Replies

1. 
   creativedestruction | Oct 01, 2020 07:01am | #1

   #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.

2. Expert Member
   Michael Maines | Oct 01, 2020 07:45am | #2

   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.

3. 
   vashonz | Oct 01, 2020 09:45am | #3

   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)

4. 
   Jon_R | Oct 01, 2020 11:15am | #4

   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

5. 
   oldhousesam | Oct 01, 2020 05:08pm | #5

   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