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Condensation on framing in spray foam ceiling retrofit

jackzylkin | Posted in Green Building Techniques on

I have an old stone farmhouse in Allentown, PA, and it is my only wish that I can figure out how to make he roof even slightly more energy efficient. Last week I bit the bullet and ripped down all the drywall in the ceiling of my 3rd story finished attic space.  I discarded the stink-bug infested R13 batts that had been up there (unvented), and scheduled a closed-cell spray foam team to come turn my drafty attic bedroom into a sealed up hot roof.

The decision to use CCSF is the only way I could figure out to get a reasonable level of draft sealing and thermal insulation in my situation — the bays are slighly less than 5″ deep (The rafters are 3×5 at 32″ O.C.), and it is a simple gabled roof.  (The foam is low-GWP “Gen 4” foam, so I don’t feel so guilty about filling the bays with it.)

I have read up on unvented roof assemblies on this site and elsewhere. The sticking point for me is that the 3x5s are nasty thermal bridges.  Since they are hardwood, I assume ~R-1 per inch, and so I will have R-5 thermal bridges covering 10% of my otherwise toasty ceiling.  My plan is therefore to come in after the spray team and fur out each rafter with strips of 3/4 or 1″ unfaced polyiso to bring them up to R-10 or so before the drywall goes up.  I am using unfaced polyiso because it has a perm rating of about 4 (vapor retarder), so I can prevent a double vapor barrier while blocking moisture transit into the framing. Sort of like what they do in this article: https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge.

Here is my dilemma — won’t the framing get cold in the winter, after I insulate the face of each rafter?  Then, won’t moisture condense underneath the polyiso strips?  In my climate zone (4/5), the code specifies a max a 60/40 ratio between the insulation inside/outside of the air barrier in an unvented roof, and on paper my plan does meet that requirement, but it still seems iffy to me.  Would gluing/foaming around each of my furring strips provide some insurance against moist air getting to the framing?

And a related question, which I think might shed some light on my problem:  In a hypothetical “normal” flash-and-batt job on a modern 2×8 ceiling, with only a few inches of foam that is then buried in fiberglass, why wouldn’t the framing right up near the foam also get cold enough to cause condensation?  That part of the framing is closer to the outside than it is to the living space, the wood provides little insulation, and it is buried in fiberglass that keeps indoor heat away from it…   Seems like these jobs would get condensation problems too.

I realize over-roofing is something to consider, but I could not figure out how to make it add up $$ wise or how to resolve various other details it brings up, and it would not do anything for my draftiness problems.

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Replies

  1. Expert Member
    Akos | | #1

    Since your rafters are 32" OC, you'll have to strap it out to hang your drywall. What you can do is strap it out with 2x3 on edge which will give your 2" of space for the spray foam guy fully encapsulate your rafters with foam. With the 2" of SPF over the rafters, it will be a pretty decent bump in your whole roof R value and less work/cost than the foam strips.

  2. jackzylkin | | #2

    Yes we are planning to add strapping on the flat -- 3/4" - 1 " thickness of it. Owing to the low-ceiling shape of the room and some of the features in there (especially in the bathroom/closet areas) we decided not to add more than an inch or so of furring. I will reconsider that thickness though and talk it over with the spray people -- I had assumed their job would be a lot easier without yet having the strapping in place.

    The foam strips will be cheap and I don't mind the work -- just want to know if the current plan will have bad consequences when all is said and done.

    1. Jon_R | | #7

      > we decided not to add more than an inch or so of furring

      Is 1" furring adequate support for drywall?

  3. jackzylkin | | #3

    Likewise: I could just get them to spray over the rafters to a 1" depth, although for some reason code requires a greater depth than that for spray foam applications on roofs. (I don't have to meet any codes really but I figure they are a good guideline for not shooting myself in the foot).

    Polyiso is more open to vapor than CCSF, so I figured it would be better for drying the framing to the inside if any moisture issues did occur for whatever reason. On further research the spray is only about half as permeable as the polyiso, so maybe it doesn't make a difference.

  4. GBA Editor
    Brian Pontolilo | | #4

    Hey Jack.

    Am I missing something? Your title references flash-and-batt, but your post says that you plan to fill the rafter bays with spray foam. If you are going to fill the rafter bays with spray foam, I think Akos made a good suggestion, which will not only get you greater overall R-value and some thermal break at the rafters, but proper fastening for the drywall.

    You asked "Would gluing/foaming around each of my furring strips provide some insurance against moist air getting to the framing?" The answer is that diligent air sealing of the new drywall ceiling is the best thing you can do to keep moist air from getting to cold framing (or foam) where is can condense.

  5. jackzylkin | | #5

    Good point -- I edited the title. For the most part the cavity will be filled with closed cell, except for the 1" space I am furring out, which will be filled with something else -- looking for feedback on that point.

  6. jackzylkin | | #6

    After reading more of the discussions between Martin and Dana on this issue, I get the sense that 1" of foam on the inside of the rafter is OK as far as moisture goes. On the other hand, the two seem to disagree whether 4.5" inches of closed-cell foam are able to provide sufficient drying (for the foam I'm using it is only .33 perm at that depth).

    I tend to think that, unless I get a roof leak, I will be safe no matter what. And my roof appears to be leak-free for now. When it is time for a new roof, I'll consider putting in a ventilated nail base for some peace of mind and to kill the thermal bridges more effectively. At the very least, it would be nice if I had a roofing membrane under those shingles...

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