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Insulation and Vapor Membrane for a Conditioned Attic

alexdorf | Posted in General Questions on

I’m in the process of plans on a 900 sqr foot ADU (small house) in Southern California. The roof is going to be a simple shed roof. The only equipment in the attic is going to be a ducted mini-split air handler. The space between the ceiling joists and the rafters the top end of the roof is just under 5′.

The plan is make the attic part of the conditioned space. We’d like to use fiberglass batts or Rockwool for the insulation. The rafters will be at least 10″. I suppose I could leave a channel behind them and the roof?

Also, considering adding a strip of vapor permeable membrane near the high side of the roof to cover a 4″gap across the width of the roof in the sheathing.

Looking for any insight from you great folks. Also is it better to go with the Rockwool?


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  1. Expert Member


    If you are conditioning the attic and moving the roof insulation to the rafters, you are building a cathedral ceiling. This article describes the various alternatives:

    1. alexdorf | | #2

      Do you think I'm better off putting the insulation on the ceiling joist vs rafters? Wondering if there's a preferred method.

  2. Jon_R | | #3

    Some info if you want to do buried ducts:

    Some info on roof construction (also check your local code):

    It's worth doing the "maintain .... roof sheathing above 45F" calculation to see if you can avoid foam.

    1. alexdorf | | #4


  3. Expert Member
    Akos | | #5

    In your climate you only need R5 exterior rigid insulation above the deck for an unveted roof the rest underneath can be any type of fluffy insulation. This is probably the highest performance most robust roof assembly, although performance might not matter all the much in your mild climate.

    You are also in warm enough climate that you can do an unveted roof with fluffy insulation only with a vapor diffusion port but there is slope limits. Provided your roof is over 3:12, this can also work. Check details here:

    In either case, you want the insulation in the rafters and the air handler and ducts inside conditioned space.

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