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Lstiburek 20% Unvented Ratio for a Vented Roof?

johnwtaylor | Posted in General Questions on

I made a inquiry with a company that makes batt insulation looking for a list of dealers. This progressed into an application/installation discussion.

In the latest email the company sent me a copy of a publication (named below) saying the 20% foam/batt ratio needs to be used for vented Cathedral Ceilings.

Insight Hybrid Attics and Hybrid Walls Simple physics…complicated language  By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

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  1. Expert Member


    I have no idea where they got that from. Not from Joe L. or any building code I know. Did they even read the article they sent you?

    You may find this useful:

    1. johnwtaylor | | #3

      Don't know if they have but I have, several times.

      The company is international so I thought they may be dealing directly with BSC and have the latest information.

      Edit: The method I used isn't one of the 5 methods. The top chords are 2x10. There is 2" air gap below the sheathing then 1.5" XPS that is air sealed. The remaining 5.5" - 6" is filled with batt insulation. The continuous insulation isn't done yet.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        I think I know what their concern is. You are using impermeable foam as your vent baffles. They are worried moisture could condense on the underside because the foam doesn't meet the ratios, whether the roof is vented or not.

        Martin addresses that issue in his article on site-built baffles:

        1. johnwtaylor | | #6

          Thanks Malcolm
          I thought 1.5" EPS is considered permeable? Essentially the same thing could happen inside batt insulation of the first example of "5 Cathedrals that Work".

          I explained to them that everything below the insulation will be air sealed so I'm not sure where they think moisture will come from.

          I'll send an email tomorrow and find out.

          1. Expert Member
            MALCOLM TAYLOR | | #7


            I don't agree with them, I just suspect that's where they are coming from.

            No doubt roof assemblies with very permeable baffles, or those thick enough to meet the ratio are safer and more resilient than those without, but that's a different question than whether ones like you are proposing are risky. I don't think they are.

            "Essentially the same thing could happen inside batt insulation of the first example of "5 Cathedrals that Work". "

            I could but there would be no consequences. That's why Joe L. writes about the "First condensing surface of interest.". The water vapour moves through the wall (and the batts) without pooling until it hits a solid surface below the dew point. If that surface has a high enough permeance, it loses moisture faster than it takes it in and suffers no damage.

  2. brendanalbano | | #2

    It sounds like the Lstiburek article they are referencing is:

    That article is about unvented assemblies, and the ratio of foam/batt is dependent on climate zone. The article has nothing to do with vented assemblies!

    1. johnwtaylor | | #4

      Yes, that's the publication by Lstiburek they sent me

  3. johnwtaylor | | #8

    Hi Malcolm
    Your assumption is right. The company manufacturing the batt insulation says 1.5" of XPS doesn't meet the 20% ration so it will be a condensing surface. They say moisture will come from inside despite being told air sealing will be done 110%.

    The 1.5" XPS (R6) will work for our (BCBC) code minimum of R26.5 for a sloped roof but I'd like more than that.

    For the time it takes I'll add another 1.5" of XPS. There's really no difference between R value of Type II XPS and batt so I'll be trading one for the other.

    Thank you for the assistance

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