Breathable Underlayment for Standing-Seam Metal Roof
oldhousesam
| Posted in Energy Efficiency and Durability on
I think I’m over thinking this a bit so help me out please. I am putting a new roof on my house. New sheathing underlayment then standing seam metal roof. Do I need a breathable underlayment (Like GAF DeckAromor) to allow any moisture that might get between the metal and the underlayment to dry to the un conditioned vented attic? or can I get away with a cheaper alternative? Thank you in advanced
P.S. Im in zone 6 of Pennsylvania on the IECC map
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Replies
#15 roofing felt tends to be the cheapest underlayment and it's permeable. The roof pitch is key in determining whether or not a synthetic underlayment is needed--they make sense for steeper pitch where the roofers need better grip, say greater than about 6:12. You need two layers of underlayment if the slope is less than 4:12. Felt works well in most cases.
The underlayment does not need to be vapor permeable. No drying is happening through the metal roofing, and it's fine to have two vapor retarders next to each other as long as there is not a space between them.
When the roofing is installed on battens to allow air flow underneath, a vapor permeable underlayment is usually a good choice.
Drying of the roof deck (osb or plywood) is the concern, with a vented attic it allows it to dry inward.
Impermeable underlayment should be fine.
15/30# felt may not be appropriate under metal roof, based on temp.
Also, open to feedback- but I don't think Ice and water is required under standing seam, not sure it would have a negative effect in your situation. (I got in an argument with my builder over this)
Anything that isn't airtight provides some drying/wetting. Metal roofs aren't airtight and there will be a little upward drying. Is there any practical advantage to far more drying downward? - the data doesn't appear to be available.
https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/171-Pinon.pdf
Thank you all for the feed back. That's good news I can get away with a impermeable underlayment( from what I have seen they are cheaper and more readily available). Like I said I think I was way overthinking it