Replies

1. Expert Member
   Dana Dorsett | Jan 03, 2020 05:23pm | #1

   "• Remove ridge vents. Replace shingles with ice and water shield on eaves & valleys. Synthetic roof underlayment, 2x 2” polyiso rigid foam panels (4” total), staggered seams and metal roof.
   • 4″+/- closed cell foam under cathedral ceiling with v groove pine planks"

   I was with you up to the point of the 4" closed cell foam. Filling the 5.5" rafter bays with R21 fiberglass (or even R19s) or open cell foam would perform BETTER than the 4" closed cell foam at a tiny fraction of the cost, environmental and financial. The roofing underlayment is more than adequate air sealing, and the 4" of closed cell foam limits the drying rate of the roof deck, buying you less than nothing on thermal performance, since it's performance is being robbed by the thermal bridging of the rafters. Do the math:

   https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

   There is nothing "green" about closed cell foam, even if blown with HFO blowing agents:

   https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

   Anything more than the minimum necessary for dew point control on more vapor/air permeable insulation is trashing the planet, and in this case, for no good reason: As little as 1" of polyiso above the roof deck is sufficient for dew point control at the roof deck and you're planning on 4". With that much exterior-R the interior side insulation can (and arguably should, from a resiliency perspective) be more vapor-open.

   >"• Open cell foam in attic including exterior wall caps and all gabels. (Fill rafters)
   o Remove old ceiling insulation"

   Unless you're installing the full IRC minimum R38 of open cell foam (~11" of half pound foam) or the 4" of polyiso will be above the roof deck in that section too, leave the old ceiling insulation in place except where needed to achieve an air seal the roof deck to the exterior wall top plates where the roof meets the attic floor. It's still doing something, and an R20-ish 2x6 rafter bay fill is woefully under-insulated for your climate.

   You may need to install a broadsheet membrane type vapor retarder on the underside of the open cell foam to avoid ping-pong moisture issues in the attic in summer.

   >"• Purchase polyio as funds become available
   i. Insulate above the deck with polyiso then cover with second layer of synthetic underlayment until I can get the metal roofing"

   At least one of those underlayments needs to be at least semi-permeable to moisture, or the polyiso can take on moisture over time.

   Used roofing polyiso from commercial building demolition/re-roofing can usually be had for very low money- cheaper than batt insulation per R, and is by far the greenest foam insulation possible, since no new polymer or blowing agents are used, only some minor carbon footprint from stripping & shipping. In my area 4" of 2lb roofing polyiso from reclaimers usually runs a bit less than a buck a square foot (<50 cents for 2"), plus delivery charges if you don't have your own truck.

   Run this search on your more local craigslist every now and then. You're likely to eventually find some a bit closer to you:

   https://atlanta.craigslist.org/search/sss?query=rigid+insulation

   https://jacksonville.craigslist.org/search/sss?query=rigid+insulation

   >"• Do I need some type of barrier/layer between the foam and pine ceilings?"

   Yes- pine plank/shiplap does not qualify as a thermal barrier against ignition, but half-inch gypsum board does.

2. 
   bk2a_3a | Jan 04, 2020 11:49am | #2

   Thanks Dana,
   I was basing the cathedral ceiling on the gba article regarding same. Making an airtight roof assembly. There seems to be two schools of thought regarding insulating unvented cathedral ceilings. I would much rather use batts VS spray foam if I can get similar performance.
   -I thought the polyiso over the decking would eliminate thermal bridging through the rafters?
   - yes I am using polyiso over the entire roof assembly. Goal being hit code requirement. R20+/- over deck (using 4" polyiso); r20+/- under deck @ rafters (by whatever means necessary).
   - thank you so much for the Atlanta/Jax cl idea. I looked in my local and found nothing. Nationwide recycling is much higher.

   Thank you again for your help today and all of your advice over the years.
   Have a great new year.
   bk