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Smart vapor retarders in unvented attic assemblies with cathedral ceiling – Zone 4A

Jason Huffine | Posted in General Questions on

First of all, I’m in Tennessee, climate 4A, in a mixed-humid climate.  The house we are constructing has a complex roof structure, with trusses, and two cathedral ceilings.  Because of this it looks like I may need to consider an unvented assembly as identified in a previous Q&A post.  It really seems that in order to do this, foam insulation, whether sprayed inside or on the outside, or both, is always involved.  In my climate, exterior foam is not really heard of and though I know it’s got plenty of years of use, I’m still wary of spray foam.  Not to mention the cost of it.  Then I ran into this article:

https://foursevenfive.com/blog/insulating-unvented-roof-assemblies-foam-free/

A foam free approach using smart vapor retarders!  I had never heard of such an animal.  I know dense pack insulation has been deemed a big no-no.  But I also noticed articles (whether from Building Science or GBA) were written in the 2010/2011 timeframe and I’m not sure when smart vapor retarders came out, before or after.  So I thought it would be asking the experts in this community.  Is this truly feasible?  Or is this really another sales pitch for something that will be troublesome down the road?

Also, if it is feasible, does it make sense in my climate?

I’m at work, so I don’t have them handy, but I’ve read the articles on the different ways to insulate cathedral ceilings in an unvented assembly.  I’d still like to ask that if this is not feasible, and you were in my shoes (and climate), what have you seen or recommend as the most physically and cost effective solution?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jason,
    Here is a link to a GBA article that thoroughly discusses the questions you raise: "Smart Vapor Retarders for Walls and Roofs."

    1. Jason Huffine | | #2

      Thanks for the link Martin. I've missed that somehow. That was a good article and spot-on. Looks like a no-no. Back to exterior foam for me I guess. Let me ask you though. When the article approaches 475's claims on an unvented roof assembly, the statement was made "Building codes require a ventilated air space between the top of air-permeable insulation materials and the underside of the roof sheathing for a reason: without the ventilated air space, moisture from warm interior air can accumulate on the cold roof sheathing during the winter." I was thinking the ventilated space requirement was for vented assemblies and that with the more approved approaches for unvented, it was not necessary. Is that not true? Is it required for both? Placement of this text was funny since this section was addressing their claim and I just want to cover my bases and make sure I'm not assuming.

      1. GBA Editor
        Martin Holladay | | #4

        Jason,
        You're right that vented insulated roof assemblies need a vent channel, while unvented insulated roof assemblies don't need a vent channel.

        That said, your question noted that you want a "foam-free approach." There is no such thing as a foam-free approach if you want an unvented roof assembly -- unless you install semi-rigid mineral wool insulation on the exterior side of your roof sheathing (an approach that is very rare in the U.S. and Canada).

    2. Deleted | | #3

      Deleted

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