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New, Vented Crawl Spaces Should Be Illegal

Most building science research condemns the classic vented crawl space, at least in certain climate zones, even if the crawl space meets code

Dark, damp, and unappealing. Crawl spaces often suffer from several problems. Venting them to the outside just makes the situation worse.
Image Credit: Mike Linksvayer / Creative Commons license at Flickr

For new homes, vented crawl spaces should be illegal. That may seem harsh, considering the entrenchment of this common, code-compliant construction detail. I am on board for most local and common practices when they’re done right, but it’s tough denying that most building science research is condemning the classic vented crawl space in a humid climate. It’s time for this building practice to be outlawed for new construction.

There are plenty of crawl spaces that seem to do fine, but even a dry crawl space can have negative effects on indoor air quality in the living spaces above. Most vented crawl spaces are not dry; they are building science disasters. Do you really think building a cave below your house is a good idea?

There is danger in breathing air that comes from open earth. Radon is one of the most alarming forms of indoor-air pollution. While radon can be an issue for all foundation types, homes on slabs (stem walls or basements) offer more control over water and soil gases like radon than houses built over a crawl space.

An unvented crawl space — or, better yet, no crawl space

In humid climates, vented crawl spaces don’t make sense because they increase moisture problems.

In an unvented crawl space, plastic sheeting seals the ground and keeps most of the soil gases out of the air. But it’s easier said than done.

The plastic should be completely sealed to the walls, piers, and any other penetrations from the ground. In areas prone to groundwater during heavy rains, the crawl space grade should be sloped to a daylight drain. Crawl space walls should be insulated, while allowing access to some portion of the wall for termite inspection.

Finally, unvented crawl spaces need some method of dehumidification or space conditioning, which adds to energy costs and maintenance needs.

One of our arguments against crawl spaces is that cramped conditions leads to inferior work from all parties involved with the home: carpenters, plumbers, electricians, HVAC contractors, crawl space encapsulation workers, energy raters, the builder, and finally the homeowner. Show of hands: Who wants to go spelunking to ensure craftsmanship and maintenance?

Proper unvented crawl space encapsulation is not cheap. Moisture and ongoing energy loads have big lifecycle costs.

Consider a basement or stem walls with slab instead

Stem walls with slabs work anywhere that is suited for a crawl space. It’s better building science, because the building envelope is more defined.

Moisture and soil gases like radon are big risks to indoor-air quality. Building researchers seemingly agree it’s easier to create a solid separation of the ground with a slab. This is largely due to the even bed of clean gravel and vapor barrier called for in good slab construction. Simple perforated drain pipes can do double duty as sub-slab drainage and passive radon mitigation systems. The taller the stem wall and more volume of clean gravel, the better the protection.

Slabs are the most common foundation type in some areas of the country, but many people still object to them. I find that opinions against slabs mostly come from those uncomfortable with concrete work. The best argument against them: it’s hard to change plumbing. Changing or adding waste lines under slabs is most intensive, but usually not a problem as the house is likely undergoing major renovation anyway.

Other possible arguments against slabs

There are a few other concerns raised about slabs. One of them is that slabs can cause joint and other orthopedic problems because they’re so hard. We have yet to see any research supporting this theory. There are studies suggesting that dogs might develop toenail problems, but that should apply to all hard surfaces, including tile and hardwood.

I disagree that wood or wood-framed floors are significantly more forgiving. Modern, code-built floors have immeasurable deflection from normal foot traffic. The only finish where I notice a difference is rug or carpet with thick padding, which is among the worst material choices for those with indoor-air quality concerns. A better strategy for joint and related concerns are orthopedic insoles.

Yes, slabs can be cold, but we include insulation under our slabs, making them close to the same temperature as the indoor air. This also reduces the chance of condensation and creates another redundant barrier to soil moisture and gases like radon.

The risks of flooding and high groundwater

Flooding problems, which are usually small plumbing leaks, are more quickly discovered with a slab. Subfloors and framing are more at risk of rot than concrete. Leaks getting past the slab drain harmlessly through the gravel. When a leak pools in a crawl space, it increases humidity, energy costs, and the risks of mildew and mold.

Higher groundwater from a heavy rain is less of a risk with raised gravel drained to daylight. With a crawl space, it’s usually tougher to get positive drainage around the house. It’s also tougher to get daylight drains from under the house because of the sub-grade conditions.

Crawl spaces contribute to higher humidity, mold, and mildew. They are also are tougher to remediate for radon and other soil gases, making them a poor choice for those concerned with indoor air quality. They also contribute to higher energy costs, which is not good for outdoor-air quality.

If you’re still not convinced, at least be sure that your crawl space is properly encapsulated and unvented. That advice goes for existing homes, too.

Eventually, vented crawlspaces will be an illegal building practice in new construction. This is why our company will not include them directly below living spaces. We will do unvented crawlspaces, but we think there are usually better options.

Brian Knight is a builder and the owner of Springtime Homes in Asheville, North Carolina.


  1. Kathleen M | | #1

    Illegal? That's a little harsh.
    Here in Portland, Oregon, we don't tend to have humidity problems with our crawlspaces, and the code dictates that we install radon vents. I'm not a fan of the blanket statement that vented crawlspaces should be illegal everywhere, particularly since concrete has insanely high embodied energy.

  2. Expert Member

    Like Brian I prefer slabs when possible, but I regularly encounter situations where they are not feasible. On sites where trees or other organic material have to be removed, the excavation often ends up being around four feet deep to hit good bearing. For a one storey structure that means an awful lot of fill is necessary if a slab is to be used. Slabs are also difficult on sloping sites. If you end up having to use the stem walls to retain soil then a lot of the advantages the blog cites disappear.
    I'm also not sure you can make such clearcut distinction between the attributes of basements and crawlspaces. A well-built crawlspace should be constructed like a low basement. The only difference being the thickness of the slab. It shouldn't be any more prone to flooding or other moisture related problems - and unlike a basement it is much less likely to become living space over time. Most of the significant damage that occurred in the large-scale flooding that occurred here in Canada over the past several years was to finished basements, not crawlspaces.
    Finally, I don't think you can so easily dismiss the value of a crawlspace as a service cavity. There is a huge difference between running the necessary plumbing to a new bathroom, or adding ductwork when there is a slab. I think it's fair to say a house with a crawlspace and roof framed with gable trusses is a renovator's dream.

  3. Dana1 | | #3

    The utility chase aspects of crawlspaces have value.
    The people who truly LOVE slab-on-grade are builders who routinely run ducts in attics, which can come with it's own sets of disasters. One might make the case that mechanicals in vented attics above the insulation should be illegal too, but unvented sealed attics with the insulation at the roof deck are a significantly more expensive and more difficult to detail than unvented sealed crawlspaces.

    The statement "Proper unvented crawl space encapsulation is not cheap." is only true for retrofits. In a new build it's not very expensive at all. The primary cost adder of properly treated NEW sealed crawlspace over a properly treated slab-on-grade is in the floor framing & subfloor. There's no rocket science, high art, or high cost to building an ICF stemwall foundation with an (insulated, where appropriate) rat-slab on top of an EPDM or heavy polyethylene ground vapor barrier. Inspection on the quality of the crawlspace construction can happen before the framing for the first floor is done, in broad daylight, without high levels of spelunker certification.

    The rat slab of a sealed crawlspace can be at the same elevation as with a slab-on-raised-gravel-grade approach to drainage, if desired, at a modest cost of a somewhat taller stem wall.

  4. morganparis | | #4

    Rats and comfort
    Here in central North Carolina we routinely prefer encapsulated crawl spaces over concrete floors for all the good reasons cited by commenters above, though usually without 'rat slabs' - do we have fewer vermin here? But I'd also challenge the comment in the article about the comfort factor. Unless you are actively heating the slab a concrete floor is going to feel colder underfoot - in certain climates this may be an asset but certainly not in mine - and as any dancer will tell you, however limited the engineering deflections a wood floor is always going to be gentler on knees and feet.

  5. Expert Member

    I almost missed Christmas dinner when a rat chewed through the leads to the fuel injectors on my truck. I hope you do have less vermin than us!

  6. turner7804 | | #6

    North/South perspective
    As a builder retired from Vermont to the South's heat & humidity, I've enjoyed checking out building practices. We followed two of our children down here who have each lived in older and new homes and built new homes. What you get from the stand point of quality has run the gamut much as it did in northern New England...good and bad builders down here also. The older homes with dirt floored crawl spaces have superior venting. Newer ones don't and I've seen 5 yr old places with frightening situations requiring very expensive remediation. The newer cookie cutter homes are not as easy to vent as the classic ranches, bungalows & capes....basically simple rectangles. One of my "kids" just built on a slab, and understands that there won't be any extensive remodeling....I find their solid stained oak floor to be very comfortable. The other recently buought a beautifully maintained 1928 rambling tudor that has full basement and crawlspace with mud slab. The house is incredibly solid and quiet wth no discernible deflection in the 1 1/2" maple floors throughout. All that aside, I learned growing up as 3rd generatikn builder, to refer to the slab as a "mud slab" vs a "rat slab". ;)

  7. ErgoDesk | | #7

    Problems all homes share
    Ever notice that this and other sites tell us how to solve problems but these problems always seem to be related to what the houses are made from, WOOD. Remove the WOOD and majority od all problems are solved, FYI.

  8. marcjb | | #8

    Glad I'm not on a slab.
    Our home in coastal BC has a 4' crawl within icf walls and a slab floor. When we moved in I spent about an hour rolling around on a dolly sealing the vents and checking the air barrier for leaks. The electric hot water heater is down there and it keeps the space pleasantly warm while a small baseboard heater gets triggered only on the very cold days. When I installed a mini-split heat pump the best locations for the two units were on opposite sides of the house. Thankfully the pipes and wire run through the crawl, avoiding the poor esthetics a slab floor would have imposed. At 4' working down there is not bad at all.

    When my wife's parents were coming to stay with us for twelve weeks we decided to convert a large pantry into a powder room since we only had the one bathroom. I think it's safe to say we wouldn't have done it if we were on a slab and we are now very glad to have that extra facility. Another example is when we bought a bidet toilet seat. It requires an electrical receptacle and adding one behind the toilet was again quite easy. Finally, the crawl also gives us about 1000sqft of warm, dry storage space. As a builder in this area, I recommend properly built crawlspaces to my clients if the site allows.

  9. synergytodd | | #10

    Slabs are fine for one story homes
    I agree with the first 5 1/2 paragraphs completely. In one story homes, slabs are fine but in two story homes, conditioned crawlspaces are the best solution for our area. I agree that unvented crawlspaces need dehumidification or space conditioning, but it doesn't necessarily add to the cost. My company, Synergy Airflow and Ventilation LLC,, performs HVAC sizing and designs, closed crawlspace encapsulation, spray foam insulation, and home energy audits. Building a home with the HVAC system located in a properly constructed conditioned crawlspace allows the smallest HVAC system many times by over a ton in our typical customer's home (compared to ducts in a vented crawlspace or vented attic). We centrally locate the variable speed compressor heat pump and centrally locate the return(s) with proper fresh air ventilation and jumper ducts. We feed air into the crawlspace and positively pressurize the crawlspace so radon and moisture is pushed outward. In new homes, we recommend commercial grade dehumidifiers but oftentimes they are not needed. We educate the homeowner to monitor the crawlspace and the interior of the home's humidity and to contact us if it rises above 60%. In existing homes, we require commercial grade dehumidifiers to be installed because many have oversized, single speed, single stage HVAC systems that don't dehumidify especially in the spring and fall. I often tell my customers concerned with humidity that let's try to get it so dry your nose bleeds. My personal home runs about 30% relative humidity and my indoor air quality is perfect. All hardwoods, central vacuum, fresh air ventilation, direct vent fireplaces, electric cooktop with downdraft with fresh air makeup kit. With all that said, most of the large 2 story homes that are built in our area are impossible to get the ductwork properly designed and installed if they are built on a slab. The ductwork for the first floor is most often inaccessible on a slab home once it is sheet-rocked. On a small one story home slab home, we are ok with the ductwork in the attic as long as the attic is encapsulated with ICYNENE open cell foam. In our area, slabs are installed in most of the cheap production builder homes and most of the custom homes are built on conditioned crawlspaces.

  10. Expert Member
    MALCOLM TAYLOR | | #11

    Could it be that a lot discussion centres on wood frame construction because, as you point out, it is what people use to build here? If you go to a UK based building discussion group they will all be trying to solve the problems inherent to masonry construction. Remove the masonry and their problems might disappear too.

  11. phuelman | | #12

    A slight twist on terms
    I agree with the underlying premise of the article, most of the key points, and much of the discussion. How we connect our homes to the ground (or not) is huge and foundation choice can certainly make or break a house. Unfortunately we see far too many bad basements, bad crawlspaces, and bad slabs. But I would admit that bad crawlspaces are very common, so nasty, and no fun to fix.

    I wouldn't outlaw a properly vented crawlspace. However, my definition of "properly vented" would be quite different than the codes allow. The space should be essentially outdoors -- not unlike a vented attic that is properly insulated and air sealed from the home. I think homes with (properly insulated and air sealed) raised floors on piers or grade beams is a viable candidate for many of the issues described in earlier posts. I would go even further and suggest that this approach can be done in cold climates, as well, though we get push back on comfort and utilities, much like the slab conversation. And tornado protection has to be addressed differently for those in tornado alley, much like manufactured housing.

  12. phuelman | | #13

    I should add that I am perfectly happy with "good" basements and conditioned crawlspaces (mini-basements), too. You simply have to properly deal with the four moisture transport mechanisms (bulk/gravity, capillary action, air flow, and diffusion), provide adequate thermal insulation, manage condensation potentials, and maintain a drying strategy.

  13. Brian Knight | | #14

    Thanks for all the great
    Thanks for all the great comments, editing assistance from Scott Gibson and GBA for posting the blog. Crawlspace versus slab always generates lively emotional discussion in this country. I standby the idea that vented crawl spaces should be outlawed for new construction but acknowledge they can work fine in drier regions. It seems most research condemning vented crawl spaces is more east coast than west.

    I'm a strong believer in serviceable cavities but think clever design negates any need to build crawlspaces. I don't think this idea of access should compromise the building envelope which is exactly what a crawl space does. Crawlspaces, including unvented or those considered properly built, convolute the building envelope in the riskiest area possible; the ground interface.

    I think GBA is a great platform for the idea that crawl spaces should be avoided. Passive house, healthy indoor air quality, and energy efficiency details are all improved by eliminating vented and unvented crawlspaces from the design of a new building.

    Crawlspace versus basement or slab can be a regional issue but is typically more of a topographical one. I think the best lots for crawlspaces are those that slope from four to seven feet across the building footprint. That's not quite enough slope for a comfortable walkout basement and takes a lot of fill to level up. Usually smart excavation and design can turn such a lot into a walkout basement with more usable square footage and a building envelope that is not confused about it's role or boundary.

    I say crawlspaces hurt the more important access issues. When builders raise a house for a crawlspace, it usually means adding steps. Aging in place and better accessibility means eliminating as many steps as possible. The idea of raising a crawl space for more headroom in a never used area while hurting everyday traffic flow doesn't make sense to me.

    We are about to start a new home that fits the 4-7' slope conditions and was originally designed for an unvented crawlspace. I convinced the owners to change the uphill wall panels of the house into pre-cast concrete panels which will allow us to sink the house into the site, eliminating the steps going up to the main entrance and bringing the big, downhill-side deck closer to natural grade. No windows were eliminated, just moved up higher in the bermed wall to allow 5' of backfill against the house.

    I have mixed feelings about basements that don't have walkout topography thus daylight drainage. I suppose sumps can have redundant backups but would tend to gravitate towards raft type foundations with daylight drainage if possible. Luckily our climate and region is not as difficult as more northern ones.

    NC requires that two story homes be zoned and we always manage to fit the required ductwork for our forced air systems in the floor truss system, even with returns in every bedroom. It's not easy, it requires extra planning but we don't need to put them in the attic and we don't need to build a semi-controlled cave.

    I disagree that crawlspaces are better suited for termite protection or inspection. It depends on many details, much like protecting building materials from moisture. I liked the comments about pier construction, some designs certainly blur the lines.

    Something I want to see discussed is the idea that crawl spaces do not belong in new passive houses. Surely existing homes can meet the renovation version of the certification, and existing homes with vented crawlspaces should strive for properly unvented conversions. From my limited knowledge and research there seems to be no new passive houses with crawlspaces. I did find this: 3 reasons to rethink crawlspaces in passive house design.

    Apparently, even unvented crawlspaces complicate the building envelope too much for passive house.

  14. Expert Member
    MALCOLM TAYLOR | | #15

    Thanks for the reply, these blogs become so much more useful when their authors come back to engage with the commenters.
    As a twist on your remarks about topographic reasons for crawlspaces: I build on Vancouver island, where many excavations end up with large rock outcroppings several feet down. This means it is a rare site that allows deepening of proposed crawlspace to make a full basement, but at the same time still would require a prohibitive amount of fill to do a slab on grade. In these cases a conditioned crawlspace makes sense to me. What would you suggest to a client as an alternative?
    I think the link you posted may say more about the way that Passivehouse calculates their energy requirements than whether or not crawlspaces make sense for a very well insulated building. One of the dangers of adhering too closely to concrete rules or standards is that you may decide to do things to gain accreditation that might be quite illogical.

  15. Brian Knight | | #16

    That's a good example Malcolm
    That's a good example Malcolm and agree that unvented crawlspaces are a fine choice for many situations. I would add that even in situations without bedrock, front entrances on the high side of slightly sloping lots tend to make sense for unvented crawlspaces. If main entrances are on contour or from downhill then it's easier to avoid them.

    For Passivehouse, I'm sure the interior area calculations are a major factor but I think the air-sealing, moisture and energy concerns are more relevant.

  16. bobwojo | | #17

    add 4 feet
    Just dig down an extra 4-8 feet or so and call it a basement. Solves all problems with heating, electrical and plumbing routing and accessibility.

  17. Expert Member
    MALCOLM TAYLOR | | #18

    And double the cost of the excavation and foundation? If I regularly made those types of decisions I'd be out of business by now.

  18. GBA Editor
    Martin Holladay | | #19

    Response to Malcolm Taylor
    So what's the upcharge from going from a 4 ft. crawl space to an 8 ft. basement? If your excavation contractor and concrete contractor are charging you twice as much for the basement as the crawl, you really ought to shop around for new subs. That's nuts.

    For excavation, a significant part of the cost of the job is just getting the backhoe on the site. Once the backhoe is there, the upcharge to dig a deeper hole isn't going to be twice the cost.

    A similar argument could be made for the concrete contractor.

  19. Expert Member
    MALCOLM TAYLOR | | #20

    Reply to Martin
    As I explained in an earlier post, it generally isn't as straight forward as just digging deeper as we often hit bedrock, but lets leave that out of the equation.The excavators in here generally charge a straight $100 drop fee. That is less than their hourly rate. So yes, the cost is in direct proportion to the depth - except that you have to add the trucking to dispose of the extra excavated fill.

    We don't use a separate foundation contractor here, the framing crew cribs up the forms, and strips them after the pour. So again, time and materials are in proportion to the size and depth of the walls. Reinforcement isn't though. It increases considerably once you go over approximately five feet. Deeper foundations also generally preclude running drains to daylight, so add another truckload of rock for pits to the bill.

    At the end of this process I'm left with a buried, taller version of my crawlspace, that no one here wants, with walls that have to be insulated to the same level, and because it may become living space, our code mandates that it has to have the electrical services roughed in. Somewhere on the plans I also have to find room for, and build, a stair rather than a hatch.

    It makes no economic sense. And if the solutions we suggest to simple problems end up appreciably pushing up the price of construction, builders like me aren't going to be able to adopt them.

  20. GBA Editor
    Martin Holladay | | #21

    Response to Malcolm Taylor
    I have no reason to doubt your report about foundation costs in your area. I'm curious to hear from other GBA readers on this issue, however. Does an 8 ft. basement cost twice as much as a 4 ft. crawl space?

  21. STEPHEN SHEEHY | | #22

    Reply to Martin and Malcolm
    Maybe a better question is whether the additional cost for a basement instead of a crawl space is worth it to the owner?
    A full height basement allows a place for a water heater, maybe a furnace or boiler, laundry, HRV, etc. While all but a laundry can be installed in a conditioned crawl space, installation and maintenance are more awkward. The laundry ends up in much more expensive real estate upstairs. You still need electrical service down in the crawl space and you still need an insulated slab.
    Of course the extra storage room for stuff is a big issue for some people. A real basement is a lot more useful.
    We opted for a slab in our new PGH. Storage is over the garage. It sort of forced us to rethink how much stuff we really need. We're extremely pleased with the polished concrete floor, which really didn't feel cold on my bare feet this AM, with outside temp at 7 F.

  22. Expert Member
    MALCOLM TAYLOR | | #23

    Sorry i've forgotten: Is your slab heated?

  23. STEPHEN SHEEHY | | #24

    reply to malcolm
    No, it is not heated. Four inches of reclaimed xps under the slab probably help considerably. I'm sure the floor is usually at about room temp.

    Can you give a rough estimate of how much more it would cost for a full basement as opposed to a conditioned crawl space, assuming normal excavation conditions?

  24. Expert Member
    MALCOLM TAYLOR | | #25

    That's a lot harder than just giving a proportion as I have done. The cost of crawlspaces differs wildly based mainly on the footprint of the house and the depth of the excavation. So it would be fair to say that for a very small crawlspace the additional incremental cost, while still doubled might not amount to that much. But again, try and find the space on your plans for a stair in such a house. If you anticipate a basement and plan accordingly, what do you do if you hit rock? persevere, or alter the house plans than have come to rely on that space?
    What occasioned my initial response is the idea that the solutions to any perceived problem can be decided independent of economic considerations. Right now a lot of energy efficient homes are built by clients with a commitment that allows cost decisions which builders such as myself don't have the luxury of making. If we decide that "green" houses are going to include things that are probably not really necessary but make a big difference to the costs, then my feeling is that they will be stuck as a boutique product.

  25. STEPHEN SHEEHY | | #26

    I was just trying to get a sense of how much additional cost we are talking about for, say, a 24x40 foot foundation. Obviously, running into ledge is a risk in some places, but that risk can materialize whether one is putting in a slab, crawl space or full basement. We are lucky to have sandy soil with no rocks at all, so excavating was simple.

    I don't dispute your point about energy efficient homes often being built by clients who can spend extra $$ on features that aren't really needed. My house is an example, although not outlandishly so. Let's face it, architect-designed custom built homes cost more than typical spec-built.

    But as you may know, most new homes built here in Maine have full basements, even lower priced homes. Many homes need a place for an oil tank, boiler or furnace, water heater, laundry and so on. In addition, basements are usually warm and can be used as a workshop, if not as real living space.

    Assuming an ordinary hole in the ground on a level lot, no ledge, no importing of fill, would a 24x40 crawl space instead of a full basement add $5000? $15,000? More? Less? I have no clue. I'd assume that around here at least, a prospective buyer would want to know how much the extra headroom might cost in order to make an informed decision.

  26. Expert Member
    MALCOLM TAYLOR | | #27

    Let me give you some ballpark figures from the last house I built - a one story, about 2150 sf, with an irregularly shaped foundation of approx. 62ft x 34ft, averaging 42" high including footings.
    - Concrete for foundation and scratch coat $ 5,800.
    - Hardware, rebar and rentals $1,700.
    - Excavation, backfilling and material $2,300.
    - Labour $4,800.
    - Total = $ 14,600.

    So if we had gone with a full basement, it would have been around 29,200.

    Again I'm not trying to denigrate basements. They make a lot of sense in some regions and on some sites. I just think there aren't enough problems inherent to crawlspaces that would lead me to a blanket policy of switching to full basements. The $14,600 saved went towards a three-bay garage and workshop. That space, and the breezeway it created on a south-facing waterfront lot, are much better value that I could have provided below grade.

  27. morganparis | | #28

    Cost delta, crawl to basement
    We currently have a house in construction whose crawl space on the downhill side is over eight feet at the deepest point and about thirty inches at its shallowest on the uphill side. The slope would clearly have allowed for a daylight basement but the decision to not go there was made on two counts. First, the site is extremely rocky with ledge visible even on the surface. In our area the cost of blasting, the slab, and building a nine foot retaining wall would have added about $45 - 50k to the budget not counting the cost of the additional space for the stair. The second reason is that the space would have had zero value to our clients, who though still active and healthy in their late fifties plan to grow old in the house and wanted all the accommodation including laundry and guest accommodation on a single level. The home will have at-grade entry and stepless breezeway connection between garage and mud room. I shudder to think what the cost of fill and retaining might be to create a slab floor in that condition. I guess we could have done a suspended slab, but why? Most of our new home clients in recent years have had the same concern for the aging in place benefits of a single level home with a flush entry and for the resilience and comfort of a suspended wood floor: most of our sites have had sufficient slope to make that most easily achievable with an encapsulated crawl space. Of course it should go without saying we've not considered a vented crawl in new construction for more than two decades.

  28. Brian Knight | | #29

    I don't think it goes without
    I don't think it goes without saying that your crawlspaces have not been vented for 20 years but glad to hear at least some good builders are aware of the difference. The vast majority of new crawlspaces I see in my area are vented, with fiberglass batts in the floor cavities. It's a travesty to me that this practice is still legal in our region and climate. Spec housing is flying off the shelves in our city and most people don't have a clue about the legacy of the attached caves they are buying into.

    We completed this house last year and hit bedrock pretty close to the surface of a basement excavation. We faced a difficult decision of bringing in a blasting contractor or having the grader rent a bigger machine with a rock hammer. Luckily, we went the hammer route and the rock gave way without too much trouble. Had it been harder or more solid granite, it would probably have been a different story. I estimate that the rock added 8-15K to the costs depending on how you figure the "free" boulders we got for retainage materials.

    It doesn't have to be basement or crawlspace. Any crawlspace lot can fill stem walls with gravel or compacted fill. From this cost you can subtract the costs of encapsulation, hatch doors and HVAC equipment and loads. A house without a crawlspace is likely to be more airtight, have less moisture concerns and energy demands. Accessibility is typically easier because you can backfill masonry higher, without concerns of staying below the wood floor band.

  29. morganparis | | #30

    Hey Brian
    If you've only seen poorly built vented crawl space I can understand your antipathy but you need to come down some time for a visit and see crawl spaces done right, tight, dry and highly energy efficient, complete with step-free at-grade or near-grade entries. All it takes is a little care and forethought.
    By the way, I'm bewildered at the idea that building on a slab avoids the need for HVAC equipment. And how (and why) you would economically fill and retain an eight-foot-plus crawl space - or as we prefer to call it, a clean, dry and inexpensive mechanical and storage room.

  30. Brian Knight | | #31

    Thanks James
    I always love seeing other's good work. Still, I would much rather admire the potential space as unfinished basement or walk-out finished space. Kneeling and crawling into the depths of a plastic encapsulated cave to ensure workmanship isn't fun. Most of the workers installing stuff, are working off raw dirt or mud which is less appealing and even more prone to sloppiness.

    A highly energy efficient crawlspace is one that doesn't exist in the first place. Crawlspaces add volume, surface area, air-leakage, moisture loads and unwanted thermal transfer to the building envelope. They are not easy to properly encapsulate and are further from the idea of a perfect building envelope.

    Most crawlspaces are not good storage areas. Regularly used storage puts the thin plastic vapor barrier of the ground at risk. The better you build an encapsulated crawlspace for storage, the more it resembles a basement or walkout level, lower floor.

  31. techbiker | | #32

    I know this thread is ancient, however I have a few thoughts to add.

    1. By "vented" are you including crawl spaces with only a powered exhaust vent? I have found very few people who are opposed to the "exhausted" crawl space. About the only complaint I can find surrounds concerns about the fan(s) failing. The fans draw a small amount of conditioned air from the living area, maintaining humidity levels and exhausting contaminates. Encapsulation should help by encouraging air to be drawn from the living space instead of directly from outside.

    2. I believe there are overlooked merits to a "leaky" philosophy. Less complicated envelopes allow for easier, faster, and less expensive initial construction and renovations. They also anticipate water intrusion and include multiple drying paths. If you can live with a somewhat potentially leaky area, a crawl space offers clear benefits such as ease of access vs. a slab. They are also (imo) more cost-effective than basements in areas with expansive clay soils. Further, a crawl space with dirt floor is great for sewer pipe access. Changing out a sewer pipe in a basement with concrete floor is likely to be nearly as bad as doing so in a slab house.

  32. GBA Editor
    Martin Holladay | | #33

    1. A sealed crawl space with a small exhaust fan is not considered to be vented. For more information on this issue, see "Building an Unvented Crawl Space."

    2. There are a few builders who agree with you that crawl space foundations make sense. (I'm not one of them, however.) If you insist on building dirt-floored crawl spaces, however, you must, at the very least, cover the dirt with well-installed 6 mil polyethylene.

    I'm not sure what you mean by a "leaky" philosophy. If you like to build leaky houses, you won't find much support here at GBA.

    1. techbiker | | #34

      1. I see. Thank you!

      2. Thank you for the quick response. My personal opinion is that slabs are generally a pain and can shorten the economic lifespan of a structure. I would have to respectfully disagree with one of Mr. Knight's points (infrastructure access w/ slab). There are many reasons one would want to access the plumbing, electrical, etc. outside of a renovation. Some follow:

      -Foundation shifts affecting slope of PVC waste pipes.
      -Water or sewer pipe leaks.
      -Access for infrastructure upgrades. Say an avid gardener buys a house with 1/2" water lines to the spigots. Many examples in the commercial world.

      The "leaky" comment was really a philosophical point of discussion. It's fascinating to explore construction methods used on older (~1900) buildings. Those builders frequently built durable, elegant, cost-effective structures with limited carbon footprints. Lots of organic (biodegradable) materials that in many cases have stood the test of time. Yes, we saw big efficiency gains in the 1980's. It now usually takes ever more polluting, expensive, and/or exotic materials to eek out slightly more energy efficiency though. Is it green to externalize the pollution from the production of these materials to some factory in China?

  33. jewilson | | #35

    I live on a log cabin with approx. 600 sq ft vented crawlspace. I am in process of sealing the floor with at least 8mil poly and using borafoam boards to insulate the walls and bands. I will seal off all openings and wrap all piers. The humidity is 80% constant unless I run the air conditioner then the humidity drops to the lower 70s. I will install a dehumidifier. I am doing the work my self in phases. Phase I is leveling up the area, which is no small feat. The contractor who built the cabin in the late 1990s left a 3'x15' ditch running down the center of the structure. He constructed 5 32"x24" piers that were never backfilled. I am leveling the soil under the house with the plan to have it on a level grade. This is difficult work as I am digging and shoveling the fill dirt on my knees under there with the mold mildew black widows and mice in various stages of decomposition. I am wearing a respirator at all times along with other protective equipment. There as no sign of standing water despite the very low areas. I was surprised at that. I have done about 1/4 of the grading work at this point. Not to bad considering the amount of digging down the high areas and using the cut to fill in the ditch.. My question is this. Do any of you have any experience or knowledge of how to compact the loose fill dirt around the piers and in the ditch. I thought about soaking the soil with a hose to compact it but that could prove problematic. Any kind of vibratory compaction rental machine would kill me by carbon monoxide poisoning under there. Just looking for advice or ideas. Any would be greatly appreciated...John E. Wilson

  34. GBA Editor
    Martin Holladay | | #36


    Q. "Do any of you have any experience or knowledge of how to compact the loose fill dirt around the piers and in the ditch?"

    A. Don't overthink this -- perfect compaction isn't necessary. If you want, though, you can use a hand sledge (a sledgehammer with a short handle) in conjunction with a 10-inch-long scrap of 2x10 -- whack the piece of lumber with the sledge.

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