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. “It’s your first line of defense against what amounts to about half of a home’s energy consumption,” says Hamilton.
Though thermal resistance (insulation) came first, air sealing has become the mantra of modern home energy-saving science, and expanding polyurethane spray foam—which addresses both, but air sealing foremost—has filled the void literally and figuratively.
“It’s like putting a jacket over a sweater,” says Steve Easley, a residential building science consultant and educator in Danville, Calif. “Air sealing increases the effectiveness of the insulation because there’s no air flowing through it.”
Before that, energy-savvy builders struggled to stuff cut sections of fiberglass batts or wedge caulking guns into framing joints to cut off air flow; now, whether from an off-the-shelf aerosol can for spot-sealing around window frames or electrical boxes or by a specialist on a full-blown job, spray foam more easily and reliably tightens the structure.
But despite spray foam’s recent popularity in housing construction (its market share of exterior wall insulation jumped from 1% to 8% between 2004 and 2009, according to the NAHB Research Center’s latest Builder Practices Survey), it is not the end-all answer to the optimized insulation question.
Not only is spray-foaming an entire house expensive—perhaps twice that of a blown-in fiberglass application—it’s also messy, delays other trades, and contains a measure of petroleum in its formulation.
And as an insulator, closed-cell foam delivers an R-6 per-inch resistance (about half that for less-dense open-cell), while properly applied fiberglass can match or better that value, if not nearly the air-sealing capacity of foam, at half the price.
Not that batts or any other insulation option aren’t without caveats; in fact, while all carry R-value (thermal resistance) ratings out of the factory, that value can quickly diminish in application. “Insulation is only as good as the installer,” says Sean Desmond, director of sales and marketing for Bonded Logic in Chandler, Ariz., which recycles blue jeans to make high-density cotton batt and loose-fill insulation. “Sometimes the choice of an insulation material is who can install it to achieve the best result.”