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Insulating 1950’s Tectum roof from exterior – Air barrier ?

scottyg927 | Posted in Energy Efficiency and Durability on

I have a house built in 1957 that was originally a summer place on a lake. Climate zone 5, just outside of Chicago, IL. Roof is a timber (3.5″x5″) frame at 32″ o.c. with what appears to be Tectum I, 2″ thick structural roof deck panels (2’x8′ long). Current roof assembly yields me a whopping R3.5, which naturally means ice dam-nation every winter as we melt everything that lands on our roof. Apparently all previous owners did not care or did not know how to remedy.

This horrible winter actually produced the first ‘leak’ if you want to call it that into our house. No water ever made it in, but a small potion of a panel where it meets the wall discolored and saturated. Since this roof deck is structural and my only line of defense to the exterior, I now have to insulate the roof from the exterior (wife loves exposed beam to interior look and won’t let me change it). I am an architect and builder and I want to geek out on this as I love this kind of a project.

I plan on adding 4″ to 6″ of polyiso to the top of the tectum roof deck and then 2x furring at 32″ o.c. screwed to each timber using headlok screws with cdx plywood over the furring. I plan to vent at the ridge and behind the gutter using a continuous ridge vent and coravent strips at fascia (I have this detail worked out already).

Since this is my first project of this type with a Tectum panel as the substrate and not plywood, I am not 100% sure on the air barrier to use on top of the tectum deck and below the polyiso. I do not believe I have any type of moisture barrier on the inside or outside of the deck. I am sure I will find surprises once I start stripping the roof. I do not want to ice and water shield the entire roof as I do not want to trap any moisture inside of the tectum panels, but not sure if a breathable membrane would be adequate under the polyiso as it is essentially my vapor barrier and could be the air barrier if sealed properly, but I like a belt and suspender approach always.

Any thoughts or comments on this would be greatly appreciated and if I am wrong about anything above, please let me know. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    I assume the Tectum panels can dry to the interior, so there shouldn't be anything wrong with installing Ice & Water Shield above the Tectum panels (and below the polyiso).

    If you want to save money, you can use the polyiso as your air barrier. If you have more than one layer of polyiso, the polyiso seams of the second layer should be staggered with respect to the first layer; it never hurts to tape the seams of both layers with high quality tape.

    Some builders worry about the possibility that rigid foam can shrink. If you are in that category, you should probably invest in the Ice & Water Shield or a European membrane under the polyiso.

  2. scottyg927 | | #2

    Yes, as far as I know, they can dry to the interior as I do not believe there is any type of 'vapor barrier' paint on it. I'd be doing some type of membrane over the tectum just because of timing as weather will never cooperate. I have not looked at cost difference between a ice and water shield vs. a pro clima product. Just has to be water-tight as there will be sometime before the final roof layer is installed.

    I was planning on taping and staggering my layers regardless, so if it does indeed shrink (have not witnessed it as of yet), I have a second defense.

    I appreciate the response and if there are good air barrier products that I may not be aware of, I am always willing to listen. Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    Another product you might consider is synthetic roofing underlayment (although that product is not usually sold as an air barrier).

  4. scottyg927 | | #4

    Was considering that, but not sure how to seal it.

    One question that did come up was volume of vent space. I am planning 1.5" above the foam for vent space, but realized that I might be choking it down at the soffits with the coravent at 3/4" thick. Is 3/4" vent space enough above the foam or is 1.5" preferred? I have seen instances showing both. if 3/4" is ok, it would allow me to add another R-5 between studs.

  5. batwest | | #5

    Hello Scott, I am currently looking at a property with the similar condition. Have you thought of using a house wrap instead of ice and water shield below the poliso? May be more cost effective, if concerned with fasteners causing moisture penetration cut ice shield strips and roll along timbers. The property I am looking at seems to be bowing between beams. Have you considered the weight load and the type of roofing to top your project with?

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