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Community and Q&A

Should I use XPS or ISO on my house?

Sidingguy2013b | Posted in Green Products and Materials on

We are planning on covering our house with one inch of rigid foam in connection with a gut renovation. The house is located in Zone 5A and is constructed with 2×4 walls that will be dense packed with cellulose.

One inch of XPS or ISO will suffice from “dew point” perspective to keep the dew point out of the wall cavity. My concern is that the ISO could potentially take on water resulting in a r-value of less than 5 and that based on my research it would have an effective r-value of less than 5 in situations in which it is very cold outside.

From an environmental perspective, the ISO seems more “friendly”, but the XPS is now waterblown and therefore has less of a negative impact on the environment.

Does anyone have any thoughts on these issues? I’m currently leaning towards the XPS, but would love the benefit of the wisdom of the folks on GBA.

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  1. GBA Editor
    Martin Holladay | | #1

    I have never heard of XPS insulation that uses water was a blowing agent. What brand are you talking about? Was it purchased in the U.S.? Who told you that the XPS is "water-blown"?

  2. nkurz | | #2

    The US Environmental Protection Agency has a good summary of the environmental impacts of different blowing agents for XPS:
    No mention of using steam for XPS, although some manufacturers have switched to CO2, which when used in this manner is quite neutral. I don't know how CO2 blown XPS compares to Steam blown EPS.

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