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Q&A Spotlight

Adding Insulation to a 1944 Roof

If it’s time to replace your roofing, it may also be time to upgrade your insulation

Image Credit: Mary Clark

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…

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5 Comments

  1. Buildingwell .org | | #1

    Limitations and options
    Replacement and renovation of existing structures certainly comes with a number of limitations as well as options. This article points out that when updating a roof construction for better insulation, you can be restricted to what you can do based on the original construction and the costs. In the end though, there are a number of options available to increase the efficiency of an existing building that these restrictions are be moot. For more information on roof renovation options, check this page.

  2. Shane Claflin | | #2

    whole house fan
    I thought whole house fans were big no-nos. You would need a thorough insulating schedule during the winter to make them air-tight. The problem of radiant heat is during the day, and not at night anyway.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Shane Claflin
    Shane,
    If nighttime temperatures drop to comfortable levels, whole-house fans are an excellent way to provide cooling with much lower energy costs than air conditioning. Be sure to choose a high-quality fan; I suggest the Tamarack model HV1000, with R-22 insulated motorized shutters, 1000 cfm, about $428.

  4. Bob Coleman | | #4

    humidity
    the other issue with whole house fans is for those in high humidity areas, and/or areas where there is not a lot of days of high heat, cool nights

    in my area, there are not a lot days one would spend all day paying to cool and dehumidify the house, and then open the windows at night even if it was cool because you'd lose the humidification and have to pay to get rid of it again

    and then laziness comes in to play.... you physically have to operate the thing, and possibly weather-rise it in areas with varying climates during the winter

  5. Thaddeus Cox | | #5

    wow
    Had no idea I was the subject of an article until I happened to Google myself... :)

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