headaches: the walls need special bracing against racking; window and door openings need to be furred out; and siding can be attached only to the studs.
We always install Tyvek, which we consider the best available housewrap. We tape all seams with Tyvek tape, following the manufacturer's instructions.
Where an interior partition meets an exterior wall, we install a vertical 1x8, 1x10, or plywood piece as a drywall nailer. This nailer needs to be continuous (not a collection of scraps), and it needs to be wide enough to provide room to tape the poly air barrier in the corner.