GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

XPS vs. EPS vs. polyiso shrinkage rates

lance_p | Posted in Energy Efficiency and Durability on

I’ve done some searching and found lots of claims about foam panels shrinking but have yet to see any hard numbers comparing the shrinkage rates for the big three foam types.

My application is continuous insulation and air sealing on the inside of a 2×8 garage wall insulated with cellulose and sheathed with OSB, CZ6A.  As it’s on the inside of an insulated wall I’m not worried about the cold performance of polyiso.

From inside to out:

Foam (continuous, taped)
2×8 stud wall dense packed with cellulose
OSB (continuous, taped)
WRB (Typar/Tyvek)
Strapping (vented rain screen)

I would like to use either EPS or Polyiso for cost and environmental reasons.  I’m leaning towards foil faced Polyiso but can’t seem to find any reliable shrinkage data for it?

Also, since I assume it does shrink somewhat, what are the best options for dealing with the shrinkage?  I was wondering if cutting 8′ sheets down to 4′ is worth it in order to minimize shrinkage of individual panels?  This way the resulting gaps would be smaller, but there would be more of them (and more tape).


If Polyiso is the best choice for that application, would it also be the best choice for insulating the inside of a poured concrete basement wall where it’s sandwiched between the concrete and an interior side 2×4 wall stuffed with fiberglass?

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

    There is no really good data on this since the processes and blowing agents have all changed compared to what was manufactured installed 30-50 years ago. The manufacturers are all aware of the issue and "working the problem" and there appears to have been incremental improvement, but it's difficult to quantify that without controlled experiments. Putting it on the conditioned space side of the assembly is also not representative of how the vast majority of rigid foam is installed, and with less temperature & humidity cycling is likely to have different shrinkage rates than foam that experiences much larger seasonal swings.

    Cutting panel sizes down won't change the shrink rates- it just distributes the gaps between more opening, making it harder to air seal initially.

    Installing the foam in multiple layers with overlapping seams is one way to manage it. Installing only reclaimed foam that has undergone most of it's shrinkage is another.

    Installing polyiso between the foundation and a fiberglass insulated studwall is fine, as long as the cut bottom edge of the polyiso doesn't rest on a potentially damp slab.

    1. lance_p | | #2

      Thanks Dana. Being a garage I'm not THAT worried about airtightness, especially since the inside air will be dry. I will be taping the OSB on the outside, using a WRB and filling the cavities with cellulose, so even if the odd gap opens up in the foam it won't be the end of the world.

      I just dug up this 6 year study that concluded in 2015:

      showing a roof in BC studied for polyiso shrinkage and r-value. They were monitoring 4'x4' sheets and noted gaps up to 6mm (1/4") after 6 years. That's actually pretty encouraging considering they are using roofing grade polyiso and their instruments recorded a 63C temperature range. I'd be using foil faced boards in an environment that won't swing much beyond 25C, maybe 30C and will probably maintain a much lower humidity level.

  2. drewintoledo | | #3

    Did you end up with polyiso on your build?
    I'm curious if graphite impregnated GPS has the same shrinkage issues as EPS. Does anyone know?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |