Replies

1. 
   creativedestruction | Jul 07, 2020 07:53am | #1

   "is it best to locate the air barrier on the lower sheathing layer as stated by taping the seams?"
   -Yes. Not sure of your climate zone, but this strategy minimizes moisture accumulation on the underside of the roofing during winter and gives the best opportunity for continuity of your air barrier if you can also tape to the wall sheathing. An insulated and protected air barrier like this sees much less movement and moisture cycling stress over the life of the building.

   "Is it worthwhile to also tape the top layer of foam as well, or is that unnecessary?"
   -It's less important than the primary air barrier covered by the foam, but taping one or both layers of foam seams can further reduce convective heat loss. Effect is likely marginal, considering you have 2 layers allowing seams to be overlapped.

   "Finally, with the interior moisture drying inward from that air barrier inward, what kind of roofing underlayment is best to go under composition shingles? Does it need to be a self-adhered membrane? Or will roofing felt work?"
   -Either works fine, but the sheathing and insulation should be dry prior to shingle or impermeable underlayment installation. Felt is permeable, but crinkles after rain exposure so you're really racing against weather either way. If your pitch is steep (>6:12), i like self-adhered underlayment. Roofers grip it rather than rip it!

   Cheers

2. 
   Jon_R | Jul 07, 2020 08:21am | #2

   There is more upward drying through asphalt shingles than most people think. Which creates some advantage for permeable underlayment.

   http://rdh.com/wp-content/uploads/2015/03/The-Problem-with-Ventilated-Attics-RCI-March-2015-Nov-5-2014-For-Printing.pdf

3. 
   creativedestruction | Jul 07, 2020 08:51am | #3

   3-4% lower winter MC at #15 felt vs. 40 mil self-adhered bituminous underlayment--good to know! Thanks, Jon.

4. 
   snc_seattle | Jul 07, 2020 08:55pm | #4

   Thanks so much for the advice and the link!