Since 1984, I have been building energy-efficient homes with special attention to air sealing. Most conventional new homes, when tested with a blower door, show a natural infiltration rate of 4 to 8 air changes per hour; my homes are rated at 0.48 to 1.0 air changes per hour (ACH50). My package of energy-saving details costs my customers only about $1.25 per square foot, and they often make back the extra cost with just three to five years of energy savings. Since satisfied customers tell their friends about their low energy bills, my homes have been in steady demand.
Building a tight home does require training your subcontractors. But subs who do quality work may be eager to learn about air sealing, since those skills make them more attractive to other energy-efficient builders.
Most builders install a poly vapor barrier under their basement slabs. But it’s also important to include poly under the wall footings to prevent the foundation walls themselves from wicking up water. Eliminating this source of moisture lessens the chance of mold and improves indoor air quality. After my foundation contractor coats the exterior of the basement walls with dampproofing, I install 2 inches of extruded polystyrene foam from the footing to the mudsill. All of my basement footings have perimeter drain tile on the interior as well as the exterior. Finally, I backfill with 3/4- inch crushed stone up to 2 feet of finish grade.
To protect the rigid foam above grade, I use a tough fiberglass material called Ground Breaker, which comes in 50-foot rolls in widths of 12 and 24 inches. These panels are tough — I’ve never put a hole in one. The top edge of the material is fastened to the mudsill with 3-inch roofing nails driven through the rigid foam, while the bottom is kept in place by the dirt backfill.
Our basement slabs are poured over 8 inches of 3/4-inch crushed stone covered Start the polyethylene air barrier under the mudsill and seal every seam by Steve Lentz with a layer of 6-mil poly. For the poly, we prefer a brand called Tu-Tuf — a high-density, cross-laminated white polyethylene. Since the poly under the slab is a vapor barrier, not an air barrier, there’s no need to tape the seams. Over the poly we install a layer of 1-inch rigid foam. If the basement is getting radiant floor heat, I increase the depth of the under-slab foam to 2 inches.
Air Sealing Begins at Framing
The most important factor in building an energy-efficient house is the installation of a continuous polyethylene air barrier (see “Practical Details for Energy Efficiency,” 2/01). This barrier needs to be as airtight as possible, as it snakes its way up from the mudsill, around the band joist, under the wall plates, up the interior edge of the studs, and under the ceiling joists.
If you wait until the framing is complete to think about air sealing, it’s already too late. Unless the framers take time to install narrow strips of polyethylene between framing members in key areas, there’s simply no way to keep the air barrier continuous. For the polyethylene to serve as a true air barrier, all seams must also be sealed with either 3M contractor’s tape or Tremco acoustical sealant. Since Tremco is a sealant, not an adhesive, there must be a solid framing member behind it for it to work effectively.
When sealing a seam without solid backing, we use tape. The red tape from 3M is tenacious and long lasting. I’ve opened up walls six years after completion and found the tape to be as good as the day it was installed.
Band Joist Details
We frame our floor system so that the band joist is flush with the foundation, while the walls are framed to overhang the band joist by 1 inch. When the band joists are later covered with 2-inch foam, they end up flush with the 1-inch foam wall sheathing.
On my houses, the poly air barrier starts under the mudsill. We staple a length of 6-inch-wide Tu-Tuf polyethylene to the bottom of the sill before it is installed, with about half the width of the poly extending beyond the sill toward the exterior. Every seam gets sealed with Tremco or tape. I cut the 51/2-inch-wide roll of sill seal in half lengthwise before I staple it to the sill, because I find that the narrower 23/4-inch-wide strip squashes down better and provides a better seal. Once the sill seal is stapled over the poly, the sill is flipped over and bolted down.
After the subfloor is nailed down and the lines are snapped for the exterior walls, we wrap the exterior of the band joist with poly. Since a band joist gets a lot of abuse during construction, we wrap it with Tenoarm, a tough 10-mil polyethylene from Sweden. Unlike Tu- Tuf, Tenoarm is transparent, so we can see the chalk lines through it where it laps onto the subfloor. The Tenoarm strips are about 16 inches wide, so they span the band joist and the 5 1/2-inch width of the bottom plate with at least an inch left for overlap. We seal the Tenoarm to the poly sticking out from under the mudsill with tape or Tremco, then fold it over onto the plywood subfloor.
Next, we extend the 2 inches of basement wall insulation up to cover the exterior of the band joist. On the inside, we insert a piece of R-19 fiberglass batt insulation up against the band joist in each bay. The 2-inch exterior foam keeps the band joist warm enough to prevent condensation on the poly.
We frame our exterior walls with 2x6s spaced 16 inches on-center, since 24-inch spacing doesn’t provide adequate nailing for siding. Once the walls are raised, the Tenoarm should peek out from under the bottom plate of the exterior walls, facing the inside of the house. On a two story house, the second-floor band joist is also wrapped on the exterior with Tenoarm. In this case, the Tenoarm extends from the interior over the top plate, up over the exterior of the band joist, and back onto the second-story subfloor.
Back in the 1970s, I used to pay an insulation contractor to blow 10 inches of cellulose into my attics. When I inspected the attic of one of my homes a few months after completion, I noticed that the cellulose barely covered the bottom chord of the trusses. My insulation sub explained, “It must have settled.” So in 1979 I decided to get my own cellulose-blowing equipment. Now I install 22 inches of cellulose in every attic, so that even after settling, my attics have a minimum of 16 inches of insulation.
To be sure there’s enough room for attic insulation, I specify raised-heel trusses. To get the necessary R-value on a stick-built roof, where the rafters come down too low at the attic perimeter, I install several layers of rigid foam insulation between the rafters above the wall plates.
To prevent wind-washing of the attic insulation above the soffit vents, we install wind breaks between the attic trusses. These are scraps of 6-mil poly, housewrap, or Tu-Tuf, stapled to the wall top plate and the roof trusses.
Cathedral ceilings. For cathedral ceilings, my minimum rafter size is 2x12, although I’ve installed wood Ijoist rafters as deep as 18 inches. In my experience, when cathedral-ceiling rafters are densely packed with cellulose insulation, no ventilation channels or soffit vents are required. As cheap insurance against possible moisture problems, I include ridge vents above my cathedral ceilings. Although this “hot roof” construction is controversial, I have done it this way successfully for years. I’ve had several opportunities to open up the ridges of cathedral ceilings completed years earlier, and in every case the rafter bays were dry and free of mold. None of the houses I’ve built have ever had a problem with ice dams or ceiling condensation. Be careful, though: This approach works only if your ceiling air barrier is airtight.
Blowing walls. To retain the cellulose insulation blown between the studs, I use a translucent permeable fabric called Insulweb. Insulweb is a spunbonded polypropylene fabric full of tiny holes that allow excess air pressure to escape; it is not intended to act as a vapor or air barrier.
After stapling Insulweb to the studs, we make one hole in each stud cavity, about 4 feet up from the floor. We start filling the cavity from the bottom, using a 2-inch rigid or flexible hose. When the stud bay is almost full of cellulose, we direct the hose to the top of the cavity to ensure that the top gets well filled.
When we finish blowing the walls, we count the bags of cellulose to be sure we’ve used enough. At the recommended density of 3 pounds per cubic foot, a 30-pound bag of cellulose should fill 1 2/3 8-foot wall cavities framed with 2x6s on 16-inch centers. We also check the density by pounding on the installed cellulose: If the cellulose moves, it’s not tight enough. If necessary, we go back and squeeze a little more in.
We use fiberglass batts in a few areas, such as behind a tub located on an exterior wall. In this case, the batts and the poly air barrier need to be installed before the tub goes in. The poly behind the tub is taped to the flanges of poly protruding from the bottom plate and the intersecting partition walls.
Electrical boxes. Despite what some insulation contractors will tell you, dense-pack cellulose does not stop air flow — it just slows it down. If you have a leaky electrical box, you can feel the air moving right through the cellulose during a blower-door test. When I started building energy-efficient homes, I was frustrated that there weren’t any decent airtight electrical boxes on the market, so I decided to design and manufacture my own. For the past 15 years, I’ve been selling the Lessco box, an airtight plastic box large enough to accommodate a standard electrical box inside of it. Lessco boxes are simple to install, so it shouldn’t take long to train your electrician; they are installed at the same time as standard electrical boxes. After the walls have been blown, we insulate the Lessco boxes by hand with scraps of fiberglass.