“Flash and batt” is an insulation technique that combines the air-sealing superiority of spray foam insulation with the cost benefits of fiberglass batts. An inch or two of polyurethane foam seals the cavity and the batt insulation adds R-value without costing an arm and a leg.
That’s roughly the plan Dave Frank is considering for the roof of a house — presumably his own house — in Climate Zone 5. But his plan contains a twist: He wants to spray the underside of the roof deck with foam and install the batts between the joists at ceiling level.
“My reasoning for this is to (a) get HVAC in the attic within the envelope where rafters can’t be properly vented, and (b) still have insulation on top of the flat ceiling drywall because there is some ceiling radiant heat,” he writes in a Q&A post at Green Building Advisor. “And there’s the added benefit of getting some of the R-value from a cheaper product.”
Frank wonders whether the code requirement for a total of R-38 in the roof will allow the insulation to be separated in this way, as long as the R-value totals for the different layers all add up to the right number.
Or is this a case where 2 plus 2 equals 3?
That’s the topic for this Q&A Spotlight.
The code is clear: Your idea won’t work
There’s not much wiggle room here, says GBA senior editor Martin Holladay.
He cites a provision in the International Residential Code (IRC), which says the air-permeable insulation (the fiberglass batts) must be installed “directly under” the air-impermeable layer (the spray foam).
“The air impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5…