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Roof ventilation

E_J | Posted in General Questions on

I am buying a house with vaulted ceilings and I’m in climate zone 5 near zone 6.  I am planning on installing at least 3″ of XPS over the existing drywall on the ceiling and then adding another layer of drywall over that.  The current roof is 2″ by 6″ and has fiberglass bats installed and has roof vents but the soffits are not ventilated well.  The roof will need to be replaced and the sheathing as well.  Do I need to worry about installing something like SmartBaffles or just leave as is and make sure the foam insulation I install is airtight?  Would it be better if it wasn’t vented at all?  If I install the smart baffles then I’m assuming I want to install them the whole length of the rafters and put in a ridge vent?  I’m worried about ice damming and whether the sheathing will retain moisture leading to rot or mold.  It does have one dormer for the front porch, otherwise is just a simple 3/12 roof.  Thanks in advance for any advice you can give, Erik.

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  1. GBA Editor
    Martin Holladay | | #1

    If you plan to replace the roof sheathing and the roofing, you have a golden opportunity to install the rigid foam where it really belongs -- on the exterior side of the new roof sheathing. This approach will result in much better performance than your approach (putting the rigid foam on the interior).

    For more information, see "How to Install Rigid Foam On Top of Roof Sheathing."

    If you ignore my advice, it would be best to install ventilation baffles from the soffits to the ridge. Ideally, you'll improve your soffit vents at the same time. For more information, see "Site-Built Ventilation Baffles for Roofs." (That article includes information on commercially available ventilation baffles as well as site-built baffles.)

    1. E_J | | #2

      Ok, so if I install the foam on the exterior under the shingles, I'm assuming I remove the roof vents and soffit vents since they would be introducing cold air to the roof under the foam? In other words I need to convert it to a non-vented roof? Also would 4" xps be sufficient for my climate zone? I'm in Iowa zone 5 but very close to zone 6. And from other posts I've seen I want to put some kind of air barrier under the foam or tape and seal the foam so it is air tight? Thanks.

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