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

Any other non-derating Polyiso brands? Alternatives to Thermax

mdb_az | Posted in General Questions on

Heyo, I’m unable to find 2″ thermax anywhere. Building in CZ6, 2×6 wall w/blown-in and 2″ rigid exterior specified as thermax. Suppliers locally can get “Rmax” brand…or should I just go with 2.5″ in whatever brand is available?

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. Patrick_OSullivan | | #1

    You need to find out why Thermax was spec'd. Thermax has thermal barrier properties that are typically relied upon for using in some situations without an additional layer. If that's why it was spec'd (perhaps because of some local fire code), there aren't many alternative options. If it was spec'd just because it's a name brand (e.g. Kleenex), I would go with another product.

    The only place I could get reasonable quantities of Thermax from was White Cap (

  2. Expert Member
    BILL WICHERS | | #2

    Thermax is usually specified for one of two main reasons, the first being that one side is rated to be left exposed. If that's the reason you need Thermax here, Johns Manville's CI Max is a comparable product.

    The second reason is that Dow claims (and has some data to back up those claims) that Thermax doesn't reduce in R value as much as some other polyiso variants at cold temperatures. I'm not aware of any comparable products in this regard, but you could always just use the usual "derate it to R5/inch" and call it good with anyone's polyiso in this case.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |