Most residential attics are purposely built drafty: that is, they’re unconditioned and vented to the outside. That’s no problem if the attic is truly isolated from the house, but in practice it often isn’t. In many homes the ducts running through the attic, as well as the ceiling plane below it, are poorly sealed, letting conditioned air blow and seep into attic space. The attic becomes a huge energy drain.
But at least one building scientist thinks there’s a better way. Brian Coble, director of high performance homes at Advanced Energy Corp. in Raleigh, N.C., believes that in some homes it makes more sense to convert the attic into a tempered space: that is, by bringing the attic into the thermal envelope without adding heating or cooling registers. The roof deck and gable walls are air-sealed and insulated, and all vents are eliminated.
While this is a great approach for new construction, it can also be a practical retrofit for existing homes. In such retrofits, Coble found typical energy savings of 18% to 20%, even in homes with properly sealed ducts.
Where and When
Builders and suppliers hearing about this approach for the first time often ask two questions: Where does it make the most economic sense, and How do I make sure it gets done right?
The first answer is that it won’t be cost-effective for every job. In an average 1,800-square-foot existing home, the homeowner can expect a bill of around $4,000 for materials and labor. By contrast, the more common remedy of plugging gaps in the ceiling plane requires just a day’s labor and $300 worth of sealant, though the results will be much less consistent. The builder can estimate the payback period by weighing costs against projected energy savings. Of course, a less drafty home will also be more comfortable, and comfort will most likely close the sale.
Big homes with complex ceiling planes, which have tricky details like double walls, tray ceilings, and mechanical chases, make finding and plugging air leaks way more labor-intensive and less reliable. If that’s not enough, the roof was probably built with little consideration for draft blocking, so there may be more gaps and holes than you expect around framing, trim, and siding.
Choosing a System
The best insulation for the job is spray foam, which insulates and air-seals in one shot. Whether to use a closed- or open-cell product depends on where you build.
In the northern U.S., use closed-cell. There, the drawback with an open-cell foam is that warm, moist air from the living space will diffuse through it and condense on the cold sheathing. Closed-cell foam, on the other hand, is impermeable enough to act as a vapor barrier.
Open-cell foams are best in the hot, humid Southeast. That’s because solar radiation can drive moist air through the roof system, and with closed-cell foam that moisture could then get trapped between the sheathing and the foam. With an open-cell foam, moist air can move through to the living space, where it will be taken care of by the AC system.
In the hot, dry Southwest either type will be acceptable, but Coble prefers open-cell foam there as well.
Prepping the Job
Before calling the insulator, the builder needs to block large holes and gaps in the attic. These include gable and ridge vents as well as between the top plate and the roof sheathing near the eaves. Most gaps can be blocked off with wood framing or panel stock. Spaces around chimneys will need a code-approved material such as fire-rated caulk or metal flashing.
The builder also will need to remove any kraft-faced batts in the attic floor. If there is a plastic vapor barrier between the drywall and joists, rip out as much of it as possible. This is crucial in humid climates so attic moisture dries to the inside.
If a single attic covers the home and garage, build a partition. Only the portion over the house should be insulated and tempered.
It’s important that the roof be in good shape before installing the foam. Replace shingles, if necessary, and repair damaged sheathing or trim. This is mostly a liability issue: If a leak begins after foaming, the builder will be blamed, even if he wasn’t actually responsible.
Duct sealing has to be part of every job; recent numbers show average duct leakage of 15% for new homes and 25% for existing homes. Even in a sealed attic, leaky ducts can unbalance the heating or cooling system, creating negative air pressures in the living spaces that pull air in from outside. If the home has an old, atmospherically vented furnace, negative pressures can cause it to backdraft carbon monoxide and other combustion by-products into the home.
Of course, the builder has less control over the quality of the insulation job. Common problems include voids, air bubbles and large variations in thickness. Inspect the job for these problems before writing the final check.