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Can anyone help with info (esp. technical documentation) about whether vented (soffit-ridge) or unvented roof assemblies are more resistant to high wind load damage?

Yakmanfl | Posted in Energy Efficiency and Durability on

A customer of mine is being told by his builder that he can’t switch to roof deck spray foam and an unvented attic because that will change (reduce) the resistance to wind loads compared to the as-designed house with soffit and ridge vents. We are in a 130 mph wind zone.

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Replies

  1. Expert Member
    ARMANDO COBO | | #1

    There are too many issues that could be involved in your answer. It can depend on the shape of the house, roof and angle to prevailing winds; framed materials and what type of tiedowns and holdowns were used or if any; wind protection form trees, other houses nearby, etc, etc. Best thing you can do is hire a structural engineer to look at the house, its plans, its site, etc, etc.

  2. Yakmanfl | | #2

    Hi Dan,

    I appreciate your answer. As always answers to complex questions are "it depends". In this case I was thinking of an "all other things being equal" kind of assessment. For the record, it's a 6:12 pitch hip roof, standard truss construction, suburban siting, and new construction code tiedowns (whatever is required in FL outside of the hurricane zone). The nut of the question is does open soffiting increase the uplift stresses under high wind condiitons versus unvented closed soffit type unvented attics?

  3. Foamer | | #3

    Kevin,
    Ask the builder to provide documentation for his claim. I would be surprised if he has any.

  4. user-795783 | | #4

    I logged in to respond to your post as I was just reading something relevant on the Building Science website: The article says "In high wind regions – particularly in coastal areas, wind driven rain is a problem with vented roof assemblies. Additionally, during high wind events, vented soffit collapse leads to building pressurization and window blowout and roof loss due to increased uplift. Unvented roofs – principally due to the robustness of their soffit construction - outperform vented roofs during hurricanes – they are safer."

    Here's the link: http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation

    DC

  5. Yakmanfl | | #5

    Thanks everybody,

    This was a little unusual question, I'll admit. The quote above from DC is from Joe Lstiburek which I had found in several places. Additionally, a comment from one of our local U of Fl professors who teaches structural engineering and wind load classes was that there are no code provisions requiring different load calculations for vented versus unvented attics. It appears the builder was just stonewalling in this instance to avoid allowing an insulation type that was not part of their standard procedure.

    Thanks again to all for your feedback.

    Kevin Veach
    Green Energy Options

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