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Community and Q&A

What steps to insulate vented attic?

HarmanRestorationLLC | Posted in Green Building Techniques on

Hey team,

I’m currently getting an hvac system installed into our 100 year old New Orleans home. I’m planning to air seal the attic, insulate the floor, then insulate the attic ceiling (roof). I’m wondering what I should do between the rafters? I was thinking putting up plastic sheeting to create a moisture barrier from the insulation, is there something else I should do, or just go ahead and fill the rafters cavities? I plan to the cover it with rigid foam afterwards. Removing the roof is not an option, and ideally would like to keep it diy.

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  1. GBA Editor
    Martin Holladay | | #1

    First, can you tell us your name?

    The work you are describing is called "converting a vented unconditioned attic into an unvented conditioned attic." Here are links to two articles that will tell you how you can insulate the sloped roof assembly:

    Creating a Conditioned Attic

    How to Build an Insulated Cathedral Ceiling

    Note that you can't install fluffy insulation between the rafters unless you either (a) include a ventilation gap between the top of the insulation and the underside of the roof sheathing, or (b) include an adequately thick layer of rigid foam above the roof sheathing.

    If you create an unvented conditioned attic, you usually don't want any insulation on your attic floor.

  2. HarmanRestorationLLC | | #2

    Thanks, I appreciate all you do here. These articles are thorough and easy to read.

    Based on those two articles I'm leaning towards creating an air channel, installing rigid foam, and air sealing before putting in some fluffy insulation, then foam over the bottom of the rafters. I'd like to keep all my equipment within the thermal envelop to the best of my ability.

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