Spray-foam has a reputation of providing superior insulating qualities compared to fiberglass batt, mineral wool, cellulose, and other insulation alternatives.
The truth is that spray-foam does not necessarily perform better than other types of insulation when it comes to conductive heat transfer. Think about the insulation products you’ve seen at your local home-improvement store. Remember that big number on the label that started with an ‘R’ (the roll of batt insulation might have read R-13, for instance)? That number identifies the thermal resistance value - or R-value - and it is used to communicate the rate of heat flow through a material. The greater the R-value, the smaller the rate of heat flow (but beware, this is not a linear relationship).
Inch-per-inch, the R-value of spray-foam insulation is not much different than some other common alternative insulation options. Spray-foam helps a structure perform more efficiently because it reduces another form of heat gain/loss—air leakage.
Figures vary, but it is not an overstatement to suggest that infiltration accounts for up to 40 percent of a home's total heat loss. Spray-foam expands to fill in cavities and crevices in a stick-framed structure—clinging to studs and reducing air leakage due to its “air tightness” versus alternatives like blown cellulose and fiberglass batt insulation, which can also settle over time. This is the primary differentiator in thermal performance when it comes to spray-foam.
So, does this distinction leave spray-foam alternatives in the cold? Not at all. But you have to adjust your thinking. Instead of considering the thermal resistance and air leakage in terms of a single dedicated product in a wall or roof, think about the entire assembly as a complete system. Air barriers, foam sealants, and good construction practices can help structures that use spray-foam alternatives to control air leakage comparably well. In such cases, a combination of tapes, gaskets, foams, caulks, and mastics will be necessary to bridge the gaps and cracks in those materials to make the assemblies airtight.
Recall the adage, “build tight and ventilate right.” As our energy-conscious building design and construction industry adopts practices that drastically reduce air leakage, do not lose sight of the fact that homes need to account for some outdoor air exchanges for healthy indoor air quality. This can be accomplished in many different ways.
However, it is increasingly common for air-tight structures to utilize high-efficiency heat recovery ventilation units.