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.
When combined with advanced or optimized framing practices, which reduce the amount of structural lumber while creating wider and deeper cavities for insulation, this “hybrid” approach can also help achieve a Grade 1 thermal envelope that the next level of the federal Energy Star Qualified Homes program standards will require. “The ideal is to insulate without gaps, voids, or compression,” says Sam Rashkin, the program’s national director.
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.
The value and popularity of a hybrid method has spawned alternatives to spray foam that address the shortcomings of that product’s formulation and application.
Specifically, water-based, one-part, spray-applied sealant frames only the joints rather than coating the entire sheathing surface at the back of the cavity to further reduce the cost (and mess) of the “flash” stage while purporting to match the air-sealing qualities of spray foam.
A hybrid approach alone may meet the new minimum insulation values specified in the 2012 Energy Star standards, as established in the 2009 International Energy Conservation Code.
But 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. “It takes it to a mature building science level,” says Rashkin. “No exposed framing [to the outside] is ideal.”
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.
The application also purports to help resist truss uplift, a key performance benefit that other insulating products can’t match. “It also creates the potential to expand living space without enlarging the home’s footprint,” says Bohdan Boyko, building science manager at GreenFiber, a recycled-newspaper cellulose insulation manufacturer in Charlotte, N.C. “It’s not just a climate-specific benefit.”
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.
“People are always looking for a numeric return on investment,” says Gary Parsons, a fellow in Dow Building Solutions Research and Development for the Midland, Mich., insulation manufacturer. “Now it’s case by case, and a builder can model different scenarios to better estimate those results.”