John Scott
7/18/2011
I was planning on installing something like Grace Ice and Water shield as a temporary room but came across an article recommending a breathable underlayment (http://www.newenglandmetalroof.com/metal_roofing/underlayment.html). What type of underlayment is recommended for metal roofs?
Todd Miller
Classic Products, Inc.
7/18/2011
I generally recommend a layer of 30-pound felt or one of the new synthetics, as well as ice and water shield where required by code. In cases where there is inability to ventilate the building / attic, a breathable underlayment can be wise. In all events check with the roofing manufacturer and make sure that what you're using meets their requirements.
Info @windowsonwashington.net
An informed customer is our best customer.
7/19/2011
The synthetics are almost as affordable as the 30lb felt now so I recommend those as a standard now. Both will work.
Guest User
7/23/2013
Do I need to worry? When my house near the gulf coast was built three years ago the entire roof deck was covered with Grace Ice and Water Shield and a standing seam metal roof was installed directly over that. Underneath open cell foam was applied to the underside of the roof deck. Will this setup be a problem? I was about to build another structure the same way. I never heard any other suggestions from the contractor or the roofer, but is felt or some other underlayment better or needed in addition to the IWS? I am not sure it was galvalume. The roofer made up the panels himself, but it looks like it.
Todd Miller
Classic Products, Inc.
7/23/2013
John, in your particular case I believe you are fine with what has been done. The foam is not allowing any ventilation through your roof deck. You're fine with what has been done. :-)
Guest User
7/23/2013
Thanks, Todd, Your reassurance is greatly appreciated. The attic which is totally sealed with foam is part of the conditioned air in the house so never too moist. However, I do have a 4" vent that comes in the attic that allows fresh air into the house when vent fans are on, but in addition I have a 85 cfm fan rotating attic air into the house and back constantly so RH is usually about 65% or less, and the attic is very dry and comfortable. If the threat to the roof deck from moisture is from below then I doubt there is much risk, as you say, in this case. For the new house should I stick with the same arrangement? Some suggest other underlayments than the Grace product, and even the Grace site says that some zinc type metal roofing may get too hot for the IWS, but they are not specific about Galvalume. The reason I am asking is that I have to choose products for my new home and wondered if I should do something else? I can do the Galvalume myself and had good luck with it in the past. I like the Grace IWS since I would be poking holes in the roof for the Galvalume and the solar panel mounts (a mitigating factor in regard to the heat on the roof, I guess) and maybe I should choose white as the color to keep heat down? Thanks for all your helpful advice. John
Todd Miller
Classic Products, Inc.
7/23/2013
Yes, the the threat is with moisture which originates inside the house. If you're either blocking that moisture by foam (and a conditioned space) or if you're venting that moisture outward, then I would not sorry at all about using ice and watershield on the entire roof.
Info @windowsonwashington.net
An informed customer is our best customer.
7/24/2013
Keep in mind that OC foam is vapor open. That being said, the fact that you are in Gulf Coast and the temps there are going to be warm, you wouldn't have the same condensation potential as a roof in Maine. Also, the plywood isn't really vapor open so the peal and stick would not be the issue as much as the moisture from inside. Air leaks carry 20X as much moisture as diffusion does so you are that much better off with a proper applied foam that is air type and installed properly.

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