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Ventless (sealed) crawlspace

RhondaRay's picture

Hi All,

Anyone have personal experience with the pros and cons of a ventless crawlspace?

We're a few days from breaking ground on our next new home, so I've been doing considerable research on this topic.

Prior new construction has been vented crawlspace or full basement, this house will have a "stoop-over" crawlspace.

Here, in western North Carolina, the trend for "knowledgeable" construction is towards ventless, with two techniques, either sprayed-on foam or a combination of 10 mil plastic vapor barrier and ridged insulation.

Here are a few sites to explain the procedure (not my endorsement, merely for illustration),2617,HPRO_20146_3463221,00.html

what are your thoughts?


(post #80837, reply #1 of 41)


The Building Sciences Institute, University of Florida and another academic institution I'm forgetting have conducted numerous studies as well as operated test houses around the country regarding vented and non-vented spaces. Their emphatic recommendations are surprising to those of us trained in the North East and practicing in the South East and West. You can find their work through googling "bulding sciences vented crawlspace", and/or attic.

Venting is meant allow drying out of any materials that get wet or retain some moisture. This helps eliminate mold, fungus and insect damage. It works very well in the North East I can attest as hundred year old structures properly vented stand year after year. It's a hard practice to change with such obvious benefits.

But, in high humidity areas, such as South Florida, New Orleans, Houston, or coastal areas, venting simply exchanges air that has contributed its moisture with fresh, slightly higher humidity air that contributes to a higher humidity level overall. Whatever needs to dry out never can because a fresh load of humidity is always available.

Read the recommendations from the organizations above for the right information but though I don't know the specific case of Western North Carolina but I'd go with either method though I prefer a spray foam total "insulation" application (total foam coverage not spot foam) as it cures two ills with a standard application. Other methods can do the same but only through diligent installation and its questionable whether even then the lifespan is as long.

As you're doing new construction I'm not sure either of the remediation vendors are applicable, their strength is in eliminating "Dirt" or basically raw ground issues. Whereas you'll have the opportunity to place a ground seal as the HGTV article lays out.

Good Luck.

(post #80837, reply #2 of 41)

Thanks for taking the time to compose an informative response.

Your efforts have led me to this

which has been most helpful.

(post #80837, reply #6 of 41)

Don't even mess with anything that resembles a crawlspace. I'm from the North and have been in SC for 17 years and detest a crawlspace. Do a full basement right and you won't regret it. See the many posts on this topic.

(post #80837, reply #18 of 41)

I agree basements are cost effective and the way to go if possible.  But what do you do if you build on rock or worse yet in an area of high water?  I am personally aware of areas where water comes within 18 inches if not 12 inches of the surface each spring.  And before you say, don't build there, I should add that this is a large area with thousands of homes and ranches.  So people build there everyday.


(post #80837, reply #21 of 41)

The vented crawl spaces in old North East homes worked only because the floors over those crawl spaces were not insulated. Heat from the house help warm the air enough to control condensation and mold.

Do it right:

(post #80837, reply #30 of 41)

I had an experience with an older home that I had worked on for a customer. The foundation was somewhat vented, but also had cracks in it as it was a limestone foundation.

I ran some new electrical wiring and used metal junction boxes and metal staples.

Part of the first floor area had fiberglass insulation in it, and some did not.

I noticed two things:

1. Where there was insulation, the metal boxes and staples had rusted in just a few months. There was also moisture on the fiberglass. It was colder outside at the time.

2. Where there was no insulation, the metal boxes and staples were just fine. No rust.

FWIW, Bryan

"Objects in mirror appear closer than they are."

Klakamp Construction, Findlay, Ohio - just south of the Glass City

"Objects in mirror appear closer than they are."

Klakamp Construction, Findlay, Ohio - just south of the Glass City

(post #80837, reply #37 of 41)

I agree.  For goodness sakes, never insult the floor.  As you say, rust is evidence of trap mositure.  Secoondly, an uninsulated floor is a BEAUTIFUL thing for alterations of plumbing, wiring, ducting, etc. in the future!  And makes monitoring plumbing for leaks easy.

My mother has a crawlspace, insultated foundation walls and rims.  IT has vents, which are open in the summer and closed during the winter.  No floor insulation.  Even with insulted heat ducts the crawlspace stays at a nice temperature, aiding in keeping things dry.  It is a pretty nice place to work if need be.


(post #80837, reply #40 of 41)

Yep insulation is installed in floors all the time to try and keep the floors up stairs warm. it's a big mistake but for some reason vented crawl spaces are still allowed in the code go figure. even if a vented crawl space is done to code it is just a matter of time before someone tries to keep the floor above warmer and insulates the floor.

The code that allows vented crawl spaces needs to be changed at least in areas where it gets cold where someone is going to try and insulate the floor. Also I am sure in humid hot areas it does not work ether but since I do not work in Florida I can't speak for that.

(post #80837, reply #3 of 41)

For years the code here required insulated crawlspaces and vents. The combination made no sense and now the requirement is that you can seal the crawlspace as long as you insulate it to the same level as the rest of the house and provide a heat source. This seems to be working.

Treat it as a short basement with the same attention to insulation and vapour barriers and you end up with a warm dry space to store things you should probably throw out.

(post #80837, reply #4 of 41)

I say i'll vent a crawlspace when I start seeing vented basements.

I put plastic down on the ground, rigid ins on the above grade walls (covered with drywall as fire barrier) and throw a duct in there.


Family.....They're always there when they need you.

(post #80837, reply #5 of 41)

I like them a lot and I'm in central NC (chapel Hill)

What I don't like is the practice of blowing air from the HVAC system or the conditioned space into the crawl. I think of crawl air as poisonous (MAD air) and want to keep it out of the house. I do a tented crawl on most of my houses

On our next house we'll be using ICF's for the crawl walls with a poly floor linked to a capillary break under the ICF's and will use one of those little dehumidifiers that exhaust warm damp air out a dryer vent rather than a garden hose to maintain low humidity. We'll have a radon vent through the roof and a sealed floor diaphragm with a foundation drain through washed rock to stop vermin to daylight.

Hope this is not too much jargon.



"You cannot work hard enough to make up for a sloppy estimate."


"You cannot work hard enough to make up for a sloppy estimate."

(post #80837, reply #7 of 41)

here are some thoughts: 

First it's not really unvented - I think the term they use is sealed or closed..  Really it's just a q of where you are going to vent it to - the interior of the home or the outside air.

If you use foam on the walls I'd advise using borate treated foam.  Termites, etc like to use foam as a highway to your floor framing.  An inspection strip (gap) will likely be required.

They sell prehung weather-stripped CS door assemblies.

Do a really good job dealing with the drainage.  If it get's wet under there you are $crewed.

Likewise do a really good job with your vapor barrier.  I'd recommend something better than regular 6 mil poly.  I'd recommend something better - like maybe Tu-tuff.  The problem with poly is that is inherently slick (greasy?) and doesn't glue very well.

Check out your 2006 NC residential code book.  They have a pretty big section on it - I think it's in chapter 4.

I've never seen that setup like ShelterN linked to.


(post #80837, reply #8 of 41)

PS - what's a "stoop-over" CS?


(post #80837, reply #9 of 41)

what's a "stoop-over" CS?


a crawlspace just tall enough to bump your head when stooped-over ;>0

(post #80837, reply #10 of 41)

I am in s.c. so we have a lot of moisture issues.Most of the work I do is remodeling and additions so, usually I am fixing mold issues and reworking poorly vented crawlspaces and living space.Here are a couple of pics of my latest project with a sealed crawl.I make the crawlspace an airtight space, pitch the ground towards floor drains, the floor drains penetrate the exterior wall and are carried away just like gutter drains. This ensures that if there is a plumbing issue which would only be a back-up or overflow the water is carried off. I turn a duct and return into the space to heat, cool, and dehumidify the space. Because the space is totally controlled there is no "bad air" in the system. If cost allows I highly recommend this method.

(post #80837, reply #12 of 41)

Could you tell us what kind of vapor barrier you use, your methods and materials for sealing the vapor barrier and methods amd material for insulating the walls please? 


(post #80837, reply #25 of 41)

a had a company do the install called sealing agents. they used 20 mil poly for the floor and roll insulation for the walls (r13) the wall insulation was faced with 20 mil poly they sealed every joint with an elastomeric style caulk, kind of like fire caulk.

(post #80837, reply #26 of 41)

Would you mind telling us the cost and size of the CS?


(post #80837, reply #27 of 41)

It was  about 3500 sq ft and the cost is fuzzy, it was about $9,000 for the sealed crawl and about 200' of french drain with waterproofing so I would venture to say it was about $2.50 per sq ft. sorry so fuzzy lots of numbers and little energy to look it up! 

(post #80837, reply #20 of 41)

That sure is a purdy vapor barrier.  What are you using? Tu-Tuf?

(post #80837, reply #23 of 41)

20 mil weaved poly.

(post #80837, reply #38 of 41)

I've always wondered why an ERV or HRV couldn't be integrated with a sealed crawlspace.  The gist of the idea being what if you put your outbound air through the crawlspace first?  That provides conditioning for the crawl as well as moving out the stale air down there.  Plus, you're not introducing possibly smelly/stale crawlspace air into the house.  The outbound air passes through the crawl and then through the ERV/HRV and on out of the house.  The inbound air comes from outside and after passing through the ERV/HRV, it enters into the living space.

So the circulation would be: outside air --> ERV/HRV --> Living Space --> Crawlspace -->outside air.

In theory, you end up with a conditioned crawl of non-stale/smelly air.  You might be able to slightly pressurize the crawl, thereby reducing the chance of soil gasses  infiltrating the house.

But you want to do it in such a manner that you're not jacking up the price of the HVAC system and preferrably don't have to go to the crawl to service anything.



"It is difficult to get a man to understand something when his salary depends on his not understanding it." --Upton Sinclair


Time is the coin of your life. It is the only coin you have, and only you can determine how it will be spent. Be careful lest you let other people spend it for you.
-- Carl Sandburg

(post #80837, reply #41 of 41)

It's a beautiful thing, why not start from the lowest point of the house with climate control thus creating harmony among seperate spaces rather than fighting one another.

(post #80837, reply #11 of 41)

I used the information on this board in deciding to build my house in 2006 with an unvented crawlspace. I used ICFs for the foundation walls had the rim sprayed with 3" of foam and covered the floor with Tu-Tuf which was sealed to the ICFs with acrylic sealant. All other penetrations were also sealed. I had my HVAC contractor put one floor register in the duct work in the crawlspace so that in the winter warm air could enter and in the summer cool air.
I'm in the middle of South Dakota where we can get to 117*F summer and -30*F winter. Although It hasn't been quite that cold for the past couple of years.
I couldn't be more pleased. I have Dura Ceramic on the floor above the crawlspace and in these cold mornings I don't even need socks on because the floor is warm. Absolutely no dampness or odors from the crawlspace either. Also I am located within 100' of the Missouri River and about 75' from the Bad River so ground water is not too far below.

(post #80837, reply #13 of 41)

>I have Dura Ceramic on the floor above the crawlspace and in these cold mornings I don't even need socks on because the floor is warm.

did you install conventional batt insulation between the floor joists?

(post #80837, reply #14 of 41)

No I did not. The entire exterior foundation wall is insulated by the Insulated Concrete Forms and the spraying of foam in the rim joist area.
There is no need to insulate anything else.
The area once sealed is treated just as a heated and cooled basement.
I also should mention that the Tu-Tuf is white so it make is very easy to light the area.

Edited 1/14/2008 8:51 am ET by 5brown1

(post #80837, reply #15 of 41)

I read that stuff about neutocreat (sp?).  It doesn't really say what is.  I'm guessing some kind of light weight concrete?  Their web site seems to focus on what the pitfalls are if you don't use their product as opposed to actually providing complete info about their product/service.  I have to wonder what the real advantage over that is vs just installing poly with 2.5" or so of regular concrete over it.  From reading their literature it sounds like the real advantage is that you would be hiring a subcontractor to do a comprehensive CS seal up job.  I'm sure it costs thousands.  I'm gonna guess 2 to 3 dollars a sq ft.  I wonder if some kind of floor drain is typically installed with it or what?

BTW - you mentioned 10 mil plastic.  Is that locally available for you?  Is it regular sheet polyethylene plastic (sometimes called visqueen) or something different?


(post #80837, reply #16 of 41)

I'm sure it costs thousands.


$8,000 to $10,000


I looked at a ventless CS over the weekend for a house that was recently finished and the day before had received a Certificate of Occupancy.


The job consisted of a 10 mil vapor barrior, ridgid insulation on the CS (block) walls, and no 1st floor insulation.


Interestingly enough, there was a 3-4" uninsulated region at the sole plate. Is this in the North Carolina code, maybe for termite inspection?


Something to research.

(post #80837, reply #28 of 41)

Well, since you haven't succumbed to my suggest of changing to a full basement, the following is, IMO, the most comprehensive article on conditioned crawlspaces. Check out the constant airflow regulators (American Aldes) coupled with backflow dampers to control the humidity during the summer...great idea.

(post #80837, reply #29 of 41)

>> Interestingly enough, there was a 3-4" uninsulated region at the sole plate. Is this in the North Carolina code, maybe for termite inspection?  <<

Yes - it is a termite inspection strip and it is required by NC residential code.  BTW - when was your project permitted?  Before or after 1/1/08?

Edited 1/14/2008 9:09 pm ET by Matt