GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Zone 2 , 3” ext insulation on roof vs 2” on roof and 1” on walls

user-7720303 | Posted in Energy Efficiency and Durability on

I am building in zone 2.  Savannah ga. I want to add exterior foam and can likely only afford either 3” on roof or 2” on roof and 1” on walls.  I have 2×4 walls and 2×6 roof.  I will spray foam open cell on both. My living room is large with cathedral ceiling.
thank you

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #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:

    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.

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

  2. user-2310254 | | #2

    Also be sure to read this article on cathedral ceilings:

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

  3. walta100 | | #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.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |