What is the correct underlayment for metal roof

Guest User
2/17/2010
I'm watching our metal roof install. The underlayment looks a lot like I&W shield. It has a sticky backing and a material (felt like) top layer. It doesn't look like breathable material. The house was built in 1954 and despite periodic leaks, poor ventilation, and minimal insulation the decking is still in very good condition. Should I be worried?
Info @windowsonwashington.net
An informed customer is our best customer.
2/17/2010
No. The roof does not ventilate through the plywood decking regardless. If you are worried about moisture accumulation under the roof deck, I would look into air sealing the attic first and you will effectively eliminate the necessity for attic ventilation and any concerns with moisture migration.
Guest User
2/17/2010
Thanks this is a shed dormer roof about 14x28 ft. There is no attic, the area directly under the roof is living space. So I'm not sure if I can do any air sealing. If the decking doesn't ventilate why is there so much talk about the need for breathable underlayment?
Info @windowsonwashington.net
An informed customer is our best customer.
2/17/2010
Here is a good site to look at the permeability rates of certain plywood. http://www.performancepanels.com/?content=app_pp_atr_perm The underlayment is designed to be vapor permeable because the plywood seams are not airtight and whatever moisture diffuses through the building will need a pathway out. If you used something like a poly plastic, that moisture would get trapped against the plywood and foster rot. If it is a shed roof, make sure that all penetrations fissures in the building envelope (in this case the drywall ceiling and where the drywall meets the floor) are caulked and air tight. That should mitigate most bulk moisture migration and the diffusion through the drywall is slow enough to be dried to inside or outside.
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