Miles TeSelle has reached a crossroads with his new house, now under construction in climate zone 5B: should the insulation in the vaulted ceiling have a facing of kraft paper as a vapor control layer?
Detailing a cathedral ceiling is a familiar topic for GBA readers. It’s an easy assembly to screw up, so it’s no wonder that TeSelle is playing this cautiously. Although he’s read what has become GBA’s go-to article on the topic, he still has questions.
The shed roof is framed with I-joists 11-7/8 in. deep. TeSelle is planning to insulate the rafter bays with R-38 fiberglass batts, leaving an airspace of 1-5/8 in. above the insulation for ventilation. He has bored holes in the webs of I-joist blocking to facilitate air flow, and he’s using rigid foam baffles at the top and bottom of each rafter bay, as the drawing below shows.
“The R-38C is below the code minimum,” TeSelle writes in this recent Q&A Spotlight, “but we have approval from the building inspector. We have insulated the walls with kraft-faced R-21.
“We do want to use the kraft-faced R-38C,” he continues, “right? To provide a vapor-retarder layer. The ceiling will have drywall with latex primer and paint.”
Has TeSelle put his finger on the issue, or is there something else going on here? That’s the starting point for this Q&A Spotlight.
The roof will be under-insulated
The International Residential Code calls for R-49 insulation in the roof for houses in climate zones 4 and up, which is about 29% more than TeSelle’s plan calls for. He explains that there’s only enough room for an R-38 batt, assuming he is going to maintain the 1-1/2-in. vent channel above it.
Although it would be…