Sealing a vented roof
In “Home Remedies for Energy Nosebleeds” (FHB #190), Bruce Harley states that if a house’s air handler is in the attic, the best thing to do is to include the attic in the home’s envelope by using spray insulation on the whole roof and gable ends. My roof was reshingled last year, and the roofer added several more feet of ridge vent and two turbines to ventilate the attic space. To seal the attic in the home’s envelope, would I need to cover or remove those venting devices? Also, what about the vented soffits?
Ben Abrams, Spring Hill, FL
Bruce Harley replies: It’s true that the venting devices conflict with the unvented roof assembly. Whether you want to remove them (and repair the roof) or just seal them with foam from the interior is simply a preference. Removing them will incur extra cost but will look better and will probably reduce the long-term potential for water leaks a bit. If you seal them, be sure that the foam contractor blocks them off from inside the attic before spraying the foam so that foam doesn’t leak to the exterior.
The soffits also should be blocked from the interior before spraying, although the need to do that will depend on accessibility and geometry; it can be difficult to access the eave area under a low-slope roof, especially if ducts are in the way. Talk with contractors about how they will handle this before making a commitment. Also, don’t forget that if you have a gas furnace in the attic, you’ll need to account for combustion-air requirements in the building code.
Before you go this route, you also might want to get a quote for thoroughly sealing the attic ductwork and the air handler with mastic, detailing or fixing any duct insulation, sealing any building air leaks between the attic and the house, and detailing/supplementing attic insulation; it might give you significant savings at a much lower cost.
I would suggest getting an assessment from an experienced third party, such as a home-energy rater (see www.resnet.us/consumer for a rater directory). They can measure the duct leakage to see how bad your system is and compare the savings that you might get from sealing the system versus insulating the roof; they also might be able to make referrals to contractors who could do the duct-sealing work.