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I live in Stuart, Florida about 5 miles from the Atlantic Ocean and the underside of my roof deck panels has Icynene Open-cell foam insulation sprayed onto it, making it an unvented space. We currently have a tile roof but are strongly considering installing a metal roof. Is there a limitation to installing metal roofing onto a roof deck that has spray foam on the bottom side of the panels? Also, can you provide a recommendation for the underlayments for this situation?
The roofing companies providing bids are recommending a 30# felt under a peel and stick synthetic top layer then the metal roof. The concern is breathability for the roof deck. I have no problem paying extra as I want to avoid bigger problems down the road.
So, this is open cell, not closed cell foam, correct? And no ventilation at all? Do you know if there is a vapor barrier behind the ceilings (such as polyethylene sheeting). I do share some concern about moisture getting through the open cell and condensing on the underside of the roof deck which will likely be cooler in temperature due to the metal cover. Yes, a non-breathable underlayment will exacerbate things but to the best of my knowledge Florida code won't allow a truly breathable underlayment. That said, a couple layers of synthetic will still be a bit more breathable than self-adhering ice and watershield. This may be a situation where putting the metal panels up on battens or on top of an entangled mesh to create a thermal break and reduce the transfer of cold to the roof deck may be a very wise choice.
Yes, it's open cell foam. There is no vapor barrier behind the ceilings. The Attic temperature in the summer is generally around 80 degrees when the house is 75 degrees and the outside temp is no more than 90 degrees. Dew Point in the Attic is a little higher (maybe 5 points max) than the house but not nearly as high as outside. The House had a full renovation and is very air tight and we run our air consistently whether we are home or away generally keeping the moisture content very controlled. Not sure any of that info helps but thought I would mention it.
My recommendation is a metal roof installed with a thermal break, typically that would be battens or an entangled mesh. It also could be metal shingles (which have an integrated thermal break). Yes, this may be a bit of a belt and suspenders approach but it is still my advice. My concern is warm moist air trying to drive out of the structure and reaching a cool roof deck at night.
Thanks for responding. The local roofers are recommending a 30# felt layer then a Peel and stick top layer. I guess they see the 30# acting as the Thermal break.
The other option is to go with what I have now which is a Concrete tile roof with a 30# felt topped with a 90# or Mineral cap Sheet. In that case, would the air gap created (Gap as the tiles layer on top of one another) beneath the tile act as the Thermal break?
I suspect they are recommending the 30# so that the self-adhering doesn't get completely "stuck" to the deck, which could potentially damage the deck if it's ever pulled off, which would be a problem with the attached spray foam. Most ice and watershield manufacturers though only specify their product to be installed direct to decking -- not over underlayment. Also, a thermal break involves a dead airspace. For me, metal is creating the need for the thermal break. Because a metal roof generally stays cooler than other roofing materials (and cools down more quickly at night), it drops the roof deck temperature which then brings up the potential for condensation.
Got it! I guess I need to discuss with the roofers. I might have to go with a concrete tile application as it will create a air space (thermal break) between the tile and the roof underlayment. Thanks!
Metal shingles also create their own thermal break due to the way they are formed.
I assume they are a barrel or S-tile shape such that they create an air gap?
There are modular metal panels press or roll formed to look like barrel tile but also panels that look like wood shakes, flat tile, slate, and shingles. All of them have an airgap between the metal and the roof deck. It doesn't take much space to create an effective thermal break -- think of the tiny space between two plates of glass in a thermal pane window.
This is great information. I will look into those options.
Thanks for taking the time on this subject it was extremely helpful.
Thanks for using our site. Here's a link to a webinar on condensation and roofing that may be of interest. https://www.youtube.com/watch?v=NYwD9CWveKY
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