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Is it necessary to insulate the ground in a Zone 5B crawlspace?

KevinEJ | Posted in Energy Efficiency and Durability on

Small, 192 sf building with a ventilated crawl space, zone 5b, Central Oregon, ground freezes in the winter. My plan is to seal vents and convert it to a conditioned space using rigid foam – R15 on walls, R20 at the rim joist. No cavity insulation between floor joists. Building has 24″ stemwalls and a 6″ tall footing. The footing and stemwall were poured flush with each other on the interior, and the dirt floor inside is level with the bottom of the footing. Planning to run Thermax (or similar) 30″ down the walls to the dirt floor, first bringing my vapor barrier underneath the insulation and onto the stemwall (to protect the bottom edge of the insulation).

Reading as much as I can, I found a question here from 2013 of someone doing something similar in Oregon. Advice given was that it would be a wasted effort if the dirt ground itself was not also insulated (to R10 and to include a ratslab to protect this ground floor insulation).

Do I need to insulate the ground of the crawl space for this to make sense doing at all?


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do I need to insulate the ground of the crawl space for this to make sense doing at all?"

    A. No. In your climate, you will never see enough energy savings due to installing horizontal insulation on your crawlspace floor to justify the cost of installing the rigid foam. As long as you have polyethylene on the dirt floor, you don't need insulation on the floor.

    For more information on this issue, see "Building an Unvented Crawl Space."

  2. Expert Member
    Dana Dorsett | | #2

    In a VENTED crawlspace, in which case there is effectively ZERO energy savings benefit from insulating the crawlspace floor.

    It's still worth putting down a ground vapor barrier to limit the amount of ground moisture and soil gases getting into that space. Most locations in eastern OR have much a ground moisture issue (except near bodies of water), but radon etc. isn't unheard of.

    When the crawlspace is vented IRC code calls out R30 minimum between the joists, or a full cavity fill (R19 minimum) if the joists aren't deep enough to accommodate R30. (R30 rock wool batts and fiberglass R30HD "cathedral ceiling" batts fit 2x8 joists.)

    To go unvented takes R15 continuous insulation (no thermally bridging studs) on the crawlspace walls, in which case the R30 in the joists can be omitted, but there is no need to insulate the crawlspace floor.

    From a long term energy savings and mold risk point of view there can be a rationale for up to about R6-R8 of foam on the crawlspace floor. See the zone 5 row of Table 2 in this document, which calls out "R7.5 sub-slab", and R15 on basement (=crawlspace, if insulated) walls.:

  3. GBA Editor
    Martin Holladay | | #3

    Kevin wrote, "My plan is to seal vents and convert it to a conditioned space."

  4. KevinEJ | | #4

    Thanks Martin. Music to my ears. For what it's worth, this is the Q&A in question, which was also asking about a conditioned crawl. Dana, your (#2) response is the one that led me to ask this question. Is this just outdated now? Or am I missing something in Dave's question that would have required him to add the additional floor horizontal insulation? Thanks! just trying to wrap my head around all of this.

  5. GBA Editor
    Martin Holladay | | #5

    It took me a little while to figure out that your answer includes a link, and that the link leads to this thread: "Conditioned crawl insulation--best way to apply?"

    I'll have to disagree with Dana on this one. I guess you can always attempt the math. Whether the work is worth it depends on the cost of your rigid foam, the cost of the labor to install it, and your local energy costs. My guess is that the payback will take decades.

  6. KevinEJ | | #6

    Martin, sorry about the link obfuscation. I'm less concerned with the energy payback as much as I am with:
    1) Will the floor above this crawl space be comfortable in the winter?
    2) Will pipes be protected from freezing if I add plumbing to this crawl space?

    Last winter, with no insulation in this building at all, the ground in the crawl space froze solid.

    The alternative plan would be: leave it as a ventilated crawl, still cut and cobble R20 rigid at the rim joist, add R30 batts to the joist bays, and add taped rigid foam covering the entire bottom of the joist bays. However, the 2' square hatch entry door in the subfloor complicates this plan (no idea how I'd detail/airseal that).

    Do you see one of these plans performing better than the other for building comfort and moisture issues?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Will the floor above this crawl space be comfortable in the winter?"

    A. Yes, if you do a good job with air-sealing the crawlspace and insulating the crawlspace walls.

    Q. "Will pipes be protected from freezing if I add plumbing to this crawl space?"

    A. Certainly -- as long as it is a properly detailed sealed crawlspace.

  8. Jon_R | | #8

    As Dana mentioned, insulating surfaces within a crawlspace effects the condensation/mold risk. Ie, you might do it to keep moist air away from cool surfaces even if it doesn't have an energy payback.

  9. Expert Member
    Dana Dorsett | | #9

    In zone 5B Oregon the mold risk is a lot lower than in most zone 5A locations. The issue becomes apparent when the temperature at the crawlspace floor is below the dew point of the conditioned space air or outdoor air, at which point moisture begins to accumulate on the top side of the vapor barrier.

    The deep subsoil temps in eastern OR are in the low 50s to high 40s F. (see: ) So as long as the dew point of the conditioned space air (or outdoor air, if ventilated from the outdoors) doesn't exceed that that temperature the risk is low to zero. Summertime outdoor dew points only rarely dwell above 50F for more than a few days in most of eastern OR, so ventilated crawlspaces work well.

    But indoor dew points can often be much higher than outdoors due to the bathing/breathing/cooking activities of the occupants. In a tight house this has to be controlled with ventilation &/or mechanical dehumidification. But the dew point will NEVER be above the room air temperature. Insulation on the crawlspace floor allows for a higher indoor dew point without risking a mold problem in the crawlspace, since the crawlspace floor would be much nearer the conditioned space air temperature.

    If you read the lead up to Table 2 in that document starting on p.9 under the heading "Economic Aspect", and the lead-in to Table 2 you will find a fuller rationale for those R-values. Martin isn't disagreeing with me, so much as disagreeing with John Straube, et al, but we all agree that many factors play into those economics. The cost of energy and the cost of the materials & installation make a large difference as to whether it's "payback = never" or not. But in your location the energy saving alone at R6 on the crawlspace floor is likely to be cheaper on a lifecycle basis than rooftop PV at $2/watt installed cost (though the insulation lifecycle might easily be longer than yours. :-) )

    That link again is:

    Whether insulated or not in almost any climate I'm personally in favor of putting a rat-slab over the ground vapor retarder in crawlspaces. That protects the vapor barrier from walking/working damage, and keeps burrowing fauna from setting up a condo down there forcing you to evict them. (That risk and the value of a rat slab also varies with location .)

  10. KevinEJ | | #10

    I read the info pertaining to the table in the BSC doc, and searched here for more understanding on dew point. However elementary, I think I'm starting to grasp the concept.

    Can you walk me through an example?
    The exterior mean temp here for Dec-Feb is 30°F (if I'm figuring that right). If air temp is irrelevant, assume the subsoil temp is 45°F. Figure the interior is kept at 68°F and 35% RH.

    With an uninsulated crawlspace floor and these baseline numbers, at what point would I run into the problem of moisture accumulating on the top side of the ground vapor barrier?

    If I change the numbers and warm the interior up to 75°F with 45% RH, using the same subsoil/exterior temps, am I closer to problems occurring?

    I certainly could be overthinking these decisions, just trying to understand the science. Thanks all for your patience and responses!

  11. Expert Member
    Dana Dorsett | | #11

    Kevin: Using my zone 5A climate location and house as an example...

    The deep subsoil temperature in my location is about 50F, and the uninsulated rat slab in the older part of the basement runs about 52-54F, even in mid-July (like right now). The air on the first floor is running about 55% RH @ 75F, even with the occasional use of air conditioning, and often hits 60%.

    The dew point of 55%RH / 75F air is 58F.

    The dew point of 60%RH / 75F air is 60F.

    Introducing air from the first floor into the basement causes moisture to move from the air into the 52-54F slab. A cardboard box placed on the slab ends up with the bottom of the box also 54F, and the bottom cardboard takes on moisture from the air. I collects enough moisture that it becomes moldy if left on the slab over the summer. Putting a sheet of 6 mil poly between the cardboard box & slab doesn't fix anything, since the source of the moisture is the basement air, not the ground. But putting an inch of EPS under the box (or storing the box on a shelf a few inches above the slab where the air is warmer) does.

    Mechanical dehumidification definitely helps, and relieves the "musty basement" smell, but a cardboard box left directly on the slab gets some mold- just not as quickly.

    The air temp in the basement is in the mid-60s year-round, but the indoor and outdoor dew points drop well below the slab temp in winter, so no moisture accumulation occurs at floor level (from the air, anyway) during the winter season.

    That's my house. Getting back to your example:

    The dew point of 45% RH air at 75F is 52F.

    If your deep subsoil temp is 45F, the uninsulated vapor barrier on the floor will be a few degrees warmer than that, maybe 48F but probably not 52F.

    If the vapor barrier is running at 48F and the conditioned space air has an average dew point of 52F over a long stretch of summer weather, the air films near the vapor barrier stays at full saturation, and the vapor barrier may even feel vaguely damp to the touch- conditions where molds can easily get started on any organic dust / grit / wood etc resting on top of the vapor barrier.

    With an inch of EPS under the vapor barrier the top of the vapor barrier will be quite a BIT warmer than 48F in summer (winter too), and could even be in the 60s F, well above the dew point of your 45%RH / 75F air, the dust on the floor stays mold-free, and the crawlspace never smells dank/moldy/musty.

    In winter, the 35% RH / 68F has a dew point of 39F, and your vapor barrier's temperature won't drop to or below the 45F deep subsoil temp. If there is no insulation between the joists, only the walls, the exposed subfloor will likely run about 65F, and the average air temp will be in the high 50s or low 60s, plenty warm enough to keep the uninsulated vapor barrier above 39F. If it's like my house the uninsulated floor temp won't change more than 2-3 F over the course of a year, and it'll always be at least slightly above the deep subsoil temp. Wintertime is never the stinky-crawlspace season (unless you have bulk water leaks- then all bets are off.)

    Is that any clearer?

  12. KevinEJ | | #12

    Dana, that does clear a lot up for me. Thanks!

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