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Under-slab insulation: XPS or EPS?

Claire Anderson | Posted in Green Building Techniques on

We’re building a slab-on-grade home with radiant floor hydronics in the floor, and are wondering what type of rigid-foam insulation is best for under-slab insulation. (We’re planning on 3″ inches of insulation, no matter what product we use.)

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Replies

  1. Robert Riversong | | #1

    Both XPS and EPS have been used successfully below grade, but XPS is still the standard because it typically has more compressive strength and higher R-value per inch.

    Your thermal design program should specify a certain required R-value for sub-slab insulation rather than a certain thickness. 3" of EPS would offer no more than R-12, while the same thickness of XPS would give you R-15 (a 25% increase).

    Where are you building and what is the heating Degree-Day total for your climate zone? In a cold climate (>7200 HDD) and for a highly-insulated house, R-20 to R-30 subslab insulation is commonly recommended for radiant slabs. Be sure to insulate the slab edges at least to the level of the sub-slab insulation, as most heat is lost from a slab at the edges to the ambient air temperature.

  2. Anonymous | | #2

    Hi Robert ~
    We're building in southernmost Oregon (Medford area), with a HDD total of 4,566. Code calls for R-10 under-slab insulation, but we'd like to boost it to R-15, since we're relying on radiant floor and passive solar heating.

    I did find a good PDF on slab insulation details at buildingscience.com. Do you have any other sites to recommend?

    Thanks,
    Claire

  3. Robert Riversong | | #3

    I assume you've seen the Oregon Residential Energy Code brochure on slab insulation: http://www.oregon.gov/ENERGY/CONS/Codes/docs/ResPub_9.pdf.

    They require R-15 slab edge insulation for all heated spaces. The slab edge insulation value can be the combined R-value of perimeter insulation and slab-edge, if it's not a monolithic pour. But since the majority of heat loss is from the edge, if you're increasing subslab insulation to R-15, you might consider increasing slab-edge insulation to R-20.

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