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

Help with Exterior Insulation in Climate Zone 5

scottwoodward | Posted in Green Building Techniques on

I’m in the process of adding exterior polyiso insulation to my small 24’x32′ garage-apartment in Strafford County, New Hampshire (Zone 5) with the first six panels of insulation on. In scouting GBA for the answer to a different question, I either missed or failed to read an important part about the lower performance of polyiso in cold climates and that builders should derate the R-value of polyiso when determining how much exterior insulation to add. I missed that the two inch standard for Climate Zone 5 applies to EPS. I purchased two inch polyiso, thinking that I was meeting the dew point requirements for Climate Zone 5. The house is framed with 2×6 framing and ZIP wall sheathing. There will be a rain screen between the insulation and Hemlock siding.

My questions are:
1.) Should I hit full stop and switch to EPS or XPS, or move to a 2.5 inch polyiso? No matter what, it looks like 2 inches of polyiso isn’t going to cut it?

2.) Or, does the fact that I’m installing mechanical ventilation (ERV) help with controlling any potential issues with the lower performance of the polyiso in the colder months?

3.) Is there anything else that can be done to offset the risk of installing only two inches of polyiso versus bumping up to 2.5 inches of polyiso?


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
    Michael Maines | | #1

    Even in CZ6, I am comfortable with 2" polyiso over 2x6 framing insulated with fluffy insulation. I would not use less than 2" and it is cutting it close on the prescribed ratios but even de-rated for temperature and age it's still more R-value than 2" of EPS or aged XPS. You can reduce the risk by keeping the indoor relative humidity to 50% or lower, and/or by using a variable permeance membrane on the interior. Or by using R-13 cavity insulation, though that wouldn't be my first choice.

    1. scottwoodward | | #6

      Thanks for the feedback Michael.

  2. kyle_r | | #2

    2 inch polyiso, even derated to ~R5 per inch, is sufficient for Zone 5. In Zone 5 you need a minimum of R 7.5.

    1. scottwoodward | | #5

      Thanks Kyle

  3. brian_wiley | | #3

    Hi Scott,

    My understanding as a non-pro is that the de-rating of polyiso is a bit overblown (pun intended). It is true that it suffers a little in cold weather, but from what I gather it’s only that outermost portion, so you’ll still be getting the full r-value for at least half of that 2”. I would assume that’d keep you well above the r-7.5 required for a standard 2x6 wall assembly in cz5, but maybe a someone with a bit more experience can confirm.

    This also may be helpful:

    1. scottwoodward | | #4

      Thank you Brian.

    2. Expert Member
      BILL WICHERS | | #7

      You are correct, and the usual "derating" done with polyiso is to "derate" it to R5 per inch from the listed R6 (or a little over) per inch the manufacturers rate it as. With polyiso derated to R5 per inch, you are now at the rated R value for brand new XPS, and better than the R value for EPS, which is usually around R4.2 per inch or so. Basically polyiso puts you a little ahead, or at least as good as, XPS or EPS, even when derated for cold temperature performance.

      BTW, I have seen this "derated polyiso for the cold" come up a fair bit in the forums over the past few months. This has encouraged me to put a bit of effort into doing some of the design work for my test rig so that I can make some measurements under actual conditions to post here. With a little luck, I'll have enough time over the next several months to be able to do more than just a "little work" and to actually fininsh the thing and be able to finally settle the age old question of exactly how does polyiso perform under various winter time temperature conditions.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |