Ways to Create an Energy Efficient Envelope

Originally published by: Building Products MagazineSeptember 12, 2011

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It wasn't that long ago that insulation was insulation. Today, thanks in large part to keener competition for buyers, rising energy prices, and stiffer building codes, there are a bevy of products designed to create and improve an actual thermal envelope. And the choice for builders is not just which one to spec, but how to combine them to optimize their respective benefits and achieve higher levels of performance.

That relatively new concept also makes energy-efficient housing more affordable. Builders across all price points can now modify their insulation packages not only to the climate, local code requirements, and the clients' expectations for comfort and energy costs, but also to the construction budget.

No matter the materials a builder uses to achieve a performance/budget balance, there are two principles that any insulation method must address: Seal up air leaks and retard thermal transfer through the envelope. Here are some practical "best practice" guidelines to help you meet your energy performance goals on time and on budget.

Sidewalls: For wood-framed perimeter walls, building scientists and budget-conscious builders advocate a hybrid approach commonly called "flash-and-batt," which combines the best combination of air-sealing and insulation technologies to affordably address the thermal envelope.

In a hybrid application, each wood-framed wall cavity receives a 1-inch layer of closed-cell spray foam to seal the joints and block airflow between the studs and the exterior sheathing.

Once the foam cures (usually the same or next day), the less-expensive "batt" layer—which could be fiberglass, cellulose, cotton, or sheep's wool in a rolled form or various blown-in applications—fills the rest of the cavity to provide the bulk of the thermal resistance value.

One alternative to closed-cell spray foam is to use a water-based, one-part, spray-applied sealant that frames only the joints, rather than coating the entire sheathing surface at the back of the cavity. This approach can further reduce the cost (and mess) of the "flash" stage while purporting to match the air-sealing qualities of spray foam.

Builders in extremely cold climates who want to earn the Energy Star label may also need to apply rigid foam panels to the outside surface of the framed walls, either behind or integrated into the sidewall sheathing, to create a thermal envelope that mitigates the conductivity of the wall framing.

Attic/Roof: While most energy experts call for HVAC equipment to be located within conditioned space, the new housing economy is driving the trend toward smaller square footages. In turn, that's forcing builders to look above the top plate to run and hide ductwork and HVAC equipment.

Problem is, an unsealed and uninsulated attic space, especially in hot, humid climates, is inhospitable for equipment and ducting to operate efficiently and maintain desired indoor comfort levels.

Open-cell spray foam—a less expensive if slightly less thermally resistant sister of closed-cell foam—applied to the underside of the roof deck from the ridge down to the top plate effectively seals and insulates the attic.

The result is a "semi-conditioned" space that is perhaps only 5-10 degrees warmer than the living areas below, enabling HVAC equipment to operate as intended in an environment that helps rather than hinders energy savings.

But for builders in moderate climates and with the heating and cooling system stationed elsewhere in the footprint, "cathedralizing" may be an unnecessary expense.

Picture-framing the roof truss cavities with a spray-applied sealant to tighten the roof assembly and throwing down a deep bed of blown-in material in the ceiling cavities adequately retards thermal loss from the conditioned living space below, which helps reduce the home's energy use for heating and cooling even if the attic is to remain unoccupied.

Basements: For the roughly half of all new homes built on a basement or crawlspace, effective insulation and air-sealing application comes down to two choices and also depends somewhat on the end use of the space.

For a conditioned, full-height basement, builders can insulate either side of the perimeter walls with rigid foam insulation panels and spray foam or elastic sealant applied to the joints and the band joist area.

Builders can also insulate the concrete slab, either prior to the pour or on top of it to create what's called a "floating" floor of a vapor retarder, R-10 foam panels (taped at all joints), and a T&G wood subfloor ready for the owner's choice of finish. "If you can plan and prep for it, insulating and sealing from the outside is faster and easier," says Gary Parsons, a fellow in Dow Building Solutions Research and Development.

For occasional-use basements and crawls, filling the floor joists with a batt, blow-in-membrane, or flash-and-batt application and air-sealing the band joist offer more affordable yet very effective thermal values to seal and enclose these areas away from the conditioned living spaces above and perhaps create a semi-conditioned area for duct runs.

Regardless of where and how a builder seals and insulates, Steve Easley, a residential building science consultant and educator in Danville, Calif., advocates adding two in-house inspections—one just before the drywall goes up and another before the exterior cladding is applied—to make sure the building envelope is thoroughly sealed against air and moisture infiltration. "A building inspector may not look that closely," he says. "But a builder should."

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