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Community and Q&A

A-frame with 2×6 construction

James_Hill | Posted in General Questions on

I have a A-frame with 2×6 construction.
Currently have 50 year aluminum shingle roof with only fiberglass insolation. 

There is felt-like material between shingles and sheeting. The inside is tongue and grove. No real issues heating cooling. 

We have no visible signs of problems, but we have a rot smell in the summer.
We live in climate zone 5. 

We are planning on a roof replacement, but we are having difficulty finding experience with unvented roofs.

 Since the space for insulation is only 6 inches, I was thinking the best would be to use structural insulated panels for sheeting and fiberglass under that. Keep it unvented. 

Insulation company strongly recommended creating a true cold roof with 2in gap. But that adds 20-30k. So looking for possible alternatives.

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  1. GBA Editor
    MIKE GUERTIN | | #1

    I upgraded my parent's A frame in the early 80s by placing 2x12s over the existing asphalt shingle roof and installing fiberglass batts with what we'd now call a vapor diffusion port at the ridge. I reroofed the house a couple years ago and the insulation, over-framing and sheathing are still in great condition. So rather than going with SIP panels you could do the same as we did. The house is in zone 5

  2. Expert Member
    BILL WICHERS | | #2

    The felt like material is "roofing felt", and is commonly installed over the sheathing under the shingles.

    You have an unvented roof with an air permeable interior finished ceiling (tongue and groove), and vapor permeable insulation in the roof assembly (fiberglass batts). This is something of a worst case scenario, and you'll find lots of articles about this that are dealing with "unvented cathedral ceilings", which is pretty much the same type of roof assembly that you have in your A frame. You probably have serious mold problems inside that roof assembly.

    My guess is that to fix the rot smell, you're going to have a lot of work to do, which likely includes replacing most, if not all, of the fiberglass batts, probably about the same amount of sheathing, and possibly a number of rotten rafters too. I like the idea of furring up the exterior to add additional insulation and create a vent channel. A vent channel would solve a lot of your problems here, but you're likely looking at a major reconstruction project to fix things.

    BTW, I very strongly recommend that at a minimum you open some of the roof assembly to check for signs of rot in structural remembers AS A SAFETY PRECAUTION. Damage is likely to be worse in the upper part of the roof. If you find rotting rafters, you have a serious structural problem that you need to address.


    1. James_Hill | | #3

      Yeah, I have quotes that include replacing all the sheeting, along with insulation quotes to remove and replace. I’ll be doing that at the very least.

      Couple questions I’m struggling with: although the current design has known flaws it has lasted 50 years with essentially no issues til now, probably due how steep it is and being metal. For 45k I can replicate a bad design but use a vapor barrier in place of the felt. (Wondering if that vapor barrier could actually increase the chance of rot from condensation, since just felt is allowing sheeting to breathe from both sides.)

      Or go with new/better design that goes above 100k.

      1. Expert Member
        BILL WICHERS | | #5

        A vapor barrier on the exterior (the "cold side") may actually make things worse. You would need it on the interior, and you'd want a vapor RETARDER, not a vapor BARRIER.

        If you're already planning to take off the exterior sheathing, then you can fix it right. Your issue is going to be with that interior T+G that is probably not air sealed at all. Maybe someone else on GBA can suggest a good way to air seal T+G from above? Maybe a fluid applied coating of some sort? Once that's figured out, you can insulate with batts, and then fur out those 2x6s so that you can fit enough insulation AND leave a VENT CHANNEL under the new sheathing. Venting this assembly is probably your easiest/cheapest way to go.

        If you can't vent for some reason, you could use exterior rigid foam instead and build up around 50% (or more) of your R value from that, which will also mean you don't need to worry about moisture drive from the interior, so air sealing that T+G becomes less of an issue, as long as you air seal the exterior (which is relatively easy with rigid foam). If you assume R23 batts in those 2x6 rafter bays, you could use 4" of polyiso on the exterior for a total assembly of R49, R26 worth of that would be from the exterior side polyiso in this case.


        1. Expert Member
          Akos | | #6

          I think the solution that Bill proposes is the proper way to go.

          My guess your roof lasted because it was covered in felt which is permeable and most metal shingles have enough gaps to allow for drying. This has kept most of the rot at bay but I would not be surprised that you'll see a mess near the top half of the roof.

          Despite the fact that it has lasted a while, the roof you have is not robust in cold climate. The lack of a proper air barrier and vapor control are the two biggest issues. If you are replacing the metal roof, the exterior insulation is the way to go. You can go with nailbase panels as well, these are similar to SIP but only have sheathing on one side, which are a bit simpler to install than rigid plus new sheathing.

          When the roof is stripped you want to detail the existing roof deck as the main air barrier as doing anything with the T&G is futile. Since the roof will have many many holes in from the shingles, the best would be a full cover of peel and stick but a synthetic underlayment with the seams taped would also work. The rigid/nailbase/SIP can than be installed over this. The side benefit of this is that it gets your place water tight even before the new insulation and roofing goes on. Make sure to air seal near the base of the roof, you want to carry the air barrier all the way down to the foundation. This is usually best done with spray foam into the soffit area from bellow but can also be done from the top if a strip of sheathing is removed.

          The good thing out of all this work, at the end of the day, you'll end up with passive house levels of insulation over most of your exterior surface, I would not be surprised if it drops your heating costs by more than 50%.

          P.S. If you want to leave it as is, replacing the felt with a vapor barrier is a very bad idea. The best would be to cover the whole roof with a permeable peel and stick (ie Slopeshiled PLUS SA or Henry VP100) and new metal roof. A largish diffusion vent at the ridge would also be a good idea.

          1. James_Hill | | #7

            Thank you for your responses, they have been very helpful.

            I am leaning towards the following:
            - Remove old sheeting and old insulation.
            - New sheeting and Blown in loose cellulose.
            - full cover of peel and stick but a synthetic underlayment?
            - 4" of polyiso on the exterior
            - Another layer of sheeting
            - Aluminum shingle
            Based on online calculators this would obtain an total R value between 45-48. Little below code, but my climate zone is 5b, so not too extreme and WAY better than current.

            Any pitfalls installing such thick foam? or better to do 2 layers of 2" stacked? Worth it to use nailbase instead?

          2. Expert Member
            Akos | | #9


            I think that is a solid plan and you'll have an excellent robust assembly at the end.

            2 staggered layers of foam is always more efficient, so if the cost is the same, I would go for it. If 4" is much cheaper, I wouldn't sweat it, your roof assembly is already high R value, the bit you loose is not too much. Nailbase is worth it if cheaper to install.

            You will still need an underlayment under the shingles. I preffer to use something permeable for this, felt can work but there are also a number of permeable synthetic underlayments you can get. This final layer doesn't need to be self adhered.

            When re-insulating the original rafter space, make sure to air seal above your wall top plates (both at the eaves and at the rake ends). This is really best done with spray foam but you can do a reasonable DIY job with cut pieces of rigid foam sealed in placed with canned foam. Getting a solid air seal is much more important for efficiency than staggering the exterior rigid, so this is the detail you want to get right.

  3. Expert Member

    Just wanted to chime in and say that you've got a very cool looking house!

    1. James_Hill | | #8

      thank you! Cool/unique looking certainly has given some unique issues, but been great overall.

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