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Is it proper to install a metal roof over an impermeable underlayment?
How does a roof perform with a permeable underlayment (such as tar paper), vs a peel and stick (like ice & water)? Is rust an issue/
Does the inevitable condensation between the layers dry out through the standing seams? Are purlins the solution to this problem, or do they themselves lead to more condensation?
A sample debate in wall construction is here:
I'm seeking for Zone 3C
Should I favor contractors using permeable or impermeable underlayment? How about if it's not plywood but 1x skip sheathing below?
If you have a properly vented attic, then there's no need for a vapor permeable (aka "breathable") underlayment. Traditional underlayments really do not breathe much ... though there are specific breathable synthetics on the market today. Purlins do not lead to condensation nor are they necessary in most cases. When condensation occurs, it is from warm moist air that originates inside the house and is trying to drive its way out. If it hits a cool surface, it condenses. If you can cent out the warm moist air, then there are no worries. Give me a call if you would like to talk this through. 1-800-543-8938 ext 201 or email me at [email protected]
+1. Roof deck breathable underlayment with a solid roof deck is...in a word...worthless.
In this particular job the roof is vented.
But the attic floor air seal is terrible, there's a huge attic stair that leaks like a sieve.
The new standing seam metal roof is to be installed over the existing 1x4 skip sheathing, so no solid roof deck.
Any moisture driven up from the attic will condense on the underlayment underside.
Climate zone 3C, so freezing temperatures only at night on occasional winter evenings with clear skies.
In that case, the breathable underlayment would be a hindrance.
Go with a standard poly underlayment in this case and get the attic floor air sealed and insulated.
Very little moisture moves through the attic via diffusion through a solid substrate. Most of it is via bulk air transfer.
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