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Community and Q&A

Styrofoam for insulating rim joists

trguitar | Posted in Energy Efficiency and Durability on

We just moved into a new home this summer, and I am finally getting around to air-sealing the basement starting with rim joists. Half of it had already been done with spray foam which keeps that area nice and warm. I don’t need to touch this area at all. But, some of the other half has been done using 1 1/2″ styrofoam panels. They are not sealed around the edges, just glued to the rim joist. There are no markings on them, so I can’t even tell who made them. They look like what those old styrofoam coolers were like. Just plain white styrofoam.

I know styrofoam is definitely not green, so my questions are these:

(1) do I remove them all, and replace with more green rigid foam, and seal around the edges?
(2) do I just leave it, add rigid foam on it, and seal it?
(3) do I just leave it, and seal it?

Thanks for your opinions!


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  1. GBA Editor
    Martin Holladay | | #1

    "Styrofoam" is a brand of XPS (extruded polystyrene) manufactured by Dow. It is always blue.

    If the rigid foam you are talking about is white, it is almost certainly EPS (expanded polystyrene), not Styrofoam. EPS is greener than Styrofoam (environmentally greener, not greener in color) because it is manufactured with a more benign blowing agent than Styrofoam. So there is no need to remove the EPS.

    For more information on this topic, see Choosing Rigid Foam.

    For more information on rim joist insulation, see Insulating rim joists.

  2. BillDietze | | #2

    If the styrofoam in already in place, then the least green thing you can do is take it out and throw it away. The blowing agent has already been released into the wild and the petroleum has already been pumped out of the ground. I vote against option #1.

  3. trguitar | | #3

    Good info. Thanks guys!

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