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Community and Q&A

To ventaliate a roof or not?

Bruce McMahon | Posted in Green Building Techniques on

I apologize for asking this since it seems like there is more on the web about this topic than one could possibly read. However, you might understand my confusion since there seems to be two schools of thought and each is convinced of their position.

I live in Ontario, Canada, zone 4 if that helps. My home is one hundred years old. It is balloon frame construction with brick veneer. The attic was renovated in the 1960’s but very poorly done. When I replaced the roof five years ago the insulation or lack of was made very apparent.

The soffits are now vented and the roof ridge is vented as well. As it stands the attic appears to have some mold behind the dry wall and no ventilation between soffit and ridge.

It is very hot in the summer and cold in the winter. Ice damming is a problem but not one that has brought water into the house.

My hope is to gut the attic this summer and insulate properly. It will be a bedroom when I am done.

The question is really about spray foam insulation directly against the roof boards. When the roof was changed they had to lay 1/2 inch plywood over the 1 inch pine boards in order to secure the 30 year shingles (in our climate that usually means 15 years!) Spray foam seems to be the best choice with old structures since the space between the rafters changes from section to section and within individual sections. The advice I have found is literally split down the middle. Half say you must vent behind insulation in order to keep your shingles intact and the other half say that heat transfer is prevented by sufficient spray foam so it is not a problem.

Help! My wife thinks I am all over this one but in reality I am confused about the whole project.
Thanks for any help you might be able to offer. Cheers

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Replies

  1. Torsten Hansen | | #1

    Welcome to the club Bruce, you are not alone.

    In the interest of full disclosure, I am a spray foam contractor, which may or may not influence my thinking. That said, our industry has installed thousands and thousands of non-vented roof systems with good results. If there was any evidence that this caused shingle damage, we as a community would have stopped doing so a long time ago.

    But, if you are gutting your attic you have a unique opportunity to have your cake and eat it too. Why not vent the roof and then spray foam? You have the soffit and ridge vents in already and it sounds like you are up to the task of putting in the necessary chutes. Use foam board or thin plywood ripped to width.

    Good luck with your project.

  2. GBA Editor
    Martin Holladay | | #2

    Bruce,
    1. Don't expect roof ventilation to change the temperature of your shingles significantly or lengthen the life of your shingles.

    2. Some shingle manufacturers will not honor the shingle warranty when the shingles are installed on an unvented roof. Others (including Elk and CertainTeed) will honor their shingle warranties on unvented roofs.

    3. If you want to keep your shingles cool, shingle color matters more than ventilation under the sheathing. White shingles stay cooler than black shingles.

    4. Unvented roofs and vented roofs both work well if the details are done right.

    5. One disadvantage of applying spray foam directly to the underside of the roof sheathing: future sheathing repairs are much more difficult. For this reason alone, many homeowners and builders prefer to install ventilation chutes and to spray the foam to the underside of the ventilation chutes.

  3. Bruce McMahon | | #3

    Thank you very much Martin. Your thoughts and ideas are excellent. Do you support the use of Soya based spray foam? Is it as effective as the poly version? Cheers

  4. GBA Editor
    Martin Holladay | | #4

    Bruce,
    Q. "Do you support the use of Soya based spray foam?"

    A. That's a hard question to answer. I support some political candidates, but when it comes to insulation materials, I prefer to answer technical questions rather than to indicate support.

    Q. "Is it as effective as the poly version?"

    A. Yes. To compare the thermal performance of different types of spray foams, look at the R-value per inch. Remember, spray foams that contain a little bit of soybean oil are not natural products. The bulk of the materials in these products still consist of petrochemicals.

  5. Bruce McMahon | | #5

    Sorry for the political jargon. We are in the middle of an election campaign in Canada. It must be on my brain. Good advice on the Soybean oil. That is not how it is advertised up here. Cheers

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