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

Polyisocyanurate vs. Phenolic Foam Board

RussellKM | Posted in Energy Efficiency and Durability on

I am retrofitting my 30-year-old timber frame/ISP covered house.  (I built the place in 1991 when no one was talking about rain screen systems.)   I’m replacing all windows but am wondering about adding foam boards to the outside of the outer OSB skin.
Is phenolic better for the environment and insulation value than polyisocyanurate?  If so, where can one purchase phenolic foam board in the U.S.?

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. user-2310254 | | #1


    Do you mean SIPs (structural insulated panels)? Where are you located?

  2. RussellKM | | #2

    Back when I bought them from Winter Panel near Brattleboro, VT., they were called double chip stress skins. Times change vocabulary.
    Yes, my mistake, I meant to type SIP. I am in north central Massachusetts. However, not sure how that addresses phenolic vs. polyisocyanurate foam board. Any ideas?

  3. Expert Member
    Michael Maines | | #3

    Phenolic foam would be better, both environmentally and in R/in, but it's nearly impossible to get.

  4. user-2310254 | | #4

    Hi Russell,

    I came across this thread and think you will find it interesting:

    I asked about climate because it helps responders give more accurate advise. I’m no expert, but the referenced discussion suggests to me that adding insulation would be risky. If it were my home, I would focus on air sealing the panels, adding a water resistant barrier, and creating an air gap (furring) to facilitate drying. Adding insulation at the roofline might have value, but I would first check out the condition of the deck. If you have a SIP roof, I would want to rule out any rot issues, which seems to be common problem with that particular system.

    1. RussellKM | | #5

      Thanks so much for the link. It is not definitive, but I am going to take the advice given in the link and go with rockwool (not easy to get right now). My roof is not SIP (loose EPS panels I made with R-50 also with double roof construction.)
      BTW there is a way to attach SIPS with no thermal bridging. Drill a hole through the outer OSB and foam until you reach the inner OSB. Then lag bolt the INNER OSB to the timber frame. After attaching the bolts, fill in the drilled holes with foam.
      Thanks again for the link and the information. Russell

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |