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Community and Q&A

Vapor retarder location

cosmosnomo33 | Posted in Green Building Techniques on

I’m looking for some options for properly protecting my cabin from water issues. It will have a metal roof attached to purlins which are on the rafters. The plan now is to insulate the rafter cativities with blown cellulose and possilby place a vapor retarder in front of that on the warm side. There will be ventilation between the insulation and the metal roof via an air gap and openings at the soffit and ridge vent. Do I need anything on the cold side or against the metal roof? Any better options for insulation? We are located in Maryland so its hot and humid summers with cold winters. It is a gable roof at 8/12 pitch. The attic area is a sleeping loft. We will burn wood for heat but no a/c. Also the walls are made of cob, which is clay, sand, and straw. The building is approx 150 square feet. Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    If you plan to to insulate your rafter cavities with blown-in cellulose, you need an air barrier on both sides of your cellulose (above and below) as well as a ventilation cavity between the upper air barrier and the metal roofing. Since it sounds like you aren't planning to install any roof sheathing, these layers can be tricky to achieve.

    The easiest way to do what you intend is to sheathe your roof with plywood or OSB, and to install your purlins above the roof sheathing. If you choose a vapor-permeable roofing underlayment like asphalt felt (not synthetic roofing underlayment), then the spaces between your purlins probably provide adequate ventilation -- even though they are oriented rake-to-rake instead of eave-to-ridge.

    In your climate, you shouldn't install interior polyethylene. Ordinary latex paint will provide an adequate vapor retarder on the interior of your roof assembly.

    For more information on these issues, see How to Build an Insulated Cathedral Ceiling.

  2. Expert Member
    Dana Dorsett | | #2

    A layer of Typar or Tyvek sandwiched between the rafters & purlins can a vapor-permeable exterior air-barrier for the cellulose. Without a rigid air barrier on the exterior you can't dense-pack the cellulose or it will pillow-out and contact the metal roofing, but low-density cellulose would be fine.

    If you are installing an OSB or plywood deck between the purlins & rafters you can dense pack against it, but it would be advisable to then use a "smart" vapor retarder on the interior, due to the lower drying rate through the wood sheathing. Relying only on a vent space above rather than below the sheathing is higher risk, since the sheathing it would directly exposed to the interior moisture drive through the insulation layer rather than the outdoor air convecting through in the channel, as in a standard vented roof stackup.

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