Most of the houses that Atlanta architect Scott West designs are contemporary, and they typically come with flat roofs. Construction often consists of 12-in. deep I-joists or open-web 2×4 trusses capped with oriented strand board (OSB) sheathing. Roofs are unvented, and the use of recessed can lights is probably unavoidable.
The most common options for insulating these roofs, West writes in a post at GreenBuildingAdvisor’s Q&A forum, would include:
- Over the roof sheathing, add an additional layer of OSB installed on tapered 2x furring strips to slope the surface of the roof to drains, and then insulate with R-30 fiberglass batts between the rafters. “Not particularly smart or green in any way,” he says, “and requires extensive air-sealing, but as with most things, the most-common way is most affordable.”
- Place 4 in. of XPS rigid insulation on top of the deck and 4 in. of closed-cell foam on the underside of the deck for a total R-value of about 44.
- Place 4 in. of XPS rigid insulation on top of the decking, 2 in. of closed cell foam under the deck as an air barrier and then add R-30 batts for a total R-value of about 62.
- Skip the XPS above the deck and use sloped decking as in the first option, then use 2 in. of closed-cell foam and fill out the remaining 10 in. of the roof cavity with dense-packed cellulose for a total R-value of about 47.
Among West’s questions are:
“Any help in assessing these options and especially in comparing real-world costs wold be greatly appreciated,” West writes. That’s the topic for this Q&A Spotlight.
First, lose the can lights