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Secondary air barrier at attic – ventilated roof deck vs ventilated attic

Chris Roche | Posted in Energy Efficiency and Durability on

In climate zone 6

I have seen many articles referencing the goal of many builders to add a secondary air barrier/wind-tight layer, but often those details end at the walls, especially when it comes to a standard truss roof assembly. Most plans I have seen for a truss roof include soffit vents, and only rely on a single air barrier at the ceiling as either drywall or a membrane such as Intello. I know the reason for this is partly due to the fact that a soffit ventilated truss roof, as described by Joe Lstiburek, is one of the most robust roof system you can have. 

If the goal is to have a secondary air barrier on all sides of the house, and you are already relying on an air tight (strapped with 2×3 service cavity) ceiling with moisture management membranes such as Intello at the ceilings, would it be safe to wrap the WRB/Secondary air barrier of the exterior walls up the sides of the house onto the roof, creating a sealed open truss attic? This would leave all sides of the exterior air-tight. The same assembly would have plywood sheathing attached to trusses, then a smart membrane (as either peel and stick or taped membrane, then strapping for to create a ventilation space and roof overhang, then metal roof.  In my case it would be a typical 8 pitch truss with r80 loose fill insulation. 

While my initial skepticism leads me to worry that if any moisture found its way into the attic, it may want to condensate on the cold sheathing and will not be dried out by the ventilation you would normally have in that space. I have read that the moisture can be removed via diffusion in warmer climates with vapor diffusion ports, but that cant be done with insulation on the attic floor.  The reason I want to have the plywood layer first, is because it just makes everything easier to install. 

I came across this exact plan (minus the plywood against the trusses) which is advocated by Four Seven Five. I have attached an image that they created showing this exact assembly. 

Does anyone have any thoughts on this idea.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Chris,

    My first question would be: what are you trying to gain? Or perhaps put another way, what problem are you trying to eliminate by stopping the ventilated air entering the attic?

    Unlike the 475 section:

    Once you move the ventilation above the sheathing, any moisture has to diffuse through it before being dissipated. That's a lot less efficient than having it leave on the ventilated air.

    Above the sheathing you end up with two competing demands. One is for the strapping to create ventilation from soffit to ridge, the other is for it to provide horizontal support for the metal roof panels.

    The benefit of leaving a gap under metal roofing appears to be climate specific. Research here in the PNW seems to show it does more harm than good.

    1. Chris Roche | | #3

      Hi Malcolm,

      Thank you for the reply. Based on what you are saying, which makes sense, I think the key difference between what I was looking to do and what 475 is offering is that they have the plywood (which has a relatively low perm rating) above the strapping ventilation channel. So the attic should have much better moisture diffusion properties through their high perm membrane. In an article written by Martin Holladay, Martin notes that diffusion does not work with insulation at the attic floor. I am not certain how that applies here.

      If what I am trying to achieve is ultra-tight air losses, (I dont know if out insanity or logic) wouldn't it make more sense to create an attic that is also air tight? Wont that prevent any leaks from the interior air barrier be less likely to draw air into the attic due to pressure differences? Or would this all be a waste of time and money which could be better spent simply making a more air-tight ceiling air barrier.

      -Edit, just thought this over. The attic is not under positive pressure, and the air leaking in is at such a negligible volume compared to the total volume of the space, so there should be nothing stopping warm air leaking into it from the conditioned space, even if the attic is also air-tight sealed, from what I understand. It sounds like wind washing would be the only enemy if not controlled.

      1. Expert Member
        Malcolm Taylor | | #4

        Chris,

        My feeling is that the place to stop the air is the ceiling, not expand the boundary through the attic. Whether that's by just being diligent with the primary air-barrier, protecting it with a service cavity, or even installing a redundant second layer, I think it's a simpler strategy with many less unknowns to trip you up.

        1. Chris Roche | | #5

          HI Malcolm,

          Thanks for the advice, you have changed my mind. Sometimes it can easy to be sold on an idea that may provide a small improvement, but is likely mostly proposed in the effort to sell more product.

          1. Expert Member
            Malcolm Taylor | | #7

            Chris,

            I think 475 are genuinely interested in advancing building science, but they seem to rely on their customers to be early adopters of the assemblies they propose. That's not something I'd be comfortable with.

  2. Mike Theis | | #2

    You already have r80. I agree with Malcolm, What are you trying to gain.
    I think this is why cellulose is often chosen over fiberglass, less wind wash of the insulation.
    Same with foam, not effected by wind wash.
    In the first home I build for myself in 1985 I had a stick framed attic. Not trusses. I rolled out Tyvec over the entire attic after the the attic was insulated. Never did it again. Don't know if it was worth it. Made me feel better at the time.

    1. Chris Roche | | #6

      Thanks Mike, it sounds like sticking with good vent channels and cellulose will provide an adequate solution.

  3. Jason S. | | #8

    To answer you're initial question, that secondary air barrier would limit convective heat loss through the cellulose insulation, but it would be too risky in climate zone 6 if the majority of that second airtight surface at the roof is plywood. The 475 detail with the vapor open membrane and ventilation above would work better, but as you implied it's more difficult to install (stand on trusses covered by a membrane? yikes...)

    I've puzzled on this same problem. I ended up just lying leftover roofing felt over the cellulose and took care with the eave baffles. It's nowhere near airtight, but I've convinced myself the convective loops are closed. Some day when I'm feeling brave I'll try the Solitex Mento.

    1. Expert Member
      Malcolm Taylor | | #9

      I don't think there is any significant convective heat-loss through cellulose.
      https://web.ornl.gov/sci/buildings/conf-archive/1992%20B5%20papers/048.pdf

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