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Green Building Curmudgeon

An Open Discussion on Closed Crawlspaces

In one week, I heard two complaints about sealed crawls

This well-constructed sealed crawl space should do the trick in keeping moisture down, cprovided the owners don't screw things up.

This post is just an observation that I hope will stimulate some conversation about the subject of closed crawlspaces. This evolved out of conversations with two builders—one a high-end custom outfit and the other an affordable builder. Both of them had been certifying their homes to EarthCraft House standards and were building with closed crawlspaces. On the same day last week I heard from both builders that they were having moisture problems in their closed crawlspaces, were reluctant to continue building them, and were considering returning to vented designs. Now, I haven’t yet visited any of their job sites, so I am working on some conjecture here, but these problems raise some interesting questions.

The affordable builder told me that all their sealed crawlspaces had HVAC systems, which were expected to maintain appropriate humidity levels. However, the builder discovered that the homeowners were often turning off the air-conditioning for much of the year as a cost-saving measure. They were willing to be uncomfortable to save money—not necessarily a bad policy—but it led to mold and mildew problems in humid months. The custom builder had problems in closed crawlspaces that did not have HVAC systems in them, and ended up having to install stand-alone dehumidification systems for moisture management.

Behavior vs. process

It appears to me that we are dealing with two separate issues: behavior and process. Assuming that the closed crawlspaces were constructed properly to keep bulk water and vapor out, the ones with HVAC systems should have functioned just fine, provided the systems were running in humid weather. The fact that the homeowners chose to turn their systems off is a behavior issue. They were not behaving as expected (does anyone?), which caused problems in the structure.

The builder could have put in a backup dehumidification system, but that would have presented a higher first cost as well as ongoing electrical costs to operate when needed. This reminds me of the quote “You can’t idiotproof things because the idiots are too smart.” This is not to demean any homeowners; in fact, I applaud them for not using their AC indiscriminately in order to save money and for being willing to live with a little less comfort. In this case, the “idiots” may, in fact, be the people who thought up closed crawlspaces in the first place, assuming that everyone would operate their homes appropriately day in and day out. No offense intended toward the smart building science folks who developed these things, but we have apparently come up against the law of unintended consequences. A well-thought-out design was taken down by someone who chose to operate the house differently than expected.

I reviewed “A Quick Reference on Closed Crawl Spaces” by Advanced Energy and noticed that among their recommendations they include “a mechanical drying system to reduce humidity” in the crawlspace. In the case of the affordable builder, they apparently met those requirements, but were foiled by the owner’s behavior; while in the case of the custom builder, it was a process issue: They did not include a method to mechanically dry the crawlspace in their projects.

So what’s the answer?

I don’t pretend to suggest that we stop sealing crawlspaces (I already had my head handed to me recently by implying that we stop insulating!), but I do think we need to take into account owner behavior and builder process when we make decisions on how to build homes.

As energy prices increase and the economy continues to stagnate, we may likely see many formerly middle-class homeowners looking for ways to save money, who are willing to live with less comfort and may use their air-conditioning less to do so. If any of them live in homes with closed crawlspaces, we will likely see more humidity-related problems in these homes. As to the other problem, it seems to me that this was a case where a very well-intentioned builder missed a key part of the puzzle in creating a closed crawlspace and paid for it by bearing the cost of installing a dehumidifier.

Can we outsmart the idiots?

In both cases, these builders are leaning away from closed crawls in future projects — a decision that I don’t believe is necessarily in their best interest, but helps them to avoid liability by reverting back to older, less-efficient building techniques. Builders should carefully consider their construction techniques and get the right advice all the way through the process to make sure they don’t miss any critical pieces of the puzzle. They must also consider homeowner behavior, how it affects building performance, and how it might change over the course of home ownership.

This last one might be too challenging for anyone to adequately manage, so do we look for alternative techniques that are, in fact, truly “idiotproof”?


  1. GBA Editor
    Martin Holladay | | #1

    Build your house on piers
    I've never understood the desire to build a crawl space.

    If you don't like a slab, and you're not willing to dig a basement, it's better to put the house on piers and let the wind blow clear under the house. At least that way you're not devising a mold factory.

    1. geoffpritchard | | #18

      Old post but I'd like to comment as a crawl space owner/builder. Slab required too much forethought of exact locations for plumbing not too mention brutal on the joints. Basements in our area always seem to be leaky messes but are at least a possibility but that reputation put my wife off of the idea for good. Also, we built on some pretty shallow soil, hitting rock at about 4' with the final foot of soil before the rock being almost impervious clay (undiggable with a shovel when dry - will suck an auger into the ground when wet) resulting in a "perched water table" in winter/spring months. So we went with a crawl space ...... and I regret it. Water comes under the stem walls this time of year (Mar) and gets under the poorly installed by me 6 mil plastic.

      Not sure why I posted this but felt the need.

      BTW -- liked you mouse mystery (as much as one could like mice in the house). We have Peromyscus maniculatis, the dreaded Deer Mouse, as our main villain. And remember -- mice don't need a mouse-sized hole to get through. They can squeeze through much smaller openings than one would imagine.

  2. greenophilic | | #2

    Trying to remember details...
    I mean, if I remember correctly, the big issue with mold in crawlspaces has to do with air permeable insulation in the floor joists, cold sub-flooring resulting from A/C operation, and plentiful moisture in outside air, which consistently reaches its dew point near the sub-floor. Am I missing something? Why do we not just provide an air and or vapor barrier on the underside of the floor joists? Care will need to be taken to not create a mold sandwich, with say vapor impermeable finished floor materials, but this seems like a possible solution. Of course, in cooling climates, you will lose the beneficial radiative heat transfer to the ground, but this just seems like a simpler solution. No incomplete insulation due to termite inspection...someone please shoot holes in this, because I feel like I am missing something.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Brennan Less
    Are you proposing this "air and/or vapor barrier on the underside of the floor joists" for vented crawl spaces or unvented crawl spaces?

    If there is no insulation in the joist bays, then this barrier -- what is it? Tyvek? Poly? -- could be cold during the air conditioning season. If it's poly, and if the crawl space is vented, it could accumulate moisture, which will drip on the floor.

    To be sure the surface of this proposed barrier is never cold, you need to include insulation. The best way to do that is to install thick foil-faced polyiso, well sealed and taped. That can work, in a vented or an unvented crawl space. It works best if the house is on piers, and the wind is blowing freely under the house.

  4. greenophilic | | #4

    Assuming insulation exists
    Martin, my assumption was that the floor joists were insulated to their full depth, and then sheathed with any variety of rigid material, from OSB, plywood, foam board...or as you propose, simply use rigid insulation and omit the cavity infill. I've worked in a number of crawlspaces (admittedly, in CA) that have used this strategy. This just seems a whole lot easier than fully sealing the crawlspace, and it doesn't fall prey to the issues that Carl is proposing in this article, which are reasonable concerns.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Brennan Less
    Your proposed details can either work or fail, depending on several factors. The worst scenario is a hot, humid crawlspace, with thin plywood protecting the fiberglass batts, and sheet vinyl flooring on the floor of an air-conditioned kitchen. In that scenario, you can get a damp, rotting subfloor.

    Foil-faced polyiso works better.

  6. greenophilic | | #6

    Response to Martin
    I totally agree, which is why I said in my initial post that choosing a vapor impermeable finished floor material would be a problem. I guess I am just wondering why this is not the preferred solution, over what seems to me, the much more difficult to detail, sealed crawlspace? The sealed crawl also doesn't get at the heart of the problem, which is moisture, air permeable insulation and cold surfaces. It's a roundabout solution, which makes me think I'm missing something, but obviously not. Thanks!

  7. GBA Editor
    Martin Holladay | | #7

    Another response to Brennan
    Here's what you forgot: ducts and plumbing pipes.

    If you have a vented crawl space, these sweat like crazy during the summer. Drip, drip, drip. Everything gets moldy fast.

    In my hypothetical house on piers, all of the ducts must be upstairs in the conditioned space, and you need a plumbing chase between grade and the first floor. Such a plumbing chase will freeze in a cold climate without heat tape, so we don't do it up north.

  8. user-659915 | | #8

    Crawl spaces, piers and basements
    To Martin: why do I like to use crawl spaces rather than pier or full basement construction? Well, full basements are problematic in many locations in NC because of the high water table - we just don't do them unless we can drain to daylight. And of course we don't have to dig the massively deep foundation trenches that y'all need for frost protection up north. So crawl spaces are a natural option. They offer accessible mechanical and duct space within the conditioned enclosure, thermal buffering from the ground beneath in the cooling season and critter protection, all at reasonable cost. Not that we disdain pier construction - it's a great option for portions of a home in certain circumstances such as when building out over a steep slope or in low country over a flood plain. But putting most or all of the average home over a well-detailed crawl space makes perfect sense most of the time.

    And we've consistently found over two decades of both new construction and retrofit installations that closed crawls beat the vented variety in both energy and air quality performance hands down, and we have not encountered the problems that Carl relates. Perhaps it's specific to certain local climate or ground water conditions, or perhaps it's just poor waterproofing or other detailing. Are you sure these builders are actually doing it properly?

    As far as saving energy by turning off the a/c goes: this is just false economy and makes the case for better homeowner education. If the Earthcraft certification means anything these homes should have a tight enclosure, proper solar gain control, decent insulation and efficient mechanicals and their energy costs should be less than the average phone bill even in the hottest weather. Alternating a/c use with natural ventilation in humid weather is not a smart thing to do: in addition to being destructive of the fabric and content of a home it can also result in greater energy costs than with continued a/c use.

  9. Ted Clifton | | #9

    Closed Crawl Spaces Done Right
    Wow, a lot of issues are brought up above, good comments!
    As was correctly brought up, and supported by several of the comments above, the warmer, muggier weather is when you are likely to have condensation problems in any crawl space. A crawl space that has AC ducts in it is even more likely to have condensation than one that does not.
    If you build on a crawl space foundation in a warm, moist climate, you better have good drainage under your floor, because you are going to condense moisture down there, vented or not. Getting rid of the moisture, before it can cause damage, is the key under those circumstances.
    I have been successfully using closed crawl space designs for about twenty years, but we do not have the humidity on the West Coast that the Gulf Coast or East Coast have. We will occasionally get a day where the dew-point is up around seventy degrees or higher, but it is rare, so moisture conditions related to that do not persist. I have been using a small fan, on a humidistat, to mechanically vent the crawl space during any periods where the dew point is that high.
    For those warm, muggy parts of the country, introducing cool, air conditioned air from the AC system into the crawl space will exacerbate the problem, unless that air is dehumidified, and recirculated. This just adds to the cost of operating your AC system. Allowing the crawl-space to warm up to the ambient outside temperature is more likely to provide condensation relief under those conditions.

    This all gets back to the reason I am a strong advocate for insulating the floor above the crawl space, as well as the crawl space walls (and possibly the floor), in a closed crawl space. During hot muggy weather, this keeps the cold floor beneath the air conditioned space farther removed from the warm moist air in the crawl space. Keep the crawl space warm, and the moisture won't condense. Let it get cold, and you have problems. On the cold side of the insulation, you want enough spray-foam, or rigid foam, to keep the dew-point somewhere in the middle of the foam, not in any fiberglass (or other "fuzzy" insulation) that may be below it. You need to understand the vapor profile of the assembly. You also need to understand what the dew point is likely to be at any time of the year. It isn't easy!

    Instead of our building codes (IRC) having a single rule for closed crawl space design, we really need a regional model, like a prescriptive path, based on sound building science, for how to correctly do a closed crawl space design. Perhaps a few of us nerds can get together and each design a model for our own areas, and share it around?

  10. Expert Member
    CARL SEVILLE | | #10

    Response to James and Ted
    James - Regarding the use of AC, even in the summers in Atlanta we have parts of many days where it is cool and dry enough to use natural ventilation for parts of the summer, although AC or dehumidification is necessary much of the time. The owners who are turning off their AC are generally very low income families that simply can't afford to pay anything above a minimal electric bill, so while they may be uncomfortable, it is preferable to not being able to pay the mortgage. As I pointed out, the best intentions can be undone by unexpected behavior. Your point about humidity causing problems with the structure and contents is important, however that is as much a problem of what we build our homes and contents out of as anything. We seem to be conditioned to use the least expensive solution to all our problems as opposed to something more durable that might cost more, leaving us with moisture sensitive materials inside and out.

    Ted - Your idea of letting a crawlspace reach the ambient outside temperature doesn't really fly in extreme humidity, that's why conditioning the space makes more sense than ventilation in most cases. Even if you don't get condensation, you can still have 80% RH which is perfect for mold growth, with or without bulk moisture condensing inside the crawlspace.

  11. user-659915 | | #11

    Response to Carl
    I understand having to choose between paying the power bill and paying the mortgage. When I wrote "alternating a/c use with natural ventilation in humid weather is not a smart thing to do" I was trying to make the point that this practice can cost more than relying on the a/c continuously. The homeowners are not just going to be uncomfortable - there's a good chance they will be spending more rather than less on their power bill to do it. And however durable the construction materials of our home, it doesn't save any money for all our clothes and furnishings to go moldy.

  12. hUd6rCjJMg | | #12

    To suggest that turning off the AC doesn't save on the electric bill is like dismissing set-back thermostats as more costly than constant indoor temperatures - neither is true.

    The only good reason for sealing and conditioning a crawlspace is if it has to be used to house mechanicals. Otherwise it's less costly and easier to terminate the conditioned space at the first floor.

    Moisture problems in both warm/humid and cold climate crawl spaces are often caused by the fact that ground temperatures are lower than air temperatures in summer - often below the dewpoint - and exposed floor framing is radiantly coupled to the ground, bringing its temperature below the dew point as well.

    If there are no moisture-vulnerable materials in the floor or walls of the crawlspace, then the crawlspace can be vented and the floor framing can be protected with a combination thermal/radiant/air barrier. As Martin suggests, the best material for this is foil-faced polyiso rigid foam board with taped seams and foamed perimeter sealing. Crawlspace floors should, ideally, have a drain to daylight.

    As to designing a home to function regardless of occupant behavior - this should be common sense. A truly Passive House is one that is livable and durable whether or not the occupants read and follow the "owner's manual" (how many of you have read your vehicle owner's manual?). There are two approaches to making a house "idiot-proof". One is to automate all functions (like those damn shoulder belts that would automatically wrap around the driver when the door was closed), or make the house work by natural law without either occupant intervention or automated (and fallible) equipment.

  13. hUd6rCjJMg | | #13

    Climate Change Shelter?
    An ancillary concern about the prevalence of crawlspaces over full basements is the dramatic increase in both the frequency and intensity of tornadoes. So far in 2011, there have been 537 tornado fatalities in the US, compared to 45 in all of 2010 (and the most since 1936).

    An analysis of the 1999 Oklahoma tornado season, which included a brutal EF5 twister, found that of 40 deaths, 133 severe injuries and 265 minor injuries, there was only one minor injury among those who took refuge in basements. In the Joplin MO area, where the May 22, 2011 death toll was 155, 82% of homes had no basements.

  14. wjrobinson | | #14

    My vote is for slabs. Then
    My vote is for slabs in the humid southern states area. Then add to all homes (in tornado and hurricane areas) safe rooms which have now been perfected and cost very little.

    Humid air and drywall is a bad mix anywhere that we build. Cheap standard drywall just isn't a good product. Neither is OSB or fiberglass bats. At least for wood we can build with something like Bluwood.

    What is a good replacement for drywall, the fiberglass faced board and what else?

  15. user-659915 | | #15

    Safe rooms
    AJ is correct that safe room shelters that provide excellent protection from hurricanes are easy to build - an interior walk-in closet with 3/4" plywood lining the walls behind the sheetrock is all it takes. But he's clearly never seen a bare slab where a tornado - quite a different beast - has ripped the entire house away from its foundation. A sealed crawlspace of reasonable depth - 3' or more - provides excellent tornado protection, optimally with interior access via a trapdoor in a closet floor.

    He's also correct that humid air and drywall are a bad mix, which is one of the reasons that we in the South use other materials for siding (!!) and take care to keep the humidity inside our homes low. And why alternating a/c use and natural ventilation on a diurnal cycle is a really bad idea.

  16. wjrobinson | | #16

    My thinking for storm
    My thinking for storm shelters is below the slab though I might be into building to the best hurricane specs. too.

    Link is to a typical company in the business of storm shelters that make sense to me. Also not that difficult to make one via the many sites online that spell out the how's and why's of underground storm shelters.

  17. Perry525 | | #17

    Crawl space, walls, floors, water vapor, condensation, rot

    If you want to keep water vapor out of a crawl space (or anywhere else for that matter) you can choose to totally enclose the space in sealed plastic water vapor proof sheet, with welded or taped seams.

    Has anyone done that? I thought not.

    So why would anyone want a water vapor proof crawl space? We don't live there, we very rarely go there. There is little there that is affected by water vapor.

    The fact is, that the walls of crawl spaces and the floors above, are not water vapor proof, because of this we cannot expect to keep water vapor out of our crawl spaces. The idea that a home owner will spend a fortune over their lifetime on running a dehumidifier to remove water vapor from their crawl space is illogical. What is needed is a simple no cost solution.

    The molecules of water vapor are very tiny, they can and do enter the surface of many apparently solid objects and they easily pass through things like drywall, paper, cardboard and mortar. An illustration of this is, imagine a cardboard box full of footballs. As you can see, there are large spaces between the footballs, it is in these spaces that the molecules of water vapor can move with ease. This is how it is with many solid looking and feeling materials, there are holes that we can see and holes that we cannot see, which are amply large enough for water vapor to slip through.

    The issue is, that water vapor is attracted to areas and surfaces that are cold (below dew point) and if these cold areas are made of wood, then the water vapor is absorbed into the wood, possibly raising its water content to the point where wood rot and mold will form.

    There are solutions. One is to keep the wood warm (above dew point) but in warm humid areas this can mean high temperatures. At a temperature of 90°F and 90% humidity the dew point is 87°F. In colder drier areas this is not so much a problem.
    Another is to use a preservative that will protect against rot, decay, wood devouring insects, mold and mildew.
    In cold areas, fixing sheet polystyrene below the joists, taking care to seal all the joints, will if the room above is heated, ensure that the joists stay above dew point. And this helps to keep your feet warm.
    Fixing a water vapor proof plastic membrane below the joists will ensure that water vapor condenses on the cold plastic where it will do no harm.(when it is below dew point)
    Damp rises to four feet above ground level in a brick, block or concrete wall, it makes sense to either insert a damp proof course six to nine inches above ground level (to keep the crawl space wall dry) or, to ensure that living accommodation is placed at least five feet above ground level to avoid joists and other wood getting wet from rising damp. Water is four thousand times better at conducting heat than dry air, wet cold wood provides a easy target for any water vapor looking for somewhere to condense.

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