eps / xps same?

andrelaplume2April 16, 2009

Many posts have mentioned using 1" XPS or 1" EPS between concrete and framing as a mositure inhibitor. I have always heard EPS had a lower R value but was cheaper. I also heard it came in 4 X 8 sheets. I never saw the product until recently in my local lowes. The tag says Extruded Polystyrene. It looks to me like styrafoam. Its 1" r value is 3.9 compared to an r value of 4 for the pink/blueboard. I can live with the lower r value since I will be adding R13 fg between studs anyway. On each side of the EPS product is a plasticy, shiny material (not foil) that has the product name and says underlayment. Are you suppose to peel this off or just leave it on...it almost like a syran wrap consistency?

Does this EPS truly inhibit mositure as well as the XPS pink board...it is cheaper at $13.50 for a 4x8 as opposed to $8.5 for a 2x8 sheets of the pink board.


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XPS (also known as XEPS) is extruded; its R Value is typically R5, not R4.

EPS (sometimes known as MEPS or polystyrene bead board) is expanded; its R Value is typically R3.85. Look at it closely, and the beads are obvious. It's the same as Styrofoam packaging material.

Building Science Corp. says either material may be used below grade for insulating basement interiors. However, XPS is the better choice if there is a possible moisture problem from liquid water penetrating the foundation. (See Link, RR-0202 at p. 13) You obviously need more EPS to get the desired R Value than you do with XPS.

Here is a link that might be useful: Building Science Corp. Research Reports

    Bookmark   April 16, 2009 at 2:39PM
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Yes, I see R3.9 vs R5...no matter since I will also use R13fg.

I have never seen any mositure, two of the 3 walls sit under a garage and sunroom, I can not imagine an issue.

So I would think the less expensive EPS would esily suffice...many are arguing I need NOTHING here and that its overkill and a waste of $$$$ since the walls are under the garage and sunroom. What do you think?

    Bookmark   April 16, 2009 at 4:36PM
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I'll stick with the building scientists. I'd go with R5 worth of EPS followed by fg. Lstiburek says R5 is the minimum you can use. (If this were a new home with a 9ft ceiling I'd probably use cc sprayed foam followed by fg.)

    Bookmark   April 16, 2009 at 7:26PM
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BTW, the EPS you describe above is underlayment for siding. For the basement, use EPS that doesn't have plastic film on it.

    Bookmark   April 16, 2009 at 7:38PM
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then I am back where I stared, no product by me. Can I use this undelayemnt and just pull the plastic off---it appears to lift off easily...in fact I was not sure if it was meant to be removed?

    Bookmark   April 17, 2009 at 10:36AM
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If the film comes off without damaging the board, I don't see why you couldn't use it. I'm surprised you can't find EPS at any big box store. If not, try a more traditional lumber yard or building supplies outlet.

    Bookmark   April 17, 2009 at 1:22PM
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Sad, another private yard just closed by me...that leaves one plus the box stores. HD had some banged up 3/4" 2X8 sheets of XPS...wish it had been 1"!

    Bookmark   May 27, 2009 at 2:08PM
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