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Mixed humid climate, conditioned crawl space

BJHuffine | Posted in General Questions on

I feel like I’ve already asked a lot of questions and many thanks for the responses thus far.  I feel like I’ve already poured through a mountain of info as I prepare for this upcoming home construction.  Today I started going through the assembly details for the crawlspace.  Basically, the land, placement, etc, all dictate the need for a crawl space.  I have been considering enclosing my current home’s crawl space and had talked with someone about it a couple years ago (who mentioned using a dehumidifier), but after reading more today I’ve begun to realize the approach to eliminate moisture issues in that space, may not actually do so completely.  In the GBA article “Building an Unvented Crawl Space” [1], dehumidifiers are mentioned for temporary use to remove construction moisture.  Even a Lstiburek article [2] showed a lot of approaches, but no mention of the dehumidifier.  I found a couple articles, [3] below explained its use as an option for example, but preferably without transfer grilles.  There seems to be conflicting ideas and neither seems to work fully.  If you use a dehumidifier the complaints are the energy use and noise.  Without, evidently humidity can still be an issue in the off-seasons Spring and Fall.  If using transfer grilles, you are supposedly equalizing the pressure, but exposing the air in the living space to that below, making any effort to ensure a tight seal critical.  And if using the supply and return approach (evidently with two transfer grilles to equalize pressure as well), the positive pressure underneath can also work against you.

So in the end, I guess my question is… what is the best approach?  Is the need for a dehumidifier over-exaggerated?  Or is it likely humidity will continue to be a problem, just not all the time?  I even saw in comments recommendations for folks to “monitor” the humidity level.  Is that right for new construction?  I would hope that if I need one, that I could go ahead and wire for it, yet there doesn’t seem to be a lot of info in this direction.  Any help would be much appreciated.


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  1. Jon_R | | #1

    Imagine a sealed crawlspace that has zero air infiltration and zero moisture permeability. A dehumidifier in it could run once and then never run again - so no ongoing energy use or noise issues!

    Imagine the above crawlspace also with perfect insulation and a little bit of natural air flow to the interior above. It would never have more moisture (absolute or relative) than the interior. Conditioning the interior+crawlspace would require ongoing energy use and noise, but no more than the interior alone.

    So either would work well. What works in the real world depends on how/where the crawlspace deviates from the above ideals. And then there is code compliance.

    1. BJHuffine | | #3


      I appreciate your response. If I understand you correctly, you are saying much the same as Martin in that if we construct it correctly, it should be okay, though you have a nudge at the code? Is that at the use of transfer grilles? So in your two scenarios, it sounds like you're comparing a sealed with no transfer grilles to one sealed with a supply/return?

      1. Jon_R | | #5

        Here are some other sealed crawlspace items to consider:

        a) where does mold odor (if any) or radon (if any) go?
        b) continuous exhaust is a noticeable energy use (although it can be energy neutral if you planned on exhaust only ventilation anyway).
        c) can you avoid the entire issue by building a FPSF slab on grade elevated with gravel? Lstiburek says: "The best crawlspace in the world is one filled with concrete and called a slab."

        From Lstiburek's other options, I like E (20 CFM continuous is 12x more than 20 CFM for 5 min/hr, quieter and uses less energy). Plus some monitoring that will remind me to close the windows and turn on the full house dehumidification when the crawlspace it at mold friendly conditions (this is likely to occur during periods of no AC use). Or if the fan fails.

        1. BJHuffine | | #7

          Interesting thought on Option-E. I was originally averting from that due to others' complaints that it becomes too tempting to not repair quickly if the exhaust fan was to ever fail. I see what you're getting at. If the HVAC unit is designed for the whole space and all the returns are done right, then it would in theory be balanced and have a constant pressure of conditioned air supplied.

          As for the slab, not sure it's a viable option as there's quite a slope where the house will sit and a cliff edge behind it, limiting what we can do with the grade. Our builder is not concerned and has built in these conditions before based on recommendations from our inspectors. But I think it will affect our ability to put a slab out. And that's not to mention the fact that it is nice to be able to work on your plumbing, etc, even if underneath the living space.

  2. GBA Editor
    Martin Holladay | | #2

    A good, dry crawl space is built with the same details as a good, dry basement. It's just shorter.

    If you plan ahead for a new construction project, and your details are good, your crawl space won't need a dehumidifier.

    Start with good drainage -- that means crushed stone in the foundation hole, as well as footing drains around the perimeter, sloped to daylight. Backfill with clean, fast-draining material.

    The ideal crawl space has a floor that is at the same height as exterior grade -- but most people don't build them that way. (If you do, it's hard to have wheelchair accessibility.) In any case, you want to be sure that the exterior grade slopes away from the foundation in all four directions.

    Manage your roof water with gutters and conductor pipes. Roof water should be directed to a location far from the house.

    Install polyethylene on the floor of your crawl space. Insulate the walls. And keep it unvented.

    1. BJHuffine | | #4

      Thanks Martin for your response. That's good to know that with the right construction it shouldn't be an issue. I do actually hope to ask to raise the floor at or above the exterior grade and filling with crushed stone underneath. I'm also hoping to slope around the perimeter with drainage tile. We'll have our challenges due to the nature of the lot (it's on a mountain back from a country road that is actually at a higher elevation than the house. But I'm hoping we can do some earth magic and still slop away from the front as well as beef up the ditch line at the road directing water to a wet weather creek that runs alongside our lot.

      From your experience, and those you've heard from, are any of the approaches to conditioning the space having the most success? The way I understand these options (again negating any application of a dehumidifier):
      1.) Two transfer grilles, no supply/return
      2.) Two transfer grilles, supply only
      3.) Two transfer grilles, return only
      4.) Two transfer grilles, supply and return

      I didn't include any of the exhaust options whether from the living to crawl or within the crawl space as that doesn't sound as robust for long term use.

      1. GBA Editor
        Martin Holladay | | #9

        If you haven't seen it yet, you should read this article: "Building an Unvented Crawl Space."

        The two approved ways to condition a crawl space are explained in that article:

        The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs. Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” In other words, install an exhaust fan in the crawl space that blows through a hole in the rim joist or an exterior wall (exhausting crawl space air to the exterior); make sure that the fan isn’t too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn’t lead to condensation problems.)

        Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)

        1. BJHuffine | | #11


          I did read that actually, but assumed the options described to be generalizations of the same options as those proposed by Lstiburek. If not, why the differences? For example, in Option 1, why only one transfer grille vs two? And none in Option 2. Wouldn't the positive pressure without a return be a bad idea? Or is this the difference between ideal and code allowance?

          1. GBA Editor
            Martin Holladay | | #13

            My article summarized the requirements found in the building code (IRC section R408.3). Here is a link to a document that quotes the code section: IRC Section R408.3.

            Either one transfer grille or two transfer grilles will work as far as the code requirements are concerned.

            You wrote, "Why only one transfer grille vs two? And none in Option 2?"

            You misread my article. I wrote, "The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs."

            Q. "Wouldn't the positive pressure without a return be a bad idea?"

            A. There is a return -- the transfer grille or duct required by code and mentioned in my summary.

        2. Jon_R | | #14

          R408.3 does not require use of the furnace or air handler. A fan (as with Lstiburek's option E) is a conditioned air supply. Even houses with mini-splits can use this option.

          R408.3 (as below) also allows the use of a dehumidifier instead. I assume Lstiburek didn't mention this useful option because code didn't allow it in 2004. Energy use can be negligible (little run time), with much or all of it offset by less AC and interior dehumidifier use.

          For a much more enlightened code, read the 2018 North Carolina R409.5. It addresses the exposed foam/smoke issues by allowing passive return vents to be omitted (return air can leak through the floor without much pressure). Or fully balanced supply and return ducts.

          1. BJHuffine | | #17

            I have to admit, I wondered if transfer grilles were used if I'd have to follow similar sealing practices as with an attic. That's interesting.

            As for Lstiburek's not mentioning dehumidifiers, I wonder if it's time our building experts took a fresh look? Especially if code is also changing and indicating that leakage plays a role. Martin, what say ye?

          2. GBA Editor
            Martin Holladay | | #18

            It's always preferable to avoid installing a dehumidifier unless absolutely necessary, because dehumidifiers use a lot of energy. If other methods of moisture control have failed, however, sometimes a homeowner has few options other than installing a dehumidifier. For more information, see "All About Dehumidifiers."

        3. BJHuffine | | #15

          Thanks as always Martin. I appreciate your willingness to help those lost in the weeds as myself, making this process a little more workable. Happy New Year!

  3. Andrew_C | | #6

    I just got through another crawlspace encapsulation this fall. If only things had been done differently the first time...
    My first thought was the same as Martin's (who I suspect has used this line before): A good, dry crawl space is just a good, dry basement, but shorter. Build it the same, and tall enough that you can move around easily (hands and knees clearance is okay, but 48" allows you to walk around bent over).
    Proper drainage, insulation, vapor barrier, and concrete floor, with built in passive radon plumbing.
    I suspect that you're going to want a dehumidifier during construction and for the first year or so afterward anyway, so just get (a reasonably quiet) one and then store it in your nice dry crawlspace if you don't need 9 month/year after that.
    Make sure your sump pit is completely sealed. Drain to daylight is better if possible.

    1. BJHuffine | | #8

      Good point about the spacing. I think we'll be okay there, though our builder has mentioned trying to get it as close as possible toward the front due to the slope of the lot. I'm sure it'll be a fun balancing act. I like the idea of the rat slab, but I'm not sure how much additional cost it would add. Is that more effective than a crushed stone base with the vapor barrier by itself? If it is, then it may be worth our considering as long as costs are not exorbitant. Another fact about our situation is that there is a lot of shale rock beneath which means we are likely to have water setting on the rock in the layers underneath. That was a huge factor for us driving toward an enclosed crawl space.

      You mention the dehumidifier. What of the options discussed have you seen most effective for conditioning the space?

  4. Expert Member
    Peter Engle | | #10

    The rat slab does not necessarily make the vapor barrier more effective, but it does make it more durable. You should also consider the benefit of being able to store things in the clean/dry crawl space if you make it tall enough and you have a smooth rat slab. An added benefit that most don't consider is the ease of getting around. If you leave a mechanic's creeper or furniture dolly in the crawl, your contractors can get around easily to make repairs and modifications. The easier the access, the better the quality of work. Make sure the slab is trowelled smooth. If you don't specify this, some masons will just rake the slab surface and that's no fun to move around on.

    1. BJHuffine | | #12

      I just had a curious thought. If we pour a rat slab, how best to know if and where a sump would be necessary? Or does the rat slab place more emphasis on the other drainage techniques? Like the crushed stone and drainage tile outside the footing?

      1. Expert Member
        Peter Engle | | #22

        Where the sump goes doesn't matter much. You should put it closest to where you want to drain the water away. The French drains are either installed dead level or with a slight pitch towards the sump. Once the water fills the gravel and enters the pipes, it naturally finds its way to the sump. If your mason is good, he'll agree to pitch the rat slab slightly towards the sump pit as well. There's nothing more frustrating than having a puddle of water sitting on the slab after you've gone to all that trouble installing French drains and sumps.

        Whether you need a sump pump depends on how high the water table is and whether you have surface water that you can't properly move away from the house using exterior methods. If you've got either of these conditions, you need a sump pump. You can also wait to see what happens. Do all of your drainage, air sealing and insulation work and install a sump pit with a tight-fitting lid. Check the pit occasionally, especially in the spring and after big storms. If you ever find water within about 6" of the lid, you probably want to install a pump.

  5. Stockwell | | #16

    I don't know where you are located, but this is the best info I found for my sealed crawl planning in the Southeast.

    1. BJHuffine | | #19

      I'm also in the South East, lower part of Tennessee, next to the borders of Alabama and Georgia. That's a lot of info I'll definitely include in my research.

      Btw, how did you handle yours from the conditioning standpoint?

  6. Stockwell | | #20

    I chose to seal it up completely and put in a dehumidifier. I am doing closed and open cell spray foam on the bottom of the floor above and encapsulating the sloped dirt floor.

    1. Jon_R | | #21

      I'd be interested in knowing how many kWh/year the dehumidifier uses, even if it's a fairly meaningless number without accounting for the reduction in whole house AC and dehumidifier use (that would come from using interior air to condition the crawlspace).

    2. BJHuffine | | #23

      After some family activities, I have given your info a read. Seems they show a relatively close result in energy savings between insulating wall and floor. In their study homes 18% and 15% respectively. However, when they broke the data up by season, the wall insulation approach was at +4%. I bet that reflects a cold floor as well as energy use to prevent heat loss due to the cold earth below.

      They also specify not using transfer grilles at all (and if I recall knowing it's not in code). They're approach is either with just a dehumidifier or by creating a slightly positive pressure underneath that they consider negligible to other effects. But I wonder if that'll be the case with a tight home? Countering recommendations is exactly why this study has been so frustrating.

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