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Community and Q&A

Cutting really thick EPS foam

clindberg | Posted in Green Building Techniques on

I have a job coming up where there is going to be 8″ of foam under slab.
The foam will be glued at the joints.
Over the years I have cut foam a number of different ways with varying levels of success.

I have used Tajima knives, circular saw, sawzall, handsaw, sharpened putty knives, table saw.

All of them worked ok, none great.
For this job I want clean, straight, glue-able cuts and I have never worked with foam this thick so I am a bit nervous about too much trial and error since it will all be custom ordered. My main problem in the past when I have had foam that is thicker than my cutting implement is having the cuts stay square.

Any thoughts or advice from folks who have dealt with really thick foam before? What has worked for you?


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  1. charlie_sullivan | | #1

    A few thoughts:
    1) Why not two layers of 4"?
    2) A hot wire cutter is how this is done in foam factories. There's a wide range of products from very cheap hobbyist stuff to very expensive production equipment. I don't know where the sweet spot is.
    3) Another thing I've heard suggested is an electric carving knife, sold for carving turkeys and the like.
    4) I asked a similar question not too long ago; if you want to see the comments there, here's the link:

  2. GBA Editor
    Martin Holladay | | #2

    My first reaction to your question was the same as Charlie's: Why won't you be installing multiple layers of thinner rigid foam with staggered seams? Staggering the seams results in better thermal performance.

    I have never heard of anyone ever attempting to use glue at rigid foam seams. The way to go is to use multiple layers.

  3. user-3133848 | | #3

    Cutting the foam is a breeze, even with a sharp hand saw. Just stay out of the wind or you'll be chasing foam bits around the neighborhood!

    Perhaps Colin found a sweet-heart deal on thick, salvaged EPS as I did at our deconstruction non-profit. $ 300 for 1600 sq ft -- however, in my case the thickness varies from 7" to 9". Since we plan to lay it on a deep bed of drain rock and cover it with a rat-slab in a conditioned crawl space, should I have a concern with a less than perfect substrate so long as the concrete finishes out flat and we maintain a 4" minimum thickness ?

  4. Expert Member

    At four to six inches thick that's some rat-slab.

  5. GBA Editor
    Martin Holladay | | #5

    If I were you, I would adjust the crushed stone layer under the 9" foam with a rake, so that the upper face of the 9" foam is level with the upper face of the 7" foam.

    And a 4" slab is just fine, if that's what you want, but (as Malcolm noted) it's more heavy-duty than the typical rat slab.

  6. user-3133848 | | #6

    Thanks for the comments. What would the minimum rat slab thickness be typically ?

  7. Expert Member

    On a good compacted substrate we usually pour 2". There are no live loads in a crawlspace. The slab is just there to provide mechanical protection for the foam (or poly), a smooth surface to crawl around on and, as the name implies, keep pests out.

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