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Polyiso over plywood – low-slope roof

mn3gjGJ4Us | Posted in General Questions on

I’m adding a 16×28 1-story addition to my 2-story house. Roof pitch is 1/4″/ 1-foot. It will be used occasionally as a deck.

I’m putting 3.5″ poly-iso on top of 5/8 plywood; GAF Liberty self-stick roofing on that.

What’s the best way to insulate between the joists?I see much discussion here of problems caused by multiple vapor barriers.

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  1. gtmtx | | #1

    Let me preface my comments by saying you did not post your location, so some issues may not apply.

    Let's start with your roofing membrane. It is a vapor retarder (if not a barrier depending the perm rating) which is on the outside. As such, any drying is normally to the interior, so I highly recommend against adding a 2nd vapor retarder (6 mil poly or similar) to your system. At 3.5" of polyiso, you are around R20 +/-, so you need to achieve at total code minimum of R30 or more depending on climate. I recommend using multiple layers to eliminate gas, voids, etc at the board joints.

    How you insulate the joists is primarily a cost-benefit to achieve the desired total R value. It appears you have an "unvented" cathedral ceiling, in which case I would recommend (lowest cost to highest) the following, "unfaced" batts, blown-in fiberglass (assume it can be covered with drywall) or spray foam (open or closed). Closed cell does create a 2nd vapor retarder, but if you provide enough of it, condensation is not possible in the ceiling cavity. If you are in snow country, ice damming is a concern, so I would err on more insulation not less and lean more towards the foam.

    On a separate note, 1/4" is not sufficient for wood framing, particularly with only 5/8" decking. If you use 3/4" I would feel more comfortable, but we typically try for 3/8" /12"on wood framing. 1/4" is the bare minimum and wood tends to deflect more over time and when (not if) it becomes less than 1/4" it will likely void the roof warranty, therefore we generally use 3/8" as our minimum.

  2. mn3gjGJ4Us | | #2

    Thank you for your reply Glen. I am in central OH - sorry, I forget that key element.

    I probably have 5/16"/ foot slope for the reason you suggest - wood sagging (and variation). I wanted 3/8" but couldn't quite get there - not enough space under a door.

    Although cost is always an issue, I am willing to spend - even on spray foam - to get the right results and eliminate condensation.

    I bought a single sheet of 3.5" polyiso on the advice of a roofer but I probably should have gone w/ my gut: 2 sheets eg. a 2" + a 1.5" would have let me stagger joints. As I can't do that now, what should I seal the edges between the sheets with? I've seen suggestions of tape and caulk.

  3. GBA Editor
    Martin Holladay | | #3

    Seal the polyiso seams with canned foam, then trim the excess cured foam and seal the seams again with housewrap tape.

    There are lots of ways to insulate the joist bays. If you choose dense-packed cellulose, any future roof sheathing repairs will be much easier than if you choose spray foam.

  4. gtmtx | | #4

    I agree with Martin on the joints, the canned foam works fine. You could always add a single layer of 1" on top to stagger the joints. Also, if this is going to be used as a roof periodically, you may consider a cover board of perlite or Densdeck by Georgia-Pacific, which would not provide any significant R-value, but reduces the "sponge" effect on low-slope roofs.

    Climate Zone 5 - 2009 Energy Code (Ohio has a state code which may be more restrictive) says R-38 is the minimum. I assumed this is a residential application.

    As Martin noted, cellulose is fine assuming it can be covered with drywall. Without a drawing it may be difficult to select a specific option, but one thing to be careful in this application is the potential thermal bridging at the connections between the low-slope roof and wall systems. Sounds like headroom is very tight so you may have an R38 roof assembly, but only have a wood header on the vertical transition.

  5. mn3gjGJ4Us | | #5

    Thank you Glen and Martin. Yes, this is residential.

    I talked to the polyiso manufacturer this AM and he said either foam or caulk will work. Foam sounds more logical and I'll try that. The roof material manufacturer said not to use take as it can loosen under the self-stick. I'm still researching that.

    I don't have room to add another 1" of polyiso so I'll work on sealing the seams well.

    Joist bay insulation: the easiest thing would be facet insulation stapled between the joists but if that will encourage condensation I'll use spray-in foam.

    Agree, the wall-roof join will be the weak point re thermal bridging. Further, it's an 1800s peg & post house so I'm dealing with house joists open at the ends. This type of house has rough-cut 2x8 oak joists set on top of a hand-hewn 6x8 oak perimeter beam (no rim joist as we'd use today. I sealed that from the inside w/ polyiso ~ 20 years ago but I want to rethink it.

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