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Non-insulated/unconditioned garage – Is ventilation required?

halleym | Posted in General Questions on

Are non-insulated, enclosed garage attic spaces required to be ventilated? I’m not sure we meet the IBC’s requirements for unvented attic assemblies if no insulation at all is present.

The garages, located in climate zone 6, will be new construction and for aesthetic/pitch reasons will either be covered with asphalt shingles or TPO roofing (with no rigid insulation below).  No insulation will be installed on the roof sheathing, in the attic space, or in the garage walls. The garage bays are planned to have air sealed gyp. board ceilings installed on the underside of shallow roof trusses. Air sealed gyp. board will be installed on the interior walls also. A portion of the garage structures will be used for storage.Lap cement board siding over drainable weather barrier is intended for the exterior cladding over wood sheathing.

If venting is required, what would be the recommended ventilation method for a low slope roof covered in TPO membrane? 

Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Halley,
    As far as I know, there is no code requirement or building science reason to include soffit vents or a ridge vent for an uninsulated garage roof.

  2. Jon R | | #2

    I can think of a few scenarios where moisture might be an occasional issue.

    a) warm wet cars that, for some period, turn the garage into a high humidity heated space. Automatic powered ventilation (garage to/from outside) would help.

    b) night sky radiant cooling that causes the underside of the roof to be below the dew point. A little bit of rigid foam roof insulation would help.

    c) wet warm siding heated by the sun. Overhangs and vented rain screens help.

    In general, airtight enclosures plus some introduced moisture are a bad combination. Do use poly under the slab (and maybe epoxy on top).

  3. Expert Member
    Peter Yost | | #3

    Hi Halley -

    You mention low-slope roof; exactly what is your slope? I am sure you know this but have to say minimum pitch per the industry standard is 2:12. But I would say that depends on your climate and site; if your garage roof sees wind-driven rain, or has a lot of penetrations or has any complex geometry meeting other surfaces, I would bump minimum pitch for asphalt shingles to 4:12.

    If indeed you have a low-slope roof (less than 2:12), the industry standard is to NOT vent your "attic." But you aren't insulating or conditioning the space below so you can do pretty much whatever you want.

    I would at least PLAN for adding ventilation later, if it turns out that moisture is accumulating in the space; you will have either Class I (TPO membrane) or ClassII (asphalt shingles) vapor permeability in your roof cladding, so you will get precious little drying potential outwards in your roof assembly. When I say plan I mean louvered vents on opposite sides of your attic, with wiring in place in case you need to add mechanical ventilation, probably controlled by t-stat or humidistat.

    But I bet you won't need it; just worth planning for.

    Petr

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