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Community and Q&A

Above deck mineral wool roof assembly

Stewart Robinson | Posted in Green Building Techniques on

I’m building a timber frame house in California (climate zone 3).  The roof assembly that is currently spec’d is as follows:

* conditioned living space
* timber frame roof trusses without any eave overhangs 
* 2x t&g
* air/moisture barrier that wraps over t&g and runs down wall providing continuous air barrier
* rigid foam panels
* 2×4 strapping over rigid foam to provide vent channel and structure to support eaves which would support soffit and ridge vents.
* osb roof decking
* wrb
* standing seam metal roof

I would love to change this assembly and replace rigid foam with mineral wool batts and also eliminate the vent channels to simplify the structure and reduce fire risk.  My proposed assembly would be as follows:

* timber frame roof trusses as before
* 2x t&g
* moisture (barrier or retarder?) that wraps over t&g and runs down wall providing continuous air barrier
* 2×8 (or 2×10 if vent channel is required) on edge to provide cavity for R30 mineral wool batts  and provide structure to support eaves
* osb roof decking
* wrb
* standing seam metal roof

So I’m wondering if I can just use mineral wool wool installed between my over-roof 2x’s?  I realize I will loose my thermal break, but its climate zone 3 after all.  If I used 2×10’s I could still have a vent channel with soffit and ridge vents.

Even better, could I use 2×8’s  so a 7.25″ batt would basically fill the cavity, loose my vent channels and soffit vents, but install a vapor diffusion port at ridge line?  This would be an ideal solution in my mind, but will it work and meet code?  I’m a bit confused about 2018 IRC 806.5 item 5.2.10 that seems to imply an air supply to the unvented attic space is required.  Does this apply to my assembly or just when there is real attic space below permeable insulation which I would have none?

I would appreciate any thoughts on how I can use mineral wool batts in my roof assembly and eliminate the vent channels and soffit vents.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Stewart,
    Your proposal is complicated, and not ideal from an R-value perspective, but it will work. You can install vapor diffusion ports in your climate in a cathedral ceiling, even when there is no attic.

    For more details on roof assemblies with vapor diffusion ports, see this article: "Vapor Diffusion Ports."

  2. Stewart Robinson | | #2

    Thank you Martin. It was your article above that gave me the idea of eliminating my soffit vents to reduce fire danger. But I'm still curious on a couple details.

    1. I would consider using foil backed osb to provide a radiant barrier under roof deck, but that would require an open space behind the barrier. Code says impermeable insulation must be directly below decking (but it doesn't say in contact with). Would a 2" gap between osb and mineral wool be allowed so I could have a radiant barrier, or is this a bad idea?

    2. Your article discusses the vapor permeability of vapor port, but should the vapor/air barrier on top of t&g be vapor permeable?

  3. Brendan Albano | | #3

    What's your motivation for venting the first assembly in climate zone 3?

    This still uses foam, but it eliminates the roof vents, which seems positive from a fire perspective:

    * conditioned living space
    * timber frame roof trusses without any eave overhangs
    * 2x t&g
    * air/moisture barrier that wraps over t&g and runs down wall providing continuous air barrier
    * rigid foam panels (I believe you could also substitute a rigid mineral wool product here like ComfortBoard 80, depending on the slope of the roof and the willingness of your contractor to install it)
    * osb roof decking
    * roofing underlayment
    * standing seam metal roof

    In your mineral wool approach, you've added a bunch of thermal bridges with your 2x framing which seems like a bummer. You've also built two redundant roof structures with this approach (the timber trusses, then the 2x roof on top of that). Not that the double structure thing is inherently a problem, it just seems inefficient from a cost/materials standpoint.

    If you're leaning towards trying to avoid foam for environmental reasons, you might also take a look at the availability of recycled foam in your area.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Stewart,
    Q. "Code says impermeable insulation must be directly below decking (but it doesn't say in contact with). Would a 2 inch gap between OSB and mineral wool be allowed so I could have a radiant barrier, or is this a bad idea?"

    A. Your reference to "impermeable insulation" is confusing. Mineral wool is air-permeable. The code section that describes the requirements for vapor diffusion ports (2018 IRC section R806.5.2.8) requires that "Where only air-permeable insulation is used, it shall be installed directly below the structural roof sheathing." The way I interpret that provision, you can't have an air gap between the top of the air-permeable insulation and the roof sheathing -- but the final arbiter of the code is your local code official, not me.

    Q. "Your article discusses the vapor permeability of vapor port, but should the vapor/air barrier on top of tongue-and-groove boards be vapor-permeable?"

    A. The vapor permeance of the air barrier above the tongue-and-groove boards is irrelevant. This type of roof assembly does not depend on inward drying.

  5. Stewart Robinson | | #5

    Brendan, thanks for your thoughts. At the time I asked the designer the same question, do I really need to vent the roof. He said he is just a believer in a vented roof structures because they are more bullet proof/reliable. But his experience is in much colder climates.
    There was also some concern that without a vent channel, the foam would need to be in direct contact with roofing deck. But with external rigid insulation, I'm not sure how we could ensure this "direct contact" requirement as well as provide a structure to support my eaves. Are you suggesting I could nail 2x’s on edge, lay polyiso in between them, and then nail the osb over that? Would that be direct contact?

    As to why I chose this complicated assembly structure, let me give you a little background. I just completed a tiny granny unit on the property in which I focused on building it tight, but with standard R21 Walls and an R30 ceiling under a traditional vented attic. The temperature in the house seems to hover between 67 to 70 most of the time with only an occasional need for heat or air conditioning. So, I'm trying to continue with a similar focus on the timber structure; make it tight and not stress about maximizing insulation values.

    Our reason behind building a timber frame house was to enjoy seeing the full timbers with the t&g over the top, sort of a modern farmhouse style. By eliminating all the roof joist tails, it would be very easy to wrap my air barrier over the t&g and down onto the side walls. This then necessitates exterior insulation and 2x's to support the roof eaves. That's how I arrived at these assemblies. But I was never happy with the soffets and ridge vents because of fire danger. Martin's article on vapor ports got me to thinking about using mineral wool batts, they don't burn!

    The roof is 9:12 pitch, so the framers are a bit stressed to work on it, and get even more stressed when I suggest building a second roof structure over the rigid foam. Roofer's have no problems with the pitch, but don't want to build the eve structure. In our area, we just don't use rigid foam over a roof deck so it is causing a lot of puzzled looks.
    That's why I came up with my proposed assembly. The framers have no problems installing 2x8's on edge directly on the T&G. So once that's done, it’s just a simple matter then to install mineral wool batts, and then the osb.

  6. Stewart Robinson | | #6

    Thanks again Martin, I did mean to say "air-permeable", just a typo on my part. Next stop will be building dept to see if they will accept 2018 IRC updates related to vapor diffusion ports.

  7. Brendan Albano | | #7

    Figure 1 and Photograph 3 in this BSC article show a way to frame your eaves where the framing is incorporated into the foam layer if you want to have direct contact between insulation and sheathing with no venting: https://buildingscience.com/documents/insights/bsi-081-zeroing-in

    My understanding is that the main purpose of a vented overroof like you have in the foam assembly you described with the foam and 2x4 vent channels is to prevent ice dams. If you don't have ice dams in your climate, but you are concerned about sparks from wildfires that could get sucked into roof ventilation channels, I'd have another conversation with your designer about the necessity of the vent channels. Ask what specific problems they solve. Simply saying they are "more bulletproof" doesn't really give you the information you need to make an informed decision unless they can tell you specifically which "bullets" the ventilation is protecting you from.

    1. Stewart Robinson | | #8

      Thank you Bendan, I've not seen this article before, it was very informational. It was also great to see my focus on construction techniques that simplify an effective air barrier validated.

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