Metal directly to foam?

Jeff Cook
10/31/2011
I am planning to install a standing seam roof shortly. One of the ways it is being done in this area, is to place sheets of foam over the shingles, then screw through the foam into the plywood decking. Will this system cause condensation problems to my new roof? Trussed roof, 1/2" 4 ply plywood decking, shingles. 6/12 pitch, well ventilated. Thanks
Todd Miller
Classic Products, Inc.
10/31/2011
The foam will not cause condensation. I would put underlayment on top of the foam. Also make sure that the metal you're installing is approved for this type of application.
Info @windowsonwashington.net
An informed customer is our best customer.
10/31/2011
What is the point of putting foam on the roof if the attic is ventilated? If the insulation layer is the attic floor, putting foam on the roof does nothing and you would be better served to remove the shingles and blow in some more insulation. I would always prefer to attach the roof directly to the deck when possible.
Jeff Cook
11/1/2011
The foam keeps the metal from resting directly on the shingles. It also serves as a thermal break, keeping the attic temperature closer to the ambient temperature. Todd, what kind of underlayment would you recommend? Thanks to both of you for your quick reply's.
Todd Miller
Classic Products, Inc.
11/1/2011
Thanks Jeff. I have to agree with windowsonwashington in that generally I do not think the roof deck is the place for insulation. Money for upgrades is better spent usually on increased attic floor insulation and increased attic ventilation. That said, there will be some benefit from insulation no matter where it is located. I would encourage you to choose a metal roof with decent reflectivity either through light color or reflective pigments in the paint. Your thermal break point is valid, but can also be achieved through choosing a metal roof that, by its design, has a gap between the metal and the underlayment. You may be required by code in your area to use self adhering ice and watershield on part or all of the roof. Other than that, really, the synthetics available today are al pretty similar. The one unqiue one is Sharkskin as it is polypropylene rather than polyethylene and as such has less shrinkage. That probably means more to the installer than it does to the homeowner though. All Best.
Info @windowsonwashington.net
An informed customer is our best customer.
11/1/2011
Jeff, I understand the idea of foam in this application, however, in applying the foam in this case, you are mixing the insulation layers. If you wanted to move the insulation layer to the roof deck than by all means, do that but make sure it is proper thickness and R-Value. Over 95% of the roof cooling happens to the outside via convective looping. If you attic insulation layer is intact and sufficient, the benefit to reducing the attic temperatures is technically nil or non-impact. If you have a secondary cooling system in the attic, that is another consideration but should be properly sealed and insulated. My issue with foam on the roof deck is nothing to do with the foam but the attachment of the panels. It is very easy to over torque the panels and create waviness in the panels. If you wanted to make that roof really efficient, consider installing it on a set of purlins for over deck venting. Feel free to contact me directly to discuss as I am not a proficient at typing. Also, don't mistake my recommendations for being critical of your approach. I just want your roof to perform as optimally as possible and get you from point A to point B.

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