Finding effective ways of beefing up under-insulated roofs is a perennial problem, affecting countless houses built to minimum energy standards.
Even roofs that meet minimum code requirements are susceptible to thermal bridging when only the rafter bays are insulated.
So imagine starting with a New Hampshire beach house whose cathedral ceilings contain not a speck of insulation. “There is no insulation except for 1/2-in. Homosote sheets between the trusses,” Michael Buckley writes in this Q&A post at GreenBuildingAdvisor. “The roofing is relatively new (5 yrs). So I would balk at tearing it off.”
Although Buckley posted his question last September, it continues to attract comments, as well as questions from other homeowners facing similar problems.
Buckley’s budget is limited. He has no objections to putting in a new ceiling, but he wants a cost-effective way of insulating the 5 1/2-in.-deep bays between the framing. He intends to do the work himself.
As GBA senior editor Martin Holladay points out, Buckley is in Climate Zone 5, where code requires a minimum of R-38 ceiling insulation. “That amounts to about 7 1/2 in. of XPS foam or about 6 in. of polyiso foam,” Holladay writes. “Or you could use about 12 inches of fiberglass batts.”
What’s the best answer? That’s the subject of this week’s Q&A Spotlight.
One plan that won’t work
One option Buckley has been considering is a combination of polyiso foam board and a product called Prodex Total 48, which consists of two layers of reflective aluminum on a 5 mm (3/16 inch) layer of closed-cell foam. Retailers handling Prodex claim the material has an R-value of 15.67, prevents 97% of potential radiant heat transfer, and even acts as a vapor barrier.
Don’t believe it, both Holladay and…