GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Crawl space retrofit: three zones, low clearance

pbout | Posted in Green Building Techniques on

We are pursuing a remodel of a 2-story house in climate zone 3, coastal California. The first floor was built around 1920, and the second floor was added in 1980.

In addition to changing interior layouts, we are trying to make it “tight, well insulated, and mechanically ventilated”, with a primary goal of higher IAQ due to dust/mold sensitivity. Thermal comfort is important too, but the climate is pretty mild.

The foundation is a perimeter type, but it is separated into three areas (there are center walls that divide the crawl space up into three zones. Access is via a side cut on one area, and through floor hatches on the other two areas, but we are losing the two interior access points. The crawl space is currently vented, and has about 14″ of clearance between the dirt and the floor joists (sketch attached). I was in there on Friday, a few days after rain, and the dirt was moist but no signs of standing water.

The original plan with the crawl space was to encapsulate with a poly lining (floor and up walls), add R-5 or -10 insulation to the interior of the perimeter walls, seal and insulate vents, air seal between floor, and mechanically ventilate.

My designer suggested that we add spray foam insulation underneath the floor, and leave it ventilated, but I’ve never seen that approach. I have higher confidence in the encapsulation method, but 14″ doesn’t seem like enough room to work.

Also, the three zones makes it a bit tougher to ventilate. If we use an HRV (HVAC system is not designed yet), it would require three duct connections. Heating now is met through a single downstairs floor furnace, and remodel will likely change this heat source to direct vented gas fireplace. No existing AC or mechanical ventilation.

What do people recommend I do with this crawl space?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    There really aren't any circumstances that would call for the use of an HRV in a crawl space.

    In general, I recommend that shallow crawl spaces be fixed by digging them out (lowering the dirt floor) to improve access.

    Once this work is done, you should follow the advice in this article: Building an Unvented Crawl Space.

    -- Martin Holladay

  2. pbout | | #2

    I think incorporating the HRV was my idea to provide mechanical ventilation, which would be required for an unvented crawl space.

    I was worried that the answer would be to dig it out for access. Ugh.

    Any thoughts on leaving it vented but installing spray foam underneath the floor? Is this a viable system?

  3. GBA Editor
    Martin Holladay | | #3

    The two options listed in the code for moving some air through a crawl space (neither of which involves an HRV) are ways to lower the humidity in the crawl by introducing some conditioning -- not attempts to introduce outdoor air into the crawl space. "Ventilation" is not required -- conditioning is.

    I doubt if you can convince a spray foam contractor to crawl into a 14-inch-high crawl space. It's time to get out your entrenching tool and bucket and start removing soil.

    -- Martin Holladay

  4. pbout | | #4

    I see, so if I won't have an air conditioner, the other option for humidity removal is an exhaust fan on a humidistat, with the fan sized for minimum required flow? And for a supply, a vent to the living area?

    With a hole in between the house and crawl space, I can see that it needs to be unvented and encapsulated.

    If we were able to get a spray foam contractor in there, likely with digging, I'm still not sure about the viability of leaving the space vented, but with the foam insulation.

  5. Expert Member
    Dana Dorsett | | #5

    In zone 3 the IRC code min spells out R19 if it's insulated at the subfloor. That takes ~5" of open cell foam, which runs about $1.50 per square foot in my neighborhood.

    But if insulated at the foundation walls in an unvented crawl it only needs R5. That's 1" of closed cell foam at about a buck a square foot, and probably far fewer square feet.

    A ground vapor retarder is a good idea even if you leave it vented. In your climate ground moisture is the biggest driver of crawlspace moisture levels. (This is in stark contrast to zone 3A on the east coast & gulf states, where the high average humidity of summertime air is a primary moisture source, usually exceeding the ground moisture drive.)

  6. charlie_sullivan | | #6

    Zone 3 extends most of the way up and down California. And in some regions there are pretty different micro-climates just 30 miles apart. So we might be a better sense of the moisture and temperature issues if you told us the location more specifically--perhaps a zip code.

  7. pbout | | #7

    Zip code is 95062, and house is 1 mile from the ocean. I agree with Dana that ground moisture is a big driver of crawlspace moisture here, but if the stem walls are not insulated, won't condensation occur?

    We have a spray foam contractor coming to look at it next week. If he says he can spray it with the existing clearance, great, but I'm still concerned about moisture issues.

  8. Expert Member
    Dana Dorsett | | #8

    The short answer is "no", there will never be actual condensation in the crawlspace.

    In Santa Cruz the deep subsoil temps are north of 60F, well above the average mid summer dew point of 53F. Dew points north of 60F are all but unheard of, and it would have to dwell at dew points pushing beyond 70F to move the needle on the moisture content of the stemwalls measurably above the ground moisture wicking levels.

    Compare the dew point averages graph on the bottom of this page:

    and the groundwater temperature map here:

    Venting the crawl space to the outdoors will ALWAYS have a drying effect for the crawl space stemwalls & floor in your location (or at least more than 99.5% of the time.)

    If you close it in, the dew point of the crawlspace air could only rise to about the temperature of the subsoil, and which point the subsoil would be taking in the moisture. With a ground vapor barrier and closed vents the dew point of the crawlspace air would track that of the conditioned space above. That might be higher than the outdoor dew points at times, but it would have to be uncomfortably humid in the conditioned space for quite awhile for the dew point of the crawlspace air to rise above the low-60s F subsoil temperature, which is what it takes to end up with condensation.

  9. Jon_R | | #9

    You should being asking about conditions that support mold, not just condensation. An option is putting a dehumidifier in a sealed crawlspace - it doesn't rely on AC to condition the air.

  10. pbout | | #10

    @ Dana, thank you for the thorough response.

    @ Jon, yes, I am concerned specifically about mold, and have just been assuming that moist air will lead to condensation, which will lead to mold.

    What about avoiding mold in an unsealed crawl space with spray form under subfloor?

    And does it matter if I use open or closed cell foam?

  11. Expert Member
    Dana Dorsett | | #11

    The indoor humidity in a vented crawlspace with a ground vapor barrier will usually be higher than the crawlspace humidity (which will track the lower outdoor humidity). So it won't really matter whether the foam is vapor permeable or not. The only way open cell foam would be a foam risk if it were allowing high humidity in the crawlspace to accumulate in the subfloor, but the outdoor humidity is (on average) lower than the indoor humidity, where all that cooking/bathing/breathing activity is going on.

    If it were going to have mold problems it would be at higher risk NOW, prior to ground vapor barriers and insulation. Does the subfloor show any signs of mold?

  12. pbout | | #12

    Currently no signs of mold. I'm paranoid in part because our current house across town has a similar crawl space and soil type, and it smells musty and dank. It is a standard vented crawl space.

    We get a lot of fog in our micro-climate, how would that affect a vented crawl space, with or without a vapor barrier?

  13. Expert Member
    Dana Dorsett | | #13

    Fog is an indication that the outdoor dew point is at the outdoor temperature. That usually only happens when it's much cooler outdoors than the 60F subsoil temperature or the indoor temperature. Moving that foggy air into the crawl space would raise it's temperature, re-evaporating the micro-droplets.

    The house across town with the stinky crawl probably has no ground vapor barrier in the crawlspace (?), and higher ground moisture content, or possibly poor grading/drainage allowing surface water to be directed under the house during rains. Typical California bungalow deep roof overhangs help quite a bit, but not if the surface slopes toward the foundation. Shallow or absent overhangs of Cape Cod style houses will often have high moisture content in soil next to the foundations, and more prone to needing surface drains or "French" drains as moisture mitigation.

  14. pbout | | #14

    Yes, our house now has no vapor barrier, and although we installed a french drain and also redirected downspouts, the grade of our property is lower than our neighbors, so they drain to us.

    For the house in question, the micro-droplets from the warmed-up foggy air will collect on the crawl space surfaces, correct? Is the only way around that to have a conditioned crawl space? Or does it matter; it is vented now, and no signs of mold; how will adding spray foam change that condition?

  15. Expert Member
    Dana Dorsett | | #15

    The micro droplets of foggy air evaporate when raised to a warmer temperature. Only if the crawl space surfaces were COLDER than that air could they accumulate.

    Insulating the subfloor will lower the average wintertime temperature in the crawl space a bit, but that's when the outdoor air is much drier (as measured by it's dew point). The crawl space is constantly being warmed by the subsoil during colder winter weather, and will always be above the outdoor temperature (and by extension, above the outdoor air's dew point, which can never exceed the air temperature.) Insulating the subfloor will RAISE the average winter temperature of the subfloor, lowering it's moisture content.

  16. pbout | | #16

    Thank you Dana, this has been very helpful.

  17. charlie_sullivan | | #17

    I was surprised to find out that the actual quantity of moisture in fog is not very much, so when the temperature increases, when it gets into the crawl space, the droplets can evaporate before they get anything wet.

    Comparing the situation now to the situation with foam added, you could end up with more moisture down there because the heat from the conditioned space above heats and helps dry the crawspace in the winter. The moisture being there in the winter isn't a major mold problem because mold growth is slow in the winter, but if there's enough moisture absorbed by the wood, it might stay damp as the temperature rises in the spring. I wouldn't be too worried about it--as Dana explains, the biggest concern is moisture coming from the soil. But if you want to follow the principle to first do no harm, I think the spray foam would hurt more than help the moisture in the crawl space.

    Some questions to ask your spray foam company:

    1) Cost for R5 of closed cell on the walls vs. R19 of open cell on the floor above.

    2) For the closed cell, can they use one of the new foams blow with HFO instead of HFC? The global warming impact of HFC is more the 1000 times worse than HFO. HFO foams include Lapolla 4G which has been out about a year and a Demilac equivalent that just came out.

    3) In the unlikely event that the foam chemistry goes bad and the result is a bad smell that won't go away, what is the insurance and contract language protection for the homeowner?

  18. pbout | | #18

    Charlie, thank you for the suggestions. I will ask those questions of the spray foam company.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |