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Community and Q&A

Is some roof venting better than none?

zqsuGB5jcH | Posted in GBA Pro Help on

I read with interest the article on roof venting by Joseph Lstiburek in the Sept. 2011 issue.
I am halfway finished with a mountain home in southern Colorado, climate zone 6. I will
insulate soon. I had planned to vent as much of the roof as rasonably possible and have
already installed ridge vents for this purpose. The article made me wonder, though, if venting is a good idea in my case, because some areas of the roof cannot be vented very well, if at all. I have outrigger rafters, for example, at several gable ends that are extended cantilever style back into the interior of the house and would prevent air flow from the soffit to the eave in the first joist space at each gable end. I also have a few valleys that would be impossible to vent directely. If the rest of the roof is well vented, will these conditions create an imbalance and possibly drive moisture/condensation to the unvented areas at the gable ends and valleys? Is some venting better than none, or should I not vent the roof at all? I could easily do the Flash-Fill approach, using closed cell spray foam and either fiberglass or cellulose. My roof is asphalt shingles, and
there is a “ice shield” layer underneath the shingles. If I don’t vent, should I remove the ridge vents? Thanks in advance for responding. John Reeder

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  1. GBA Editor
    Martin Holladay | | #1

    OK, option B is flash-and-batt. What is option A? An insulated sloped roof with vent channels and what?

  2. zqsuGB5jcH | | #2

    Yes, option A would be a sloped roof with a ridge vent at the top and soffit vents along the bottom, plus a baffle system in the vented joist (TJI) spaces. There is no attic. It is basically "Cathedral" type ceilings. My concern is that some of the joist spaces will not have air flow, especially at the gable ends where there are lots of windows, due to the outrigger rafters that support a roof overhang. The total area I could vent would be about 70% of the whole roof. Thanks, John Reeder

  3. GBA Editor
    Martin Holladay | | #3

    To compare the two suggested approaches, we need to know what type of insulation you plan to install. That's why I asked -- "An insulated sloped roof with vent channels and what?"

    Fiberglass batts? Something else?

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