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Roof Air Barrier

idahobuild | Posted in General Questions on

We plan to act as our on general contractor during for our home build next year.  The construction will be in zone 5 (50 miles east of Boise, ID).
I had been planning to make the air barrier wrap the entire exterior of the home at the sheathing (a sort of monopoly-style) from the slab, up the exterior of the walls and continuing to the peak of the roof. With 2 inches of rigid foam on walls and 4 inches on the roof (both over the airbarrier).  But with the cost of lumber being what it is I don’t know if I can still do the insulation on the roof as I’d need to add lumber over the top to secure the roofing (metal).  I would really like the house to be as air tight as possible and rely on an ERV to move/ventilate in a controlled manner.

Can I still use the roof as an air barrier if all of the ceiling/attic insulation is below the roof? I ask because trying to seal around all of the ceiling penetrations (lights, speakers, plumbing, electrical, network) seems near impossible.

Thoughts….
Thanks.

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Replies

  1. the74impala | | #1

    You can do it. I have been doing it for my retrofit. Easier on a new build though.

    But it is not cheap. Probably not cheaper than what you originally planned.

    You can do a Mento outer wrap, overlap the outside walls, then continue with Intello on the ceiling. Then build the partition walls inside that. You probably want to use 1x to protect the Intello from screws, so you are still adding lumber price on top of the membrane and tape/caulk/trouble.

    You could just do drywall or plywood or foam board on the ceiling and can foam in the attic, and save a bunch of money, but probably not as tight as the Intello done well.

    1. idahobuild | | #4

      Tom,
      Thanks for your thoughts.

      You pretty much hit the nail on the head with the additional cost for tape and caulk and trouble with sealing a ceiling/interior air barrier. All of the penetrations in the ceiling and walls that would need to be sealed are the main reason I am considering keeping the air barrier at the roof. I think that the problem that I am going to have with the roof air barrier is the airflow/moisture issue that I may encounter with the insulation on the attic floor - as WaltA mentions in his response.

      I'm am trying to come up with ways of mitigating any moisture/mold issue that may arise from the placement of attic insulation on top of the ceiling rather than at the top cord of the trusses -- thus the comment regarding the ERV. I am not sure if placing one of the ERV ducts in the attic is sufficient to keep the space dry.

  2. Danan_S | | #2

    Can you use cheaper 1x lumber battens over the roof foam boards to mount your metal roof to? Thought I saw a video of Matt Risinger doing that. I think they doubled as roof footholds during construction.

    1. idahobuild | | #5

      Hey Dan,
      I did have the 1x as part of my design--that is until I looked up the specifications for the standing-seam, metal roof. All the ones that I could find seam to require placing the metal sheets on top of a completely covered roof. I think that Matt R. confirmed with the metal roof installer in one of his videos that he was straying from the installation specs for the material on his roof. He was okay with it, but I feel like I should follow the manufacturers specs which, in my situation would basically require two complete layers of plywood sheathing on the roof.

      Like this:
      Bottom - OSB Roof Layer to support the rigid foam
      Next level up - 4" of Rigid foam
      Next up - OSB Roof Layer to support/fasten the metal roof
      Next up - Air Barrier
      Next up - Rain Screen material
      Top Layer - Metal Roof

      So, I was thinking "Okay, the exterior roof insulation is going to be costly (4" insulation and 2 layers of OSB) -- plus I'd have to insulate over the garage with 4" of insulation as well -- to keep the roofs on the same plane.

      Does what I'm thinking make sense?

    2. Deleted | | #7

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    3. Deleted | | #8

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  3. walta100 | | #3

    The question comes down too will you have a conditioned attic or a vented attic?

    If you want a conditioned attic you need to move the insulation to the roof line and spend the money to buy and operate the equipment necessary to keep the attic at more or less the same temp and humidity as the rest of your house.

    If you seal the attic but try not to control the temp and humidity the dew point of the air will sometimes get above the temp of the surfaces of the enclosure. The enclosure will get wet when that happens now you are Russian roulette with mold and rot. How wet will it get and how often are unknowable. Some win and some lose do you feel lucky enough to bet your health and house on it.

    Walta

    1. idahobuild | | #6

      Walt,
      I agree that you're first question is what makes or breaks the design. In my design the attic is not-vented, but not really conditioned either. I am wondering if including the ERV and an ERV-register or three in the attic would keep things dry enough. OR what amount of insulation would need to be at the top cord of the trusses to mitigate the moisture. I have seen a table referenced on this site (and others) regarding the percentage of insulation on exterior when there is insulation on the interior and exterior, but no mention of how to handle an air barrier on the roof with insulation on the floor of the attic.

      I spoke to one architect here in town who was too busy to take on small, residential design work that had mentioned the air barrier on the exterior with the ceiling/drywall as the interior barrier. That just seems impossible to seal up all of the holes and then I'd have HVAC up there too.

      I'm not sure of the best way to handle it. I may just go with some sort of ceiling air barrier (Intello) linked to the exterior barrier (Delta Vent SA). Then just spend a bunch of time (which I'll have plenty of) to check and double check the penetrations--with a few blower door tests for good measure.

      Thoughts...

  4. walta100 | | #9

    What is so appealing about your idea to you? I don’t see it.

    Let’s move the insulation to the roof line because that will give us 20-50% more surface area to lose heat and I have lots of money buy more insulation to cover all that needless aera.

    Let’s move the insulation to the roof line because that will almost force me to use foam insulation that costs the most dollars per R. I can’t stand that cheap fluffy insulation it is almost 100% post-consumer recycled materials, I must have that virgin foam it pure oil and the least green thing I could put in or on my house.

    The only reason I can think of if your dead set on putting the HVAC equipment in the attic space.

    Seem to me what you are proposing is risky and unsanctioned in the code books, I doubt any licensed professional will bet their reputation on your plan by put their stamp on the plans. If your government body require a stamped set of plans as most do, even the ones do not inspect, still look to see a stamp on the plans. That could be a problem.

    The real question is will it work in the real world. The only answer you can get will be maybe or sometimes. People that have tried this idea often post to this board asking why it is raining in their attic or why is it so hot in the attic we got a good 4 inches of spray foam.

    Seems to me you are looking for a free lunch in that you are unwilling to pay to condition the air in your unvented attic. I say you are playing Russian roulette with mold and rot.

    Seems like ID is a pretty dry state and you may get lucky but maybe not.

    Walta

    1. idahobuild | | #11

      Thanks WaltA.

      BLUF - I am going to look into reducing penetrations in the ceiling (lights, fans, appliances) and putting the air barrier at the ceiling.

      Details:

      Walt A - Answer: What is so appealing about your idea to you? I don’t see it.
      Walt D - Response: No, appeal. I was just wondering if it was an option to have air barrier at roof deck and insulation at ceiling -- sounds like it's NOT a viable solution.

      Walt A - Answer: Let’s move the insulation to the roof line because that will give us 20-50% more surface area to lose heat and I have lots of money buy more insulation to cover all that needless [area].
      WaltD - Response: Not appealing - but was considered (minus the "lots of money" part (Walt Davies not Walt Disney).

      WaltA - Answer: Let’s move the insulation to the roof line because that will almost force me to use foam insulation that costs the most dollars per R. I can’t stand that cheap fluffy insulation it is almost 100% post-consumer recycled materials, I must have that virgin foam it pure oil and the least green thing I could put in or on my house.
      WaltD - Response: There was a plan to use exterior rigid foam, but the added cost of lumber, needed to create the extra nailing surface, and rigid foam seem to be pushing that out of reach.

      WaltA - Answer: The only reason I can think of if your dead set on putting the HVAC equipment in the attic space.
      WaltD - Response - I had considered having the HVAC in the attic, seems like, at the very least, if there is duct work it would be up there. I am now going to have a conversation with Manual J/D folks about mini-splits/cassette options.

      WaltA - Answer: Seem to me what you are proposing is risky and unsanctioned in the code books, I doubt any licensed professional will bet their reputation on your plan by put their stamp on the plans. If your government body require a stamped set of plans as most do, even the ones do not inspect, still look to see a stamp on the plans. That could be a problem.
      The real question is will it work in the real world. The only answer you can get will be maybe or sometimes. People that have tried this idea often post to this board asking why it is raining in their attic or why is it so hot in the attic we got a good 4 inches of spray foam.
      WaltD - Response: I thought it to be a risky business as well, that was part of the reason for my post on here - edumacation. :-) I've seen the post about moisture in various place. I will have an ERV and dehumidifier - which is part of the reason I want to get the air barrier in good order.

      WaltA - Answer: Seems to me you are looking for a free lunch in that you are unwilling to pay to condition the air in your unvented attic. I say you are playing Russian roulette with mold and rot.
      WaltD - Response: No free lunch requested. The attic insulation with roof-deck air barrier idea was something I needed feedback on....and have now received quite a bit of helpful info. I couldn't find anyone talking about how to handle that design idea -- looks like the reason for that is that it is not a good idea -- or difficult to adequately implement at the very least.

  5. Expert Member
    Akos | | #10

    You can use an air barrier at the roof line provided it is vapor open (10 perm+) and there is a vent channel above it. This way any moisture that makes it into the attic space can dry through the barrier to the outside.

    For a while Dupont was promoting a system where the air barrier was at the roof line. This used house wrap over the rafters with 2x2 nailed on either side to form the vent channel. To me this looks like a pretty fussy detail, hard to build as you can't walk on the roof and easy to tear during the install.

    I think the simplest is an air barrier above the ceiling. This can be something as simple as a well detailed 6 mil poly, one of the fancier air barriers membranes or any sheet good that can be taped (ie OSB/CDX/foil faced rigid).

    You can also look at moving or eliminating the holes in your ceiling. You can get mud in in-wall speakers that are invisible and can be mounted into an interior wall and instead of pot lights use surface mount slim LED puck lights that install into onto a standard ceiling device box.

    If you really must have all the holes in the ceiling, a slim service cavity formed by 2x3 or 2x4 on flat under the ceiling air barrier is much less work than dealing with the roof line air barrier.

  6. Expert Member
    MALCOLM TAYLOR | | #12

    IdahoBuild,

    I second Akos' suggestion that a service cavity makes more sense than moving the air-barrier up to the roof. You may want to make it deeper in some areas , like hallways or service rooms, if you want a plenum for ducts

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