Replies

1. Expert Member
   Dana Dorsett | Jun 05, 2021 10:00pm | #1

   You can probably afford more exterior foam if you go with reclaimed/used roofing polyiso, which tends to run about 1/4 - 1/3 the price of virgin stock goods.

   With 5" of roofing polyiso (R28.5 labeled, R24-R25 derated for age) you could even skip the R20 open cell foam in the rafter bays and still meet or beat IRC 2018 code on a U-factor basis (though there may be some benefit to 3" of o.c. foam for air tightness). At 4" (two layers of 2" , seams staggered) you'd still need the full 5.5" of oc. foam.

   This place in Knoxville has some odd sizes of used foam in good/great shape all palletized for shipment:

   https://atlanta.craigslist.org/atl/mad/d/knoxville-polyiso-rigid-foam-insulation/7328451065.html

   Looks like with that vendor you could double-layer the 2-1/4" stuff (4.5", ~R23) at about a buck a square foot before shipping. A 5/8" CDX or OSB roof deck would be installed over the foam using pancake head timber screws a minimum of 3/4" longer than the foam thickness to ensure penetration of the structural roof deck. Any standard roofing layups would then go on to the nailer deck. It would also need a facia board at the edges to trim out & protect the foam. (At 3" of exterior foam a commercial building 4" drip edge could work in lieu of a facia board.)

   There is plenty of 4x4' and 4x8' sheet stock stacked up in salvage warehouses all over the US. Nationwide Foam always has something in stock in their regional depots for cheap.

   https://nationwidefoam.com/roofing-insulation.cfm

   Reporposed Materials does too. They currently have some polyiso in their South Carolina facility:

   https://www.repurposedmaterialsinc.com/polyiso-insulation/

2. 
   user-2310254 | Jun 06, 2021 09:14am | #2

   Also be sure to read this article on cathedral ceilings: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling.

   Avoiding penetrations like recessed lights will go a long way to keeping the assembly safe over the long-term.

3. 
   walta100 | Jun 06, 2021 11:09am | #3

   I see the use of spray foam in new construction plans as a red flag pointing to poor design choices, bad planning by someone looking for an easy fix with little regard for the cost in dollars or to the environment. If you avoid using spray foam you are likely to end up with better performance at a lower cost.

   Consider making the walls taller with a flat ceiling full of cheap fluffy insulation while resist the temptation to put the HVAC equipment in the attic. I find a room with a 12 foot flat ceiling just as dramatic as shorter wall and sloped ceilings.

   If the sloped ceiling is a must have consider a scissor truss with room for fluffy insulation.

   My guess is either of my 2 options would have a higher R value while costing less to build.

   https://www.greenbuildingadvisor.com/article/insulating-scissors-trusses

   Walta