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Community and Q&A

Crawl Insulation–Walls or Floors?

Stockwell | Posted in Energy Efficiency and Durability on

Zone 4, Asheville NC build on a steep slope. Because of the slope, we will have a sub-basement crawl that is as much as 10′-12′ on the low side and 2′ on the high side. The floor will be dirt with the same slope as the hillside. It will be sealed with 12mil poly. The local energy rater said I could have either the walls insulated, or the floor insulated–it makes no difference. It will have a duct feeding conditioned air to it per code, and a dehumidifier. Basement floors will be quartersawn white oak over advantech. I would value your opinions on where to put the insulation for my particular situation. Is the 50 degree earth a benefit or a negative? The builder talks about needing only R-10 or R-15 either way–is that sufficient? The floor uses trusses, if that makes a difference. I would be fine crawling around down there and screwing foamboard into the bottom of the trusses and taping it up if that is most ideal. Would love to hear your thoughts.


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  1. GBA Editor
    Martin Holladay | | #1

    In North Carolina, there is no contest here. You need to insulate the crawl space walls (especially if you plan to condition the crawl space). For more information, see Building an Unvented Crawl Space.

  2. Stockwell | | #2

    That's what I will do! Is R-15 of CC foam enough?

    Thanks and welcome back Martin.

  3. Stockwell | | #3

    Just bumping this as my final plans get closer. Would 2-3" of CC foam be enough insulation for the walls? The flooring above the crawl will be hardwood--will that have a chill to it in the winter? If so, what to do about it?

  4. Expert Member
    Dana Dorsett | | #4

    A mere 2" of closed cell foam would BEAT the IRC 2015 code for R10 continuous insulation for a crawlspace or basement wall. Three inches might be considered overkill.

    BA-1005 calls out R15 as the likely maximum economically rational level for zone 4. See Table 2, p10:

    Most HFO-blown closed cell foam would be R14 @ 2", and clearly good enough. HFC blown closed cell foam would be about R12, and still good enough (though nastier to the environment.)

    BA-1005 also calls out R7.5 on the crawlspace floor. A couple inches of reclaimed EPS below the ground vapor barrier and a 2" concrete rat slab above might still be "worth it", if you have sufficient head room to work with.

    There seems to be a few foam reclaimers working in your area. Anything under a slab would need to be polystyrene (EPS or XPS) and NOT polyisocyanurate. You can use polyiso on the interior side of the foundation walls though (in lieu of spray foam), as long as the cut edge at the bottom is on the vapor barrier, not dirt or concrete.

  5. Stockwell | | #5

    Unfortunately, the slope is 40-something degrees in the crawl, so no chance for foam on the ground and rat slab. What's the alternative? Lay down reclaimed EPS under the vapor barrier and call it a day? If so, I assume I would tape all those seams? What about if the terrain is not amenable to foam board--can one use the same EPS on the bottom of the trusses?

  6. Stockwell | | #6

    Martin has answered me before that foam on the ground will never pay off in energy savings. I am more worried about comfort of those walking above. Trying to prevent perpetually cold feet in winter.

  7. Expert Member

    If you are conditioning the crawlspace, the level of insulation and its location, will influence how much energy you use, but not how the floors above feel.

  8. Stockwell | | #8

    I just read this

    which would appear to indicate that one can seal up a crawlspace and provide only dehumidification and you are not required to provide air. Sounds like I could put down the poly vapor barrier, insulate the floor and rim joist, and add a dehumidifier and be fine without worrying about air flow. I misunderstood that I would need to provide conditioned air to the space if it was closed.

  9. Expert Member
    Dana Dorsett | | #9

    Insulating the floor has little or nothing to do with energy cost savings, it's all about moisture control.

    Insulating between the 50F subsoil and the crawlspace reduces the amount of mechanical dehumidification required to mitigate mold risk. The quick & dirty arm-waving math model:

    With 50 F subsoil and a low-50s F floor the dew point of the crawlspace air needs to stay low enough to not accumulate moisture on the cool floor via diffusion or air leakage from the outdoor air, or the conditioned space.

    The midsummer outdoor air dew points in Asheville are north of 55F more than 90% of the time:

    With a 53F vapor barrier moisture accumulates slowly via diffusion & air leakage from the outdoors over the summer, unless removed with a dehumidifier.

    Putting R4 under the vapor barrier the floor the crawlspace temp rises to something like 70F or higher, and the floor temp will be more like 65F or a bit higher. The dew point of the outdoor air is above 65F less than half the time over the summer, only 60% of the time during the peak few weeks.

    With R8 under the vapor barrier the crawlspace temp might hit the low mid-70s, and the floor temp will be closer to 70F. Outdoor dew points in Asheville are north of 70F only 5-6% of the time even during the most torrid weeks of summer. Diffusion drying to the drier air conditioned space above is likely to be sufficient and NO dedicated mechanical dehumidification would be needed to prevent moisture accumulation and mold. Whether the insulation alone is enough depends on the vapor rardency the floors above, the vapor retardency & air tightness of the crawlspace walls and the dew point of the conditioned space air above, but it's still going to make a huge difference in the mold risk even without dedicated dehumidification.

  10. Stockwell | | #10

    Thanks Dana. Makes perfect sense. If I can get the excavator man to smooth out the slope enough, I can get down the foam board.

  11. Stockwell | | #11

    Keith--you got it right except the slope is probably closer to 40%! There are other circumstances at play that might interfere with your idea. In some spots on the uphill foundation wall, the footing is 13' wide and there are 8' shear walls. With the requirements for undisturbed earth around those. its gets tricky to step your way down.

  12. keithhoffman22 | | #12

    Makes me wonder Kevin what a floor deck would set you back compared to more excavator time. I understand it would have to have multiple levels given your slope but that sounds better than a 20% slope in your crawl space with slippery and delicate foam board and poly. Or am I misunderstanding your plan?

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