Thaddeus Cox’s 1944 Cape Cod in Portland, Oregon, has a roof that needs some attention. Not only is the roof under-insulated, but it’s currently covered in two or three layers of asphalt shingles installed over the original layer of cedar. Roof sheathing consists of 1-in. thick boards.
The International Energy Conservation Code recommends R-38 for the roof in this Climate Zone 4 house, far more than the R-11 batts Cox thinks are currently in place.
But reducing winter heating bills isn’t Cox’s first concern, as he explains in this Q&A post at GreenBuildingAdvisor.
“They are already pretty low with our ancient oil furnace, and should be lower after our gas conversion,” Cox writes. “What I’m most interested in is reducing the summer temps in the attic bedroom, presumably by reducing the heat transfer from the sun-heated roof.”
His plan is to tear off the existing roof down to the 2×4 rafters and then rebuild. The question is how.
“Given the extremely limited depth of the rafter bays, I could get a few more R’s using spray foam or rigid foam, but I’m not sure that it would be worth the added expense,” he says. “One roofer has suggested adding pre-fab channels, edge vents and a ridge vent in order to ventilate the roof, but this just cuts the space available for insulation even more.”
Cox’s roof project is the subject of this week’s Q&A Spotlight.
Beef up the insulation
Increasing the R-value of the roof is an obvious place to start, if only to reduce summer heat gain in the second-floor bedroom. He has more than one option.
Even if Cox uses high-density foam, at roughly…