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Community and Q&A

Foil-faced polyiso for site-built baffles

dmbaca | Posted in GBA Pro Help on

First, thanks to everyone here for all of the advice and knowledge I’ve gleaned as a reader for the past year.

I’m 3 months into construction on a new home (CZ 4A) for my family and recently made a big change… I was planning an unvented roof assembly with spray foam on the underside of the deck, but I became concerned about the difficulty of the application and potential problems now that we’re in the dead of winter. I changed the design to a vented roof with cellulose on the ceiling and dense pack cellulose in the walls, but I’m trying to make sure I have the cathedral portions of the ceiling right.

My roof assembly is asphalt shingles | synthetic felt or I/W shield depending on location | 5/8″ OSB | 1 or 1.5″ air gap | 1″ foil faced polyiso | 8.5″ dense pack cellulose. The foil face is turned up (towards the roof).

This is how I interpretted the Fine Homebuilding “Roof Venting” article. Did I interpret it correctly? Just want to make sure with the foil faced polyiso being a vapor barrier.


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  1. GBA Editor
    Martin Holladay | | #1

    As long as you pay attention to the airtightness of your ceiling assembly, you should be fine. Your proposed ceiling assembly will have an R-value of R-38, but it will perform at a lower value, because there will be thermal bridging through the rafters. It's an OK roof -- not a great roof, but an OK roof.

    For more information, see How to Build an Insulated Cathedral Ceiling.

  2. user-901114 | | #2

    David, I brought this topic up in the past.. I'm getting rid of the thermal bridge by furring out my rafters with gussets. Had I to do it over again and was using dense pack, I would have built the baffles out of somthing more rigid with a higher perm, just for when air gets by my warm side barrier. Less to worry about. But I'm in zone 6 and I'm a little concerned my rafters being cold and condensing.. I'd also put my foam on the underside of the assembly. It's tedious work either way.

    Also, I made my baffles extend past my wall plane. If there's ever a leak it will hit my poly and drip out of my soffets. In theory..

  3. dmbaca | | #3

    Thanks for the feedback, guys. Martin, I've read the article you referenced and learned a great deal from it. I agree the assembly has drawbacks, but I switched from spray foam to dense pack too late in the game. I did use the Accu-Vent baffles on the majority of the house where I have an unconditioned attic. Only about 10% of the roof has a cathedral ceiling. Stephen, I agree about extending the baffles past the wall plane. I was able to do that on the back of the house, but unable to do it on the front. I was working on it a few days ago and just had to tell myself that I've got to let the roof be the roof. I'm a structural engineer and designed the house myself...this website has been really valuable over the past 15 months. Thanks again.

  4. davidbaca | | #4

    Again, thank you both for your replies. As a follow up, I have a few concerns/questions for which I would appreciate your input. I just don't have enough experience with these types of issues to have the confidence that things will probab:

    1. I have runs longer than 8' (actually about 12'), therefore, I chamfered the edge of the foam (in the 22.5" direction) and foamed the gap together. Last week there was water coming through a couple of these joints. I am 99% convinced it was related to the temporary propane heat (temps were in the low 30s), and that once I have the dense pack and drywall air barrier in place, all will be okay. Does this logic seem reasonable to you all? I never observed a roof leak prior to the poly iso being installed.
    2. Since water was coming through a few joints, I started thinking about condensation and frost issues. For condensation to occur, the foil faced polyiso (foil face up) would have to be cold during the summer, which I can't ever see happening. But I don't think I understand frost...will frost occur on the top of the polyiso (like it does on top of my car, for instance) and then run down the polyiso into these joints when the air temp warms up? If so, I just don't understand how you can make this system water tight (due to the joints in the polyiso). To use the car analogy...will the frost only "fall" on my roof (similar to the hood of my car) while the poly iso remains dry (similar to the engine underneath the hood...which I don't think I've ever seen frost on). I did observe that metal handrails did not have frost on them. Am I overthinking this?
    3. At the time I installed my baffles, I was not able to run them out past the face of the wall. Rather, they stop 1.5" in from the face of the wall at a band board (they abut the top of the band board, and I foamed the joint completely). If you think I will have issues with water running down these boards based on my earlier question, I will need to tear out portions of the job to make it right. But then I'm still left with my Accu-Vent baffles, which curve down onto the top plate, and all of the other foamed joints in my polyiso.
    4. From a roof leakage point of view, I see how it makes sense to "shingle" the Accu-Vent baffles. Is this a hard and fast requirement? I installed them starting at the top plate because I thought it made sense from the perspective of keeping insulation from blowing in between the gaps.

    Any help you can offer would be greatly appreciated. I've lost so much sleep and probably about 8-10 lbs due to the stress of this whole project.


  5. GBA Editor
    Martin Holladay | | #5

    Assuming that you don't have a roof leak -- and it doesn't sound like you do -- here is what is going on:

    1. You haven't finished building your roof assembly, so you don't have a tight air barrier.

    2. Warm, humid interior air is leaking through your partially completed roof assembly and is contacting the underside of your cold roof sheathing. It is accumulating there (and on the protruding points of your roofing nails) as frost (which later melts) or perhaps as liquid water (which drips).

    To stop this from happening, you need to complete the construction of your roof assembly by installing ceiling drywall and dense-packed cellulose insulation. Once that is done, everything will be fine -- assuming that you do a careful job of air sealing at the drywall layer -- because you will no longer have any warm, humid air leaking out of the top of your house.

  6. davidbaca | | #6

    Thanks Martin...that was quick! I was 99% convinced this was the case, but I don't have enough experience to be 100% certain. Any input on questions 2 & 3? Like I said in previous posts, I've followed the FH instructions and read as many of your articles as I can find (including the Venting Cathedral Ceilings, ADA articles, etc). I even removed and relocated the electrical ceiling boxes and can lights from a couple of these bays to have the greatest shot of having ADA. If you're ever in St. Louis, I'd be glad to show you the house and treat you to dinner.

  7. GBA Editor
    Martin Holladay | | #7

    I'll try again.

    Q. "For condensation to occur, the foil faced polyiso (foil face up) would have to be cold during the summer."

    A. Nope. This is a winter problem, not a summer problem.

    Q. "Will frost occur on the top of the polyiso?"

    A. No. The coldest surfaces are the protruding points of the roofing nails and the underside of the roof sheathing, so that's where the condensation (or moisture accumulation) will occur.

    Q. "I just don't understand how you can make this system water tight (due to the joints in the polyiso)."

    A. You don't have to make the ventilation baffles watertight. To limit condensation, you just have to stop air leaks through your ceiling.

    Q. "If you think I will have issues with water running down these boards based on my earlier question, I will need to tear out portions of the job to make it right."

    A. Don't worry. If you pay attention to airtightness, you shouldn't have to worry about water leaks.

  8. user-901114 | | #8

    I agree with Martin. My water leak comment was related to future roof leaks. Will be difficult to find the source when you've got tight chutes I'd imagine. And that is not the source of the moisture from what I'm reading. So get some sleep and hang that air barrier asap.

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