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9 Helpful?

Building an Unvented Crawl Space

Sealed crawl spaces have less mold and stay dryer than vented crawl spaces — and they often save energy

Posted on May 13 2011 by Martin Holladay

Residential foundations vary widely from one corner of the U.S. to another. Builders in some regions love basements, while builders in other regions swear by slabs on grade. Although most builders have a theory to explain these regional preferences, the main reason for these variations is habit, not logic. In areas of the country where basements are rare, there usually aren’t any technical barriers to building basements; and up north, where basements rule, it’s perfectly possible to build on a slab.

Slabs have several virtues: they are inexpensive and they keep all of a home’s living area above grade, away from dampness and mold. Basements also have their virtues: they keep plumbing pipes from freezing, provide a good place to install a furnace and run ductwork, and provide a useful area for storage.

Crawl spaces cost almost as much as a basement, with none of a basement’s advantages

Crawl spaces are more of a puzzle, and it’s hard to come up with a reason to like them. I’m sure that as soon as this blog is published, a builder from North Carolina will write in with an eloquent defense of the crawl space. I don’t have a dog in this fight, however, so if you really want a crawl space, go ahead and build one. Just be sure you get the details right.

If you’re perverse, and you want to build a damp, moldy, nasty crawl space, just do two things: insulate the crawl space ceiling with fiberglass batts, and vent the crawl space to the exterior. If you live in the Southeast, within a few short years the fiberglass batts will begin to hang down at odd angles like drunken stalactites. Every summer, the open vents will introduce huge amounts of moisture into the crawl space. You’ll end up with a classic moldy crawl space — one that represents a significant source of moisture for the house above.

In a hot, humid climate, venting a crawl space is counterproductive. During the summer, the outdoor air in North Carolina holds more moisture than the cooler crawlspace air. When humid outdoor air enters the crawl space vents, it soon hits cool surfaces — concrete blocks, water pipes, and air-conditioning ducts. Condensation forms and begins to drip. The more you ventilate, the wetter the crawl space gets. If you set up a fan to double the ventilation rate, you’ll just make the pipes drip faster.

Crawl space vents can also cause problems during the winter, when they introduce outdoor air that can cause pipes to freeze.

Unvented crawl spaces are permitted by building codes

In most areas of the U.S., sealed crawl spaces work much better than vented crawl spaces.

Most building codes permit the construction of unvented crawl spaces. In the 2006 International Residential Code, requirements for unvented crawl spaces can be found in Section R408.3. If an unvented crawl spaces has a dirt floor, the code requires exposed earth to be covered with a continuous vapor retarder with taped seams: “The edges of the vapor retarder shall extend at least 6 inches up the stem wall and shall be attached and sealed to the stem wall.”

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 spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. 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.)

Advantages of unvented crawl spaces

Unvented (sealed) crawl spaces:

  • Stay dryer than vented crawl spaces;
  • Protect pipes from freezing;
  • Require less insulation than vented crawl spaces (since the area of the perimeter walls is less than the area of the crawl space ceiling); and
  • Bring ducts within the conditioned envelope of the home — an improvement that usually results in energy savings compared to vented crawl spaces.

According to researchers who conducted a careful study of vented and unvented crawl spaces in North Carolina, homes with sealed crawl spaces with insulated foundation walls use 18% less energy for heating and cooling than identical homes with vented crawl spaces with insulation between the floor joists.

A crawl space can work well in a dry climate

However, similar energy savings cannot necessarily be expected in dry climates. Researchers comparing the energy performance of homes with different crawl space designs in Flagstaff, Arizona found that homes with insulated floors used less energy than homes with sealed crawl spaces and insulated foundation walls. According to a report on the research, “This seemed counterintuitive; ducts are a notorious source of heat loss. With all the Flagstaff homes’ ductwork in the crawl space, one would expect better performance from the warmer, wall-insulated crawl spaces. But according to Cyrus Dastur, the Advanced Energy building scientist who directed the research, those homes’ lack of floor insulation let heat radiate from the first floor to the crawl space, robbing more heat from the house than was saved by keeping the ductwork warm.”

While vented crawl spaces often perform poorly in the humid states of the Southeast, they perform well in most Western states. According to an article in Home Energy magazine, “In the drier regions of the West, and even — surprisingly — in the marine climates of the Northwest, vented crawl spaces work acceptably most of the time. The hot-dry conditions in summer and the cold-moist conditions in winter do not cause the same problems that hot-humid conditions cause in the rest of the country. ... The Washington State University Extension Energy Program (WSU-EEP), as part of its work for Building America, monitored four test houses in Vancouver and Moses Lake, Washington, for over a year and found that the vented crawls rarely, if ever, reached dew point and that they remained above 80% RH only for brief periods of time.”

Creating an unvented crawl space

If you live in a humid climate, and you still want to build a crawl space — or if you are trying to correct problems in an existing moldy crawl space — here’s how to go about it.

  • To help keep the crawl space dry, correct any grading problems on the exterior so that the grade slopes away from the foundation.
  • Remove all rocks and debris from the crawl space floor, and rake the dirt smooth. Ideally, the crawl space floor will be higher than the exterior grade, although keeping the grade high on the interior of a crawl space is not always possible.
  • If the home is located in an area where radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. is common, install a passive radon collection system in the crawl space floor.
  • If the crawl space is subject to water entry, be sure to slope the floor to a sump equipped with a drain or a sump pump.
  • Install a durable vapor barrier — for example, a 20-mil pool liner or Tu-Tuf poly — over the floor and extending up the crawl space walls, to within 3 inches of the top of the wall. Leave a 3-inch-wide termite inspection strip at the top of the wall.
  • Attach the top of the vapor barrier to the wall with horizontal battens, secured to the wall with masonry fasteners.
  • Seal the seams of the vapor barrier material with a compatible tape or mastic; many builders use duct mastic embedded in fiberglass mesh tape.
  • Consider installing a 2 in. or 3 in. thick concrete slab (a “rat slab”) to protect the vapor barrier.
  • If this is a new-construction crawl space, and you can’t afford a rat slab, you may want to install a temporary (sacrificial) second vapor barrier — usually a layer of 6-mil poly — on top of the permanent vapor barrier; once construction is complete, this temporary poly is rolled up and discarded.
  • Unvented crawl spaces have insulated walls; no insulation is needed at the crawl space ceiling. Insulate the interior of the walls and rim-joists with rigid foam — many builders use Thermax, a polyisocyanurate foam that does not require a thermal barrier or ignition barrier — or spray polyurethane foam. Another option: insulate the exterior of the foundation walls. If your crawl space has stone-and-mortar walls, you can’t insulate the walls with rigid foam; the only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam. Install at least as much insulation as required by the 2012 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. for basement walls, namely R-5 for climate zone 3, R-10 for climate zone 4 (except Marine Zone 4), and R-15 for Marine Zone 4 and climate zones 5, 6, 7, and 8. (Crawl space walls should be insulated in the same matter as basement walls. For more information on this topic, see How to Insulate a Basement Wall.)
  • Install a floor register in the floor above to allow air to flow between the living area and the sealed crawl space below.
  • Install an exhaust fan or a forced-air register to meet code requirements for conditioning the crawl space. Be sure that the fan does not exceed air flow requirements for the size of the crawl space, since exhaust fans carry an energy penalty.
  • Install good lighting — most crawl spaces will benefit from at least six fixtures, spaced evenly across the crawl space ceiling — controlled by a switch located near the entry door.

New-construction crawl spaces often require temporary dehumidification to remove construction moisture. Once the home is dried in, it’s a good idea to install a stand-alone dehumidifier in the crawl space and run it for three or four months until the interior relative humidity stabilizes.

Any combustion appliance (for example, a water heater or furnace) in a sealed crawlspace should be a sealed-combustion unit.

For further details on building a sealed crawl space, consult the resources listed in the “More Information” sidebar.

Even well-detailed crawl spaces may not make sense

A well-detailed crawl space is a thing of beauty, as shown in the photos (below) of renovated crawl spaces. (Perhaps such crawl spaces aren’t beautiful to all eyes, but they are to mine. I used to work as a home inspector, and I have spent far too many hours crawling under houses, in damp caves littered with debris and animal droppings.)

That said, it’s important to emphasize that even a well-detailed crawl space represents a problematic foundation design. Bill Rose, a renowned building scientist and a research architect at the Building Research Council at the University of Illinois, remains skeptical of crawl spaces. “I’m very cautious about crawl space construction — maybe it ought to be abandoned,” said Rose. “It would cost as much as a basement to get a crawl space right.”

One problem with sealed crawl spaces: the air quality in such a crawl space may be poor unless the crawl space is equipped with an exhaust fan that runs continuously. Of course, such a fan can be part of a whole-house exhaust ventilation system; but when (not if) the fan eventually conks out, the homeowner is unlikely to notice. That raises concerns over air quality in the crawl space — and also in the home above.

According to the previously cited Home Energy article, “Additional radon testing showed that radon levels in the closed crawls — with a relatively low dilution rate — were roughly 10 times the levels measured in the vented crawls.”

If a builder chooses to condition a sealed crawl space using “Option 2” from the two code-approved options — that is, by installing a forced-air register rather than an exhaust fan in the crawl space — it’s easy for any radon or moisture in the crawl space to circulate throughout the house.

The more you think about crawl space problems and crawl space remedies, the better a slab on grade begins to look.

Last week’s blog: “Alternatives to Clothes Dryers.”

Tags: ,

Image Credits:

  1. Aire Solutions
  2. Charles Buell - Buell Inspections
  3. Crawlspace Doctor

May 13, 2011 9:24 AM ET

NC crawl spaces
by Hunter Dendy

First crawl space justification from North Carolina- flood elevation requirements.
Second- high water table negates a basement,so a conditioned crawl is sometimes the only/best way to get the ducts in the thermal boundary in a multilevel house.
(you asked)

May 13, 2011 10:09 AM ET

Another reason to do crawl space...
by Armando Cobo

Great article, but I'll give another reason to do crawl space. Very expansive soils w/ high PVR.... most times you have to do a peer and beam foundation. Very common in TX.

May 13, 2011 10:39 AM ET

Edited May 13, 2011 10:40 AM ET.

Love/hate crawls
by David Meiland

Almost no one has a basement here in NW WA, most are crawls, some are slabs. I will say one thing for crawls--if you want to remodel, access is easy compared to a slab. Folks with slabs need to plan on leaving the bathrooms and kitchens right where they are.

What's the detail for termite viewing? You want to insulate the interior of the perimeter walls, but you need to leave a 3" strip bare directly under the mudsill? Is this continuous around the entire house?

May 13, 2011 10:47 AM ET

Response to David Meiland
by Martin Holladay

I guess (like me) you live north of the termite line. Aren't we lucky?

Termite inspection wreaks havoc with the continuity of a home's thermal envelope. Those insects are nasty little creatures -- another reason to live up north.

May 13, 2011 11:00 AM ET

# 1 Best Use of Crawlspace
by John Brooks

Storage space for large quantities of drywall mud
Just Kidding...

I do think slabs are better for Universal Design & "Aging in Place"


May 13, 2011 11:03 AM ET

Personally anti-slab
by David McNeely

I think I'll run a new cat6 to the office, or a dedicated electrical circuit, or a coax to a new t.v., or maybe I want a hose bib where the builder was too cheap to install one, and I absolutely need to run a gas line so I can get rid of the electric range... and a dozen more reasons off the top of my head.
I have refused to buy otherwise desirable houses because they needed updating, but were on a slab. All houses need updating, and needs are changing more quickly than ever.

May 13, 2011 11:31 AM ET

All you slab-haters out there...
by Martin Holladay


May 13, 2011 11:43 AM ET

Sealed Crawlspace Ventilation for Superinsulated House
by Daniel Ernst


Thanks for the thorough blog.

For those of us who are unlucky enough to live with termites, this is a quandary . . .

For a sealed crawlspace, how would you meet the intention of the IRC ventilation requirement, given PH airtightness, minisplit heating / cooling, and HRV?

Do you know of any PH projects that use a sealed crawlspace, or are they all built on FPSF?

May 13, 2011 11:54 AM ET

Response to Daniel Ernst
by Martin Holladay

I know of one Passivhaus with a crawl space -- the Freas house in Olympia, Washington. The builders decided to insulate the floor joists above the crawl space with blown-in fiberglass, and to keep all of the mechanical equipment and ductwork upstairs.

May 13, 2011 12:00 PM ET

Basements ;-)
by Daniel Ernst

Quotes taken from the book: "From The Ground Up," by John Cole and Charles Wing

"A full basement foundation is essentially a large and expensive concrete-lined well that we try to keep dry."

"It is difficult for me to comprehend how any harmonious relationship with the natural world of your site could lead you toward a full cellar. One does not follow the other."

"There are no two ways about it – I agree with Rex Roberts – the cellar is second-class space. It’s naturally dark and damp, and to achieve anywhere near the same livability as above-ground space, it will end up costing as much or more per square foot."

May 13, 2011 12:15 PM ET

the fine print
by 5C8rvfuWev

Martin says: "Another option (for CS insulation): insulate the exterior of the foundation walls."

I add, referring back to our SE obsession with termites -- Be sure to check w/local code officials re: termite protection for insulation before going too far along with plans for exterior insulation. Many will not permit it under ANY circumstances even though some new product developments involving pesticides and sealed (in plastic) insulations offer promise for the future.

Termite inspection strips: fortunately, the Delta-T in zone 3 is significantly less extreme than elsewhere. I've lived in New England; and I've lived in GA; and I'll trade y'all some kudzu and termites any ol' time.

RE: slabs -- the latest in snob zoning. Several counties in this area won't accept slab on grade going forward. Slab construction will, in the futre in these counties, be required to be raised slabs. 18" above grade is "suggested."


May 14, 2011 4:06 PM ET

Edited May 14, 2011 4:10 PM ET.

Martin has it almost right...
by Ted Clifton

There are way too many issues here to get into all of them with the limited time I have available, however, there is one that is way too important to leave out: The RIGHT way to do a closed crawlspace is to insulate the floor of the house the same way you otherwise would (R-30 fiberglass batts in our neck-of-the-woods), THEN insulate the crawlspace as needed to assure that the total UA from the crawlspace to the ground or ambient exterior is LESS than the UA from the house through the floor. This assures that the crawlspace will always remain above half of the difference between the outside and the inside, which in most climates will assure a dry crawlspace. This formula can be adjusted as needed for different climate zones. The big payoff is that when the delta-t through the floor is less than half what it would be with a vented crawlspace, the heat lost through that floor is half, usually resulting in a 7% to 15% reduction in total heating or cooling load on the house.
One other note, I NEVER allow air from the house to go into the crawlspace in my climate, it is way more moist than the outside air, and would CAUSE condensation. Make sure you know what the relative humidity and dew-point of your typical inside and outside air is, and design your mechanical ventilation accordingly. I use a small bath fan that operates on a humidistat to bring more cool, and therefore dry, air in from the outside. This is an exhaust-only fan, using passive leakage to supply make-up air, assuring that any radon or other undesirable elements are not forced into the living area, as they would with a positive pressure system. In a humid climate, you would want to bring the air in from the house, as Martin describes in his article. In more than twenty years of doing closed crawlspaces, I have yet to see the fan ever come on, it is only there for the code official...If you have done your calculations right, you will NEVER get condensation in a closed crawlspace.
Regarding insulating the outside of the walls, this is the best place for the insulation if you can protect it well enough. Try a sprayed-on polyurea coating, it is just like a heavy vinyl coating, and I don't think many termites or other bugs even could eat through it.

May 14, 2011 11:06 PM ET

Considering Martin doesn't
by James Morgan

Considering Martin doesn't like the crawl space on principle he does a pretty good job of describing how to do it right, and kudos for pointing out the variations in regional conditions that affect performance. Echoing Hunter's comment, it's said here in the Carolina Piedmont with its high water table that either your basement will leak and be full of water or it won't leak and your house will float up out of the ground. If the slope is enough for a walkout, fine - in other conditions a nonventilated crawl space takes a lot of beating.

May 15, 2011 6:16 AM ET

Edited May 15, 2011 11:32 AM ET.

Response to Ted Clifton
by Martin Holladay

Concerning your two suggestions:

1. I don't doubt that your proposed insulation method -- insulating the floor above the crawl space as well as the crawl space walls -- results in a dry crawl space and good thermal performance. Many builders and homeowners will balk at the added expense of insulating twice to create a double thermal envelope, but for those who are willing to go that route, I'm sure your proposed insulation method will work. To me, the complicated solution just highlights the inherent disadvantages of crawl spaces.

2. In my article, I explained the two options listed in the International Residential Code for conditioning crawl spaces. One option is to install a fan that exhausts air from the crawl space along with a grille that provides makeup air from the house. Although you don't like that method, it is one of two code-mandated options, and it works in most climates. The question of how to condition a sealed crawl space is enormously contentious; suffice it to say that no method of crawl space conditioning is without drawbacks, and there are strong arguments on all sides of this issue. If you have a system that works well for you in your climate, and if your local building inspector approves of your method, then go for it. There are some risks to your approach -- the possibilities of poor indoor air quality or high radon being among them. But if you have satisfied yourself that your homes don't have these problems, you are good to go.

Again, these contentious issues remind me of why slabs on grade look good.

May 15, 2011 10:44 AM ET

Slabs on grade are cut into
by aj builder, Upstate NY Zone 6a

Slabs on grade are cut into and modified all the time. Hilti makes all the tools to do the job dust free. We also use large barn fans and gas powered water equipped diamond saws. Actually just did a cut Friday to add a floor drain. Two crew and one hour done. I like cutting through floors to make improvements or changes to bath and kitchen plumbing and do so often as by far the best way to do the work efficiently. Last year we redid a dental office changing much of the plumbing layout. The slab was cut, work done and slab patched the first day, done. I just did the plumbing, but loved learning how easy a slab can be to work through given the right tools and techniques.

Great article Martin, much needed info. And I second the idea of building slabs raised to deal with water.

May 15, 2011 12:05 PM ET

crawlspace w/ no crawl no space
by j chesnut

We're working on a deep energy retrofit and have two conditions I'd like to describe and ask if anyone sees any redflags.
We are in cold climate zone 6.

Condition 1-
On a 14' extension to an existing home we are pondering a sealed crawlspace completely filled with insulation (i.e. no air space). Footings are located below frost line (42") and a 8" concrete block stem wall is insulated to the exterior to match the retrofit of the existing basement. 12" I-Joists sit on a sill on the new stem wall and tie back to the rim joist of the existing basement. Ground is excavated to only 2" below the new I-joist flooring. Continuous 6mil poly covers the ground and up the sides of the basement and stem walls and rigid EPS insulation is laid on the poly. I-joists laid and cover with Inusl-Web then dens packed w/ cellulose insulation. Floor sheathing is next covered by a self leveling compound that will function as the air barrier. Any red flags? If this were a cold climate wall the vapor retarder would be on the wrong side but the vapor drive dynamic down into towards the ground must be different then between conditioned spaces and outside air.

Condition 2-
The existing structure had been added onto in the past. This former addition sits on an uninsulated slab on grade - may have been a former attached garage slab. This slab is 14" below the main level floor plane. For the retrofit we are proposing hanging a floor above the slab to time out with the main level. On the existing slab we would (similar to condition 1) lay a 6 mil poly vapor barrier, lay EPS insulation and between the floor joists dense pac the cavities with cellulose. Same issues as Condition 1 but here we have much less financial flexibility to move to a typical sealed crawlspace or basement if this proposal draws redflags.

May 16, 2011 12:01 AM ET

Slabs Rule except with Expensive Land
by Kevin Dickson, MSME

Some neighborhoods may have such expensive land (say $60/foot and up) that the economics dictate that you should build a basement. If you don't, someone will eventually buy that home and scrape it off so they can have a basement. It's one of the few things that you just can't retrofit. Some neighborhoods also have zoning codes that limit the height to one story or 1.5 stories. Basements make sense there too.

Otherwise, thickened edge slabs are better:
1. Energy detailing is easier.
2. Slabs add some thermal mass to the main floor, which reduces diurnal temperature variation.
3. The most durable flooring of all is concrete. It's also the most economical of all. Stain it and seal it, and you're good. (Mine is 6 years old and I can't believe it looks like the day we did it. Hardwood usually shows some wear areas in that time.) And it's still trendy as a finished floor.
4. If you can build a 3rd floor, the house footprint can be smaller, which makes the yard bigger.
5. The 3rd floor of a slab house costs less per square foot and is worth more than a basement.

May 16, 2011 6:12 AM ET

Response to J Chesnut
by Martin Holladay

I see a contradiction in your description. You say you are proposing "a sealed crawlspace completely filled with insulation," but you also say that you will use "I-joists ... with Inusl-Web then dense packed w/ cellulose insulation."

So, I don't get it. Do you want to fill the space below the Insul-Web with cellulose, or just the space above the Insul-Web -- that is, the joist bays? If you're just filling the joist bays, then you're not building "a sealed crawlspace completely filled with insulation," because there will be an air space below the Insul-Web. But if you are completely filling the crawl space with insulation, why bother with the Insul-Web?

May 16, 2011 8:22 AM ET

by j chesnut

Thanks for your response.
The idea in both conditions is to completely fill the space between the I-Joists and the space below the I-Joists with insulation. The space inbetween the I-Joists with cellulose, the space below with rigid board insulation. The space below the flooring joists would be minimal ~1.5" -2.5". In order to gain some extra R-value and have a material less susceptible to moisture damage seemed like a good idea to try to fill the space below the I-Joists with rigid EPS. So the rigid foam would sit tight or slightly below the bottom of the I-Joists and the dens-pacing of cellulose would happen from above the floor joists. Insul-web would be used to contain the cellulose insulation as it was being installed from above and then later covered by the floor sheathing.
I think these conditions could be well detailed and executed in the field. With the rigid insulation below the I-joists it seems like the I-joist themselves will stay warm and not become a surface for "condensation". What I can't wrap my head around is if something in the floor assembly were to become wet for whatever reason how can we know if there is sufficient drying potential in the up direction back to the interior finished space?

May 16, 2011 8:41 AM ET

Living on a slab
by shane claflin

My house is on a slab, and I can tell you, it is significantly cooler inside than the ambient temp outdoors in zone 5a. The only way it could be a "thermal" mass is with in-slab radiant. Otherwise, it is a "cooling" mass.

May 16, 2011 8:47 AM ET

Edited May 16, 2011 8:48 AM ET.

Response to Shane Clafin
by Martin Holladay

Thermal mass works during the summer as well as the winter. On hot summer days, interior thermal mass will prevent the indoor air temperature from rising as quickly as it otherwise would. Similarly, during the winter, on very cold days the interior thermal mass will prevent the indoor air temperature from dropping as quickly as it otherwise would.

An insulated concrete slab does not require in-floor radiant tubing to work as thermal mass.

If you are unsatisfied with the performance of your floor, it's worth asking: did you remember to install continuous horizontal insulation under your slab? If so, how much did you install?

May 16, 2011 10:14 AM ET

Second response to J Chesnut
by Martin Holladay

Okay, now I understand your proposed plan.

I say, "Don't do it."

If you want to use wood framing, you need enough of a crawl space under the framing to inspect the joists -- that is, enough room for a small adult to crawl in without spelunking equipment.

If you don't have enough room to crawl, you need to pour a slab. Fill the area with gravel (compacting as you go with a plate compactor) until you reach the level where you still have enough room for a layer of rigid foam insulation and a 4-inch slab.

May 16, 2011 10:51 AM ET

But it looks good on paper ;
by j chesnut

But it looks good on paper ; )
Thanks for the feedback. Figured there had to be a reason I haven't heard of anyone doing this.
Back to the drawing board.

May 18, 2011 5:30 PM ET

Edited May 18, 2011 5:38 PM ET.

In South Jersey we have to many crawlspaces
by Frank Bovio

I have commented before on this issue. I have a passion for conditioning crawlspaces. As Martin said the first thing my crews do when they are working a retrofit is clean the floor. I do the vapor barrier a little bit different, i use 8 mil/12 mil/16 mil scrim reenforced vapor barrier. The mil difference for me is how bad the earth is in each crawl. Some dirt floors have a lot of fill dirt with to many rocks to remove (so I go with 16 mil and a before than I might even lay some poly to pad a path to crawl to appliances (sometimes I may double up on vapor barrier). I good scrim vapor barrier will make a world of difference with overlapped seams of 12 inches, buttered with muck or (what I use) number 15 white vapor mastic by RCD. The vapor barrier is always installed 12 inches over the ground level (always unless the dirt is as high as the band joist & yes I have seen it). Once the vapor barrier is installed depending on budget I drop the thermal barrier down the walls with either polyisocyanurate sheathing typically 2 inch (R-12) or Thermex sheathing (use nothing but Thermex sheathing when there are gas fired appliances in a crawl) (R-14) . I havent been able to use anything thicker than 2 inches due to budget numbers but will post one day when I install 3 to 4 inch poly iso. I have used 2 part spray foam (high density/closed cell/1.75 P.C.F., many names same game). We always make sure the nad joist is either sprayed with 2 part or we seal all infiltration/exfiltration points with 1 part spray foam (we only use 16 pounders with a bottem load gun). Once the band joist is done we cover it with glass wool batts. All seams of the poly iso are taped with UL181 tape and than painted with number 8 gray mastic.
last but not least we install a supply vent (when possible) and a return vent. the air change is enough to make the crawl a wonderful place to dwell (if your into that kind of thing) and really, really makes a huge difference in the winter months.
As for vented crawlspaces what I have my crews do is cut around them when installing the polly iso than I have them make a removable plug for the vents. This makes the vents useful if there is ever a flood in the home or a pipe leak. Pop off the plugs let the crawl dryout and place the plug back in.
I have had nothing but rave reveiws from my customers from our work in crawlspaces and those reviews are what make me stand by my methods.
I am always willing to discuss these methods with anyone who wants to! Yes I do consider myself as the green mad wizard!! I hope my feedback is of value to our community! thanks for reading.
Frank A. Bovio

Thermex Installed, And Sealed. 10 Mil Scrim Reinforced Vapor Barrier Installed And Sealed..JPG Have Bovio's Green Team Spray Foam Your Crawl Space, Walls Can Also Be Insulated With Spray Foam..jpg Knauf EcoBatt Glass Wool as an Ignition Barrier (R-12). Over Spray Foam Insulation..jpg 037.JPG 050.JPG

May 18, 2011 6:05 PM ET

South Florida coastal stilt homes
by Mario Carballo

Slabs vs. basements vs. crawl space....did we forget stilt homes? We live near the coast in South Florida in a stilt house that's about 4 feet above the ground (actually sand) due to flood requirements. This style allows for storage space, saves on fill and if built higher....garage parking underneath. Ours has no insulation under the floor or any walls. Completely open. Coastal breezes keep the space cool and airy with no signs of moisture but I'm always wondering if I should insulate the floor ....but how. Spray foam seems to be a good option but don't know the effects of salt air and humidity when the foam is exposed to the elements.


May 18, 2011 7:07 PM ET

permits and conditioned crawlspace
by mike keesee

Has anyone ever had a building official require a pe mit for a conditioned crawlspace based on the assertion that you are adding new "conditioned" space to the existing home? Here in California a building owner is required to pull a Title-24 permit, that is perform Title-24 energy documentation showing that the addtiion conforms with the state's T-24 energy requirements, for additions. SMUD, the utility I work for, supported a just completed a project with Habitat for Humanity where we "sealed" and insulated a 18" crawlspace - (it was nasty), and lo and behold the city official required them to pull the Title-24 permit because it was an addtion. Has this happened to anyone else in the country? Thanks for your comments.

May 18, 2011 9:54 PM ET

floor register in unconditioned crawl space?
by Erica Downs

Martin - in the instructions above for "Creating an Unvented Crawl Space" one of the steps listed is "Install a floor register in the floor above to allow air to flow between the living area and the sealed crawl space below." Can you please clarify why you need air flow between the living space and crawl space, if the crawl space is supposed to be 1) unvented, 2) unconditioned, and 3) the walls are insulated with rigid foam and there is a poly vapor barrier?? Thanks!

May 18, 2011 10:22 PM ET

Unvented crawl space over sold
by Glen M

First let me say, if constructed as outlined here, an unvented crawl space works, however, I believe they are over sold. Do they work, yes, do they require very specific attention to detail that is common to the construction trades today, not even close.

My parents live in update NY with part of the house with a full basement and another on a vented crawl space, which has slab below with uninsulated cmu walls. The crawl space is insulated with 10-12" of fiberglass batts on the underside of the floor and has two vents. This crawl space was built in the early 70s and never, I repeat, never have there been any moisture problems. Yes, the slab likely retards the moisture flow from the ground to the space, but this house was not built to any Fine Homebuilding standard.

I now live in Houston (i.e. hot and humid) on a raised floor house on "drilled" piers, which as noted here has other some advantages and is commong practice in "expansive" soils. This is obvisouly not common practice in the Northeast. I have 4" of spray foam on the underside, no slab below, with perimeter lattice all the way around the house so there is plenty of air movement. Periodically I inspect the crawl space particularly during the summer and have no issues, in fact, it is actually pleasant (as much as that is possible) during the hot/humid summers.

Obviously different regions require different solutions, but I believe the amount effort to truely seal a crawl space and maintain it (I have to run an exhaust fan, come on) in most situations (except for extremely cold climate, ducts in the space) is over sold. For my money, build the perimeter however you want (cmu, cast-in-place, etc), provide adequate venting (i.e. exceed code) and use closed-cell spray foam on the underside, call it a day.

As far as termites are concerned, remember they need a food source + water, remove one and no termites. What typically is installed down here is a terminte shield which is essentially a sheet metal cap which contrary to most people's understanding, is not a barrier but an obstacle that they have to build around. Since they have to build around it, their "tunnels" would be visible from a periodic inspection of the foundation.

May 19, 2011 5:34 AM ET

Edited May 19, 2011 5:35 AM ET.

Response to Erica Downs
by Martin Holladay

Q. "Can you please clarify why you need air flow between the living space and crawl space?"

A. As stated in the article, the requirement for a floor grille to provide air flow between the living space above and the crawl space below is a code requirement found in the 2006 IRC, section R408.3. If you choose Option 1, and install an exhaust fan in the crawl space, the grille is the source of conditioned makeup air from the crawl space. If you choose Option 2, and install a supply register in the crawl space, the grille allows the air in the slightly pressurized crawl space to flow to the house above, at a slow rate, thereby mixing the crawl space air with the home's conditioned air.

Some builders have had good success with sealed crawl spaces that lack an exhaust fan, a supply register, or a grille connecting the crawl with the living space above. Omitting conditioning sometimes works and sometimes doesn't. In all cases, however, it would be a violation of the 2006 IRC.

May 19, 2011 8:58 AM ET

Good Crawl Space Articles
by Kohta Ueno

For anyone interested in more information on building sealed crawl spaces, my usual go-to articles for explaining the process come from The Journal of Light Construction:

Building a Sealed Crawlspace (Jeff Tooley, October 2003) for new construction
Fixing a Wet Crawlspace (Jeff Tooley, August 2004) for retrofits

Yes, they're JLC articles, not Taunton. Shh! ;)

Tooley had a good point on having a "sacrificial" layer of polyethylene during construction, to account for tears and debris from construction, and some advice on drying framing before sealing it all up.

Also, if you are ever working with contractors who are new to unvented/sealed crawl spaces, be sure they have some of the basic information in hand. I once dealt with a contractor where the plans were "thrown over the fence" to him. He built a polythylene ground cover (read: a plastic "bathtub") inside the foundation before the building was dried in (read: a plastic bathtub open to the sky). He was later surprised and angry when he built an OSB box 3 feet above pools of standing water (read: his first floor framing) and got mold on it. Yes, to me, this was an [eyeroll]--"You did **what?!**" situation. But if folks are used to building things one way, they might not visual the consequences of changing their procedures.

May 19, 2011 9:24 AM ET

Edited Nov 24, 2012 4:48 PM ET.

Response to Kohta Ueno
by Martin Holladay

Thanks for recommending the two JLC articles. I agree with you -- these are great articles -- and I'm happy to second your recommendation. For anyone who wants to read them, here are the links:

Building a Sealed Crawlspace.

Fixing a Wet Crawlspace.

I have no problem recommending articles from the Journal of LIght Construction. It's a great magazine -- I worked there for years.

In case readers missed it, more links to useful articles and Web sites can be found in the sidebar near the top of this page. The sidebar is titled "More information"; it appears near the article's 4th paragraph.

May 19, 2011 8:10 PM ET

conditioned crawl space
by Greg Long

Here in Michigan crawl spaces are normal, our approach is sealed-conditioned. Usually a single heat run 6" is normal. Sometimes a vented door to existing basement space. We have trained the local inspectors to accept this, as they still think in terms of outside vents of old. We only use ICF's for foundation walls, so insulation is complete to below grade. Sealed vapor barrier, sometimes radon vents, as needed by testing. Add a SIP floor above and you have a bone dry and warm crawl space.

May 30, 2011 12:01 AM ET

Getting around the code issues with sealed crawl spaces
by Ted Clifton

For many years before the building codes recognized closed crawl spaces, we were constructing them as "basements with no habitable space". There is still no code requiring a basement to be heated, or to be provided with ventilation from the living space above.

With the installation of an exhaust fan, and no deliberate opening to the living space, radon issues would be negated, because the crawl space would be in negative pressure. The living space above should be fitted with a balanced air handling system, creating a slight positive pressure, especially if there is an attached garage. The garage should also be in constant negative pressure. House positive, garage and crawl space negative. Simple.

Jun 20, 2011 3:15 PM ET

Edited Jun 20, 2011 3:38 PM ET.

Sealed crawl space w/out air conditioning
by Jill Neubauer Architects

I work on Cape Cod, which is very humid almost the entire year. We also build a lot of houses (fewer today than five years ago) without central air conditioning.

I am a proponent of sealed crawlspaces, but how do you handle the situation when you cannot deliver "conditioned" air to the crawlspace during non-heating periods?

Jun 20, 2011 3:25 PM ET

Response to Chris Harris
by Martin Holladay

I would monitor the conditions in the crawl space during the summer to see if any signs of dampness develop. If necessary, you can install a small stand-alone dehumidifier at a cost of about $300. I would adjust the controls so that the dehumidifier runs as little as possible -- just enough to control the moisture.

Jun 20, 2011 3:44 PM ET

Response to Martin Holladay
by Jill Neubauer Architects

Thanks Martin,

So you would still be of the opinion to go w/ a sealed crawlspace, provide ventilation as described by code and the article and then use a standalone unit to handle excess moisture during warmer months?

Jun 20, 2011 3:50 PM ET

Edited Jun 20, 2011 3:51 PM ET.

Second response to Chris
by Martin Holladay

The building code tells you how the house must be built; however, the building code doesn't tell you how to operate the house.

The IRC lists two options for conditioning a crawl space, which I called Option 1 and Option 2. In your case, I would go with Option 2: "Option 2 requires that the crawl space have a forced-air register delivering 1 cfm for each 50 square feet of crawl space area."

The code does not require a house to have air conditioning. Just because there is a register in your crawl space, doesn't mean that it will be operating in the summer. The dehumidifier is an optional way to keep the crawl space dry if observations or measurements show it to be necessary.

Of course, you should check with your local building official before assuming that my recommendations make sense.

Jun 20, 2011 4:15 PM ET

I guess my assumption is that
by Jill Neubauer Architects

I guess my assumption is that if you are creating a "conditioned" crawlspace, the time of primary benefit of being conditioned is during the most humid months, presumably when you would typically be running an air conditioner (in most parts of the country for at least some amount of time).

I was just was wondering if you would still realize the full benefits of a sealed crawlspace if there was never a chance of introducing truly conditioned air during those most humid months. Or if you are just trading one way of adding humid air to the space for another (i.e. if all of the windows are open from June to Oct., you are introducing outside air to the crawlspace regardless of "sealed" or "unsealed".

I understand there certainly are other benefits of moving the thermal envelope to the building envelope rather than floor plane, better overall air sealing, etc. but was just curious when air conditioning is not a factor. It sounds like you would recommend a sealed space in this case and I guess I have talked myself into it as well.

Thanks for your help.

Jan 8, 2012 1:29 PM ET

ThermoCon insulation in a crawl space?
by Dennis Cornhill

Is ThermoCon a suitable material for insulating the inside walls of an unvented crawl space?

We are building a house around a previously unheated cabin that sits above a fairly shallow crawl space. Moisture has not been a problem in the 20+ years we've owned the cabin, but now we will be heating not just the old cabin but the crawl space as well. The crawl space walls have a rough surface of rock and mortar, and the floor is dirt (with occasional tree stumps and boulders).

Working room is tight in places, and a sprayed insulation would be much easier to apply than building a Thermax layer over the walls and into the rim joists.

Because I don't want to use a sprayed polyurethane, an insulation contractor has suggested ThermoCon instead. The ThermoCon website does not mention crawl spaces as a recommended use for ThermoCon, but overall the website is not that informative either.

Jan 8, 2012 1:43 PM ET

Response to Dennis Cornhill
by Martin Holladay

Your question is best directed at a technical representative from ThermoCon. Their telephone number in Houston is 800-979-4914.

Jan 16, 2012 4:28 PM ET

Edited Jan 16, 2012 5:04 PM ET.

A few comments to update
by John Ring

The IRC has always called them "Closed" crawl spaces, and I consider it to be the better and more accurate term. A "Sealed" space would be as tight as a balloon - neither possible nor desirable. Here in NC the 2009 IRC the Closed Crawl Spaces section is R409. R409.5 has five options for Space Moisture Control: 1. Dehumidifier, 2.Supply Air, 3. House Air, 4. Exhaust Fans, 5. Conditioned Space.
I've been functioning as a consultant re existing crawl spaces for a number of years and my approach for most spaces is to use a 6-mil plastic vapor retarder. It has all the perm you need. If there is equipment in the space I install a four foot wide piece of 16 mil from the crawl space door to the equipment and along the front of it. If there is already insulation in the floor structure I touch it up as necessary. If not, I install interior foundation insulation - I like R-10 rock wool panels - leaving the required three inch termite inspection gap. (I'm also a certified termite inspector, and 3" is much more than necessary, but I do the 3" for liability reasons.) If the Wood Moisture Content of the floor structure is less than 12% at the time of installation, that's it. If the WMC is 12% or above, to dry out the space I install a $200 dehumidifier gravity drained to the exterior or to a condensate pump. In either case I provide the client with a Temperature- Relative Humidity monitoring system with a master in the house and up to three remotes in the crawl space (less that $50). I tell my clients to keep an eye on the master monitor and if the Rh in the crawlspace exceeds 65% with any regularity I follow up by either installing a de-hu if there isn't one, or if there is I go back and find out what went wrong. 90% of the time, once the space is dried out no further action is necessary. Monthly calls to new clients and my annual inspections for the first several years. reassure the clients and keep things under control. That's it. Relatively simple and modest in price - and it works.
I haven't addressed the matter of water control or drainage - critical factors to be properly controlled to be successful. Martin did that very well in his original article. However, if at all possible, I want a gravity drain from the space. Around here the time when you may need the drain the most may be when you don't have any electricity, and battery back-ups for the sump pump have a limited charge.
A couple of asides. First, I trained with Jeff Tooley (son of the esteemed John Tooley - Building Scientist extraordinaire) for a week 11 years ago, and he really knows this topic well from a lot of hands-on experience. Read his previously mentioned articles. Second, in the 1968 NC Building Code in the back of the book there are several drawings that show how to properly do a closed insulated crawlspace! Who knew? Must have really messed with the 99.9% of the builders that were so sure that all crawlspaces had to be ventilated, and the more the better!
I will finish with this; You MUST recognize that except for doing your best to keep the water away from the crawlspace to begin with, there are no absolutes in the closing of a crawlspace. Every one is different particularly with regard to their geographic location. With a bow to Marcus Pollio: That crawlspace that would be fine in Albany, Georgia, would not be appropriate for Albany, New York, nor for Albany, California.

Feb 20, 2012 10:44 PM ET

ThermoCon insulation in a crawl space?
by Dennis Cornhill

I spoke to several people about the use of ThermoCon in a crawl space and got a range of opinions from "certainly" to "doubtful."

So in the end because we were not sure that ThermoCon would last for the long term in a crawl space, we insulated the rim joists and the foundation walls with 2" of XPS covered by 1.5" of Thermax instead. It was a tedious job working in a confined space, but the result looks good and will hopefully be an effective insulator for the long term.

Sep 18, 2012 12:11 AM ET

Edited Sep 18, 2012 12:54 AM ET.

cost effectiveness of insulating crawlspace walls
by John Eyles

I am amazed to read, in the cited North Carolina study, that insulating the crawlspace walls provides only an additional 3% of energy savings (18% versus 15%) over encapsulating/sealing the crawlspace but using fiberglass insulation in the floor joists. I am convinced that I should encapsulate my existing crawlspace, but I'm not sure if it's worth the added expense to insulate the walls (for my 1700 sq-ft house, the cost is $3900 versus $1850). Probably not worth it for the 3%. But it's hard to believe that moving the HVAC ductwork into the conditioned envelope would not yield far greater savings. Even ignoring the ductwork effect, the square footage of the crawlspace walls is about half that of the floor.

Sep 18, 2012 4:29 AM ET

Response to John Eyles
by Martin Holladay

There are many benefits to sealing a crawl space other than energy savings. However, if you are chiefly motivated by energy savings, then you are right to hesitate. Anyone who expects a huge saving from sealing a crawl space and insulating the crawl space walls is likely to be disappointed.

Sep 18, 2012 3:31 PM ET

Not mainly energy savings
by John Eyles

Martin, thanks. I have expressed myself poorly. I am chiefly motivated by the moisture issue. I intend to encapsulate the crawlspace, no matter what. But as long as I'm having the work done, I'm tempted to go ahead and insulate the block walls also. I don't think it makes much difference whether I do that, or just leave the floor batts, as far as moisture control. So it becomes a question of whether the potential energy savings is worth the approx $2K added cost to insulate the walls with 1.5" Thermax. If the savings is only another 3%, probably not; but as I say, that figure seems to conservative to me. I suppose a big part of the calculation is how you factor in the floor of the crawlspace (which would be unfeasible for me to insulate at this point). I'm thinking it helps in the summer (since the dirt is pretty cool) but hurts some in the winter - but I'm heating with free and plentiful wood then anyhow.

Nov 24, 2012 1:25 PM ET

Wet crawl space on unheated cabin
by Dan Lindberg


I was just linked to this discussion and find it very helpful - thanks, I just wish I had seen this 20 years ago.

I have a "rework" project facing me. Years ago I designed and built a cabin in Northern MN, and put it on a crawl space, (it would have been pillers). I built it in the (I believe) prevaling fashion back then; footing at 48+ inches, 7 course block wall, backfill wall, poly on fill, passive vents in wall, and glass in floor joists.

Turns out it is a good way to grow mold.

So, my plan is to dig it out, vaper barrier and rat slab, insolate the inside of the block walls, seal, maybe add active vent out, passive makeup air.

But the cabin is unheated most of the time,
any thoughts or red flags for this plan?


Nov 24, 2012 4:26 PM ET

Response to Dan Lindberg
by Martin Holladay

I think your plan is fine. Just use common sense: monitor conditions in the crawl space to be sure it isn't too humid. As long as it stays dry, you don't have to worry.

If it seems damp, you have to start analyzing where the moisture is coming from to figure out how to address the issue. But since that hasn't happened yet, I wouldn't worry yet.

Nov 26, 2012 10:52 PM ET

Cabin con't
by Dan Lindberg

Well, there is plenty of mold growing now, and I suspect that the mosture is/was coming in through the vents, as the fill is/has been very dry anytime I've gone down there.

But, my question is should I remove the insolation in the floor joints, and/or do I insolate the rat slab? The concern is freezing of the foundation during winter when it is unheated.


Jan 24, 2013 2:03 AM ET

the article said: Install a
by erik olofsson

the article said:

Install a durable vapor barrier — for example, a 20-mil pool liner or Tu-Tuf poly — over the floor and extending up the crawl space walls, to within 3 inches of the top of the wall. Leave a 3-inch-wide termite inspection strip at the top of the wall.

i thought never ever put poly on basement/crawlspace walls..?

Jan 24, 2013 9:41 AM ET

Edited Jan 24, 2013 9:46 AM ET.

Response to Dan Lindberg
by Martin Holladay

Q. "Should I remove the insulation in the floor joints?"

A. Yes. If you have fiberglass batts between your floor joists, and if your crawl space shows signs of mold, you should remove the batts and throw them away (assuming, of course, that you are sealing the crawl space vents and insulating the crawl space walls).

Q. "Do I insulate the rat slab?"

A. The payback for rat-slab insulation is going to take much longer than for the wall insulation. Most crawl space retrofits don't include insulation under the rat slab.

[Editor's note: Comments continue on Page 2. Click the "page 2" tab to read more comments.]

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