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What is the best way to insulate a crawlspace for a more comfortable hardwood floor above in climate zone 5?

Hello GBA,

I am in Northern NJ, climate zone 5, and have a crawlspace question that I have been debating with a colleague (ok….my boss).

I have a 600 sq ft addition with a cold hardwood floor that is over a vented crawlspace. The crawlspace has leaky, poorly insulated duct work for the forced air heating/cooling running through it. There is currently poorly installed R-19 fiberglass pinned into the joist bays, attempting to insulate the floor above. The crawlspace floor is dirt with a poorly installed 6 mil poly liner in place. The crawlspace walls are about 3′ high.

The question, as always: to insulate the ceiling or the walls?

The boss wants to remove the existing FG and spray foam the underside of the floor, then close down the crawlspace vents.

I want to spray foam the walls, install a better poly liner, and drop a supply register in the crawlspace to make a conditioned crawlspace.

The boss thinks spray foaming the walls will still allow for too much cold to radiate up from the dirt floor, and that addressing the walls will not adequately address the comfort issue on the hardwood floor above.



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  1. Riversong | | #1

    You're right - he's wrong.

    With the ductwork in the crawl, it's essential to bring that space into the thermal envelope or there will be not only much wasted HVAC energy but also pressure imbalances in the conditioned space that will encourage infiltration/exfiltration in the house (more wasted energy, discomfort and moisture problems).

    Sealing the floor and conditioning the crawl space will not only control the ground moisture source, but raise the ground temperature, reduce radiant losses from the floor, and reduce conductive and convective losses as well - making the floor far more comfortable and the house far more efficient.

    It's a no-brainer, so feel free to tell your boss he has no brain ;-)

  2. Dennis Brown | | #2

    How much more of a benefit would he get if he were to also put down a poured cement slab and underneath that slab would be 2" XPS and vapor barrier? The XPS provides a thermal break from the ground. Slab is kind of pricey but just curious about the net benefit.

  3. Riversong | | #3

    Almost no benefit at all unless the slab is insulated at the edge, where most slab heat loss occurs. But, unless you're planning to spend a lot of time crawling around there, it wouldn't be worth the expense, let alone the hassle of pouring and screeding a floor in a 3' high space.

  4. Bob Ellenberg | | #4

    Built an insulated sealed crawlspace in zone 5 with wood floors above and loved it. Much easier to seal the walls of the crawl than the floor so the heat loss is less--warm floors and lower heating bills.

  5. Scott Razzino | | #5

    How did you seal the walls? I am looking at that task with my cinder block walls. The choices seem to be rigid foam or poly plastic.

  6. Art Vandelay, AIA/LEED AP+, Zone 5 | | #6

    I'm planning on using 2 or 3" of a closed cell spray foam, running from rim joist down to the newly sealed (w/poly) dirt floor.....

  7. Dennis Brown | | #7

    I had a discussion about this very topic.

    An update from that discussion is that I do need to cover XPS with drywall based on what the local bldg code guy has told me.

  8. Paul Price | | #8

    Old mass concrete house (in Ireland but similar to above with wood floors over vented crawlspace). Plan to do external insulation wrap including XPS on exterior foundation wall down to footing. Crawl space has slab floor.

    Given above logic we are thinking to block vents and then crawl space is part of warm house. Does this make sense? ie. isn't insulating outside the masonry better in this case? Or do we need to put vapour barrier and insulation on to slab and up rising wall?

  9. John Hess | | #9

    I am considering putting a root cellar under a house that I am planning to build. Because it will be used to store food stuffs I wish for it to stay cool, therefore I won't insulate either the floor or walls. However, I wish to insulate the wood-framed ceiling above, which is the floor for the living space. If the outside of the cellar is well-drained should I have any problems with moisture accumulating in the framing/insulation of the main floor?

  10. GBA Editor
    Martin Holladay | | #10

    John Hess,
    I suggest you start a new thread with your question. That way more people are likely to see it, and it will avoid confusion for people trying to answer the question raised on this thread. You can post a new question here:

  11. Art Vandelay, AIA/LEED AP+, Zone 5 | | #11


    Any thoughts from you about the comfort issue query I raised above? To insulate the walls, or to insulate the ceiling, with occupant comfort at the hardwood floor the primary concern?


  12. GBA Editor
    Martin Holladay | | #12

    Insulate the walls, not the ceiling.

    Follow all the usual recommendations for sealed crawlspaces, and you'll be toasty (and energy-efficient).

  13. ROY HARMON | | #13

    Here's a sample of what can happen when walls are not insulated with no ventilation. One of the projects we are currently working on.

  14. Riversong | | #14


    As long as the crawlspace slab is dry, insulating on the outside provides thermal mass benefits and reduces the freeze-thaw cycling in the walls. It also helps stop water at the outside, which is the preferred strategy.

  15. Art Vandelay, AIA/LEED AP+, Zone 5 | | #15

    One last question on this topic....

    Lets say the ceiling of the crawlspace is already insulated (poorly) w/R-19 fiberglass, and we now want to insulate the crawlspace walls. Do we need to remove the fiberglass from the ceiling, or leave well enough alone?


  16. GBA Editor
    Martin Holladay | | #16

    You don't need to remove the fiberglass batts. But here's my prediction: eventually the batts will sag and begin to fall down and generally look raggedy. If you are doing a good job of insulating the crawl space walls and sealing up air leaks, you might as well get rid of the batts.

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