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Cost vs Benefit: Under-slab insulation, Zone 4?

oldbungalow | Posted in Energy Efficiency and Durability on

Looking into 2″ XPS or similar underneath the slab before pouring.
For Zone 4, approx 17 degrees F outdoor heating temp (99%) and 91 degrees F outdoor cooling temp (1%), what are the benefits of slab insulation?

For winters, if the foundation is 8-9 feet deep, which is below the frost line, is there sufficient cold through the slab to make the insulation worthwhile?

For summers, isn’t there a cooling benefit from ground temps, that might be lost with the insulation?

other benefits (moisture?) from slab insulation?

The basement will be a walkout, so I can sort of see the benefit at the walkout end where the slab is at ground level. Builder said it’s standard to insulate adjacent to walkout, but not sure about rest of the floor.


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  1. Nojak | | #1

    Not sure on the heating cost savings, but if you can make the floor warmer, it will be a lot more comfortable on your feet during the winter. Basement floors always feel cold to me, even with carpet. Unless they have radiant heating, which just feels awesome.

  2. Expert Member
    BILL WICHERS | | #2

    I'd do it, you never really get a second chance at something like this. If you want to keep costs to a minimum, you can usually find reclaimed XPS (and EPS) pretty cheaply in most areas. You can use EPS here too, but you need one of the better grades that is rated for compressive strength the way XPS usually is.


    1. Expert Member
      Dana Dorsett | | #3

      >"You can use EPS here too, but you need one of the better grades that is rated for compressive strength the way XPS usually is."

      That's a common misconception.

      A residential slab does NOT require a higher compressive strength EPS. Even Type VIII roofing EPS is fine under a 3-4" slab.

      The only place higher density EPS would be needed would be under footings, which would have to be engineered.

      Since XPS loses thermal performance as it's climate damaging HFC blowing agents leak out it's only warranteed to 90% of it's labeled R (R9, for 2"), and at full depletion will eventually drop to 84% (R8.4 for 2"), the same as EPS of similar density. In most markets Type-II EPS (1.5lbs per cubic foot nominal density) costs only about 75% per labeled R as Type-II XPS (eg Foamular 150), so 2.5" of Type-II EPS (R10.5) should even be cheaper than 2" of XPS (R10), and will outperform the XPS for life, since it's blown with low-impact pentane, most of which escapes the foam at the factory, and has stable performance over the decades.

      In climate zone 4 up to 2" of EPS (R8.4) would be cost-effect against energy use reductions on a lifecycle basis. Even more EPS is rational if using cheap reclaimed foam. See Table 2, p10 of this document:

      A primary value for basement slab insulation in zone 4A is to keep the slab above the summertime dew point temperatures. Without the insulation the slab will take up moisture from the summertime air, creating mold conditions on things resting on the slab such as rugs, cardboard boxes, etc. Lack of slab insulation is a common source of "musty basement smell" during the summers. As little as R3-R4 is usually enough to mitigate against that issue.

      The amount of "free" sensible cooling from a cold slab in summer is overcome by the requirement for mechanical dehumidifcation to keep the basement smelling fresh- it's not really a cooling season advantage in zone 4A. It might be in zone 4B, but it's a pretty small benefit at best.

      1. Expert Member
        BILL WICHERS | | #4

        Dana, it’s my understanding that type I EPS isn’t suitable for under slab use, but type II and better are (that’s what I meant by “better grades”). Am I correct here? I didn’t see you mention type I at all.

        I wish the box stores would start carrying type II EPS. the usual choices are crumbly type I, XPS, and a few thin options in polyiso. I think that’s one of the reasons most DIY people are only used to using XPS in most applications.


  3. RussMill | | #5

    Forget cost benefit. After, 20 years myself, 60 years of my dad builing in zone 4. THE EXTRA COMFORT IS WORTH EVERY PENNY YOU ARE GOING TO SPEND!!

    Our extreme temps over the last 50 years would show a benefit i feel certain. However, a 5 year weather/temp graph WOULD NOT show much benefit.

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