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Is “low GWP XPS” insulation really that much better?

jollygreenshortguy | Posted in Green Building Techniques on

The recent GBA article “Passive House Today” referred several times to “low GWP XPS insulation”.
I’ve been avoiding XPS in favor of EPS due to XPS much larger greenhouse warming potential (GWP). I understand that manufacturers are revising their XPS formulations to reduce the impact of XPS.
But is it really comparable to EPS now?

For example Dupont is making a big noise about the rebranding of its Styrofoam now that it has converted its formulation for all its Styrofoam products.
But how much better is it and how does it compare to EPS in terms of GWP?
Are there any independent 3rd party tests to quantify the improvement?

Also, the 2 areas where I typically spec EPS are as exterior insulation on roofs, and slab-edge and underslab insulation for foundations.
Is there any practical advantage (durability, longevity, etc) for using XPS over EPS in those applications?

And I guess while I’m on the subject, where does polyiso fit into all this?

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Replies

    1. jollygreenshortguy | | #3

      Thank you for the link. Yes, I'm familiar with that article and read it when it came out in 2021. That's part of the problem. Manufacturers are making changes and claims now that may put articles from 3 years ago out of date. But unless we get some new independent analysis it's hard to know how much is real and how much is greenwash. I wish I had the knowledge to do an update on that article. It's a good one.

  1. stamant1 | | #2

    don't know the GWP for the new formulation. For the installer Dow/Dupont's switch to ST-100 amounts to a change from blue board to grey board. Dupot does have an XPS with a slightly higher R-value than the 5 per inch, but it is less common and more expensive.

    In the commercial world generally i am only seeing EPS as geo-foam below grade in planters.
    for below grade vertical walls, I see 25 PSI. Underslab insulation for plazas receiving vehicular or pedestrian traffic and occupiable space below is almost always XPS with a diffusion open condition created by an air layer.

    For conventional roofs, poly-iso is generally preferred since the increased R-value per inch for poly-iso means the tapered system is less thick at the high points of the roof. EPS is generally cheaper than Poly-iso as a material, but also for installation since you can get thick blocks. This cuts down on the number of fasteners or quantity of adhesive which can add up quickly. [you can get a 14" block of tapered EPS with one layer of adhesive easily. to get 12" of poly-iso for an equivalent R-value is at least 4 layers of adhesive.] [discounting specified max thickness and required seam offsets.]

    For IRMA configurations, I am not familiar with EPS, but do see a lot of XPS under green roofs and terraces. I know that EPS had a reputation for absorbing water in IRMA configuration, but don't know if XPS would suffer the same without a open diffusion layer.

    mostly however, it boils down to total system warranty. if you have a roofing manufacturer that has a system with EPS warrantable for 20 years, and the project is extremely cost sensitive then EPS definitely bears looking into. If durability and long term performance is more important, then most projects are opting for XPS or poly-iso insulating systems.

  2. Expert Member
    Michael Maines | | #4

    One of the bigger problems with all foams is that the industry has decided that they only need to report the GWP of the blowing agents, not of the entire product. Doing so helps users feel better about what is usually poor decision making.

    This is a current document: https://www.dupont.com/content/dam/dupont/amer/us/en/performance-building-solutions/public/documents/en/dupont-optimization-assessment-styrofoam-reduced-grey-vs-blue.pdf. It shows--assuming a 75-year use-in-service, which is dubious--that their legacy XPS has a GWP of 100 kg of carbon emissions for every 1 kg of product used, and that their new version has 6kg of carbon emissions for every 1 kg of product.

    That is indeed a big move in the right direction, but it's still a lot of carbon, over 40 kg--almost 100 lbs of CO2--for a typical 4'x8'x1.5" sheet.

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