Winter is the season of icicles and ice dams, at least on houses whose roofs are poorly insulated and air-sealed.
Or, in the case of a nature center in Wisconsin, not insulated at all. Walt Ott is inquiring about the best way to fix a building with a completely uninsulated cathedral ceiling.
Writing in a Q&A post at Green Building Advisor, Ott outlines his strategy: on the underside of the existing 2×6 tongue-and-groove ceiling, add two layers of 2-inch-thick polyisocyanurate insulation, two layers of 3/4-inch-thick polyiso insulation, and then a layer of 1×6 tongue-and-groove boards as the finish ceiling.
There are details yet to be worked out, but is this plan fundamentally sound? That’s the question for this Q&A Spotlight.
And if this topic is ringing a bell as we approach the depths of winter, a second Q&A post deals with similar issues.
Nope, this just won’t fly
Ott’s approach has a major problem, writes Dana Dorsett. “You can’t use stacked-up polyiso against the roof deck,” he says, “since it would block the drying of the roof deck toward the interior.”
He can, however, use unfaced expanded polystyrene board insulation (EPS). Type II EPS, 5 1/2 inches thick, would act as a Class II vapor retarder with a permeance rating of about 0.54 perm, Dorsett writes. That’s about the same as the kraft paper used to face batt insulation.
And that, he adds, “would be sufficient for protecting the roof deck from excessive wintertime moisture accumulation, yet provide a path for seasonal drying during warm weather.”
Type II EPS would give Ott a roof R-value of at least 25, enough to make a “huge” difference.
When it comes time to re-roof, Ott could…