Insulating 1950's Tectum roof from exterior - Air barrier ?
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!
Posted Fri, 02/28/2014 - 11:06
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