Recently a new member,”Axiscat” (we’ll describe him as “Axis”)posted a concern on the GBA forum that worried spray foam. A skilled HVAC contractor operating in climate zone 4, Axis depends on ACCA Manuals J and S to calculate heating and cooling loads and to effectively size devices. The computations are normally based on the ACCA (Cooling Specialists of America) Handbooks J and S, which in turn consider the size and area of the structure, the number of windows, the air infiltration, and the R-value of the insulation. (We featured a discussion of the Handbooks here a few weeks back.)
Axis writes that he’s having an issue fixing up the minimum code requirements for insulation with the claims of the insulation contractors. “Recently I have been encountering insulation professionals spraying about 6 in. of open-cell foam (0.5 lb/ft3) at the roof deck to develop an encapsulated attic area. Being in climate zone 4, I thought the minimum attic insulation is R-38. What I don’t understand is how the building code enforcement individuals enable a R-22 (R-3.8 × 6-in.) insulation value in an attic? The response the insulation individuals give me is that it is comparable to a R-such-and-such of fiberglass.”
Axis composes that he’s aware that an R-value is a measurement that is independent of the materials it explains. And he knows that spray foam will reduce the quantity of air seepage, however that’s an element that’s computed separately. As he puts it, “Thermal conductivity of foam × location + seepage = Thermal conductivity of fiberglass × area + seepage.” He goes on: “Subtract seepage from both sides of the equation and it is obvious that a R-22 foam assembly has more loss than a R-38 assembly. Not to mention the overall surface area when lathering …
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