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Crawl Space Ventelation

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I was at a recent seminar that had a very good presentation about crawl spaces. The presenter stated that crawl spaces should not be ventilated. It should have a sealed vapor barrier but should not be ventilated. The reason given is that it allows humid air to enter and condense and it doesn’t dry properly.

This goes agenst everything that I was ever taught.

I just thought I would get some opinions from a few southern boys since we don’t have a whole lot of crawl spaces up here.


Bruce [?]

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Sealed crawl spaces are being promoted here by some conservation "think tanks" associated with universities.

I've even heard that one local electric utility has made the sealed crawl space part of its "seal of approval" for energy efficient rating.

What I haven't heard the spreaders of this new gospel mention is how an annual termite inspection is possible when all the inner foundation walls are draped in poly.

I sort of think there's not enough "science" on the situation to make one or the other theory of crawlspace ventilation unassailable. So it becomes a matter of whether you "beleive" things to be so.

Just my thoughts on the matter. I still think the open-air circulation reasoning holds the most water.

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Hi Bruce,

I'm not a southern boy, but I probably spend more hours a year in crawlspaces then most people spend with their kids.

Your presenter is correct if he is speaking about areas with high humidity. Humid/hotter exterior air is always going to move toward cooler/drier areas such as crawlspaces. Once it enters the crawl, it may cool quickly to its dewpoint and condense out all over everything - insulation, joists, sub-floor, pipes, vapor barrier, posts, beams, etc.. This can and does promote rot and is conducive to insect infestation. Joe L. talks about this stuff on his site at http://www.buildingscience.com.

However, if he is saying this works everywhere I think he's got his head tucked up his butt, because the only way it can work everywhere is if one has an absolutely dry crawl and perfectly sealed crawl and the crawl is made part of the conditioned space of the house. That's a tall order and the skills needed to achieve that are usually far beyond those that the 16-year old on the job site has when he's sent into the crawl to lay the barrier.

Over the past two years, Jeff Tooley has written about two or three articles about sealing crawlspaces and how to install really good vapor barriers. You'll find them online at The Journal of Light Construction Online (http://www.jlconline.com/cgi-bin/jlconline.storefront). You should check them out.

Here in Washington State, I occasionally find crawls that have not been vented. Here, unless they are either covered with a slab or a very carefully detailed vapor barrier, they have problems caused by trapped moisture. So far, the ones with concrete floors that are vented into the house and made part of the conditioned space have been the best - albeit dusty - and the others with vapor barriers all had issues stemming from moisture, because the installer couldn't figure out how to install the barrier properly (and probably didn't try).



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Mike & Jim,

I haven’t mentioned it before but until just a few months ago I was a radon mitigator as well as a home inspector. So to those who may not be familiar, one method (if there’s a crawl space) is to seal 6 mil. poly to the walls and vacuum the air from under it. That also eliminates 99% of the issues as well as removing the soil gas.

Sometimes I would add venting for radon reduction purposes. Another method is to pressurize a crawl space by blowing air in with a fan that runs continuously. Thankfully that isn’t used very often but I can see that method doing damage unwittingly.

I’m going the have to rethink my crawl space inspection it seems. It may be difficult to get it across to the sellers that they have a crawl space problem because “it’s been fine for yearsâ€

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There has to be some means of drying the crawl space. As Mike noted, this can be done by making the crawl space part of the conditioned space or by venting. The link Mike provided to the Building Science Corp web page is a great source of information. The builders guide for various climates you can find on this link are well worth the investment. Only 5-10% of the homes I inspect (in Minnesota) have crawl spaces. However, I have seen a ton of problems in poorly vented or improperly sealed crawl spaces.

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I'm still working through this. It is becoming accepted building science; one should be taking these new theories into account when analyzing any crawlspace.

I think that these theories are much more critical in new construction w/ all the vapor impermeable materials & modern construction practices.

Common sense still plays a role. If the crawl is 80 years old, vented, and bone dry, that's all you need to know. If you are in a wet climate w/ high humidity, venting the crawl CAN make a mess of things. You should be recommending sealing the vents, installing vapor barriers, & conditioning the space.

As Mike said, much of this stuff falls apart when it hits the job site due to incompetent tradesman. At minimum, you should be thinking about this & communicating it to your customer. If you ever find yourself in court for some crawlspace moisture issue, your ass could be on the line should you decide to cavalierly disregard these theories.

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Mike is absolutely right. What the knowledgeable Joe L. mentions in his well written articles for attic and crawl space ventilation in his climate may be right. I don't know, I don't live in his climate. I know my climate and enter 4-6 sub areas a week, getting to see a variety of conditions and problems. As Mike stated Joe's proposal is not universal and won't fly in my climate. More ventilation is better IMO. And so are vapor barriers on the bottom. There is a huge performance difference between areas with hot humid nights and climates where the temps radically drop 30 plus degrees in the evening.

This stuff is all starting to remind me of the Arkansas project and the catastrophes that followed as the structure rotted away.

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Originally posted by Bruce Thomas


More about the Arkansas project please.

Bruce :)

My attempts at outlining the details are not accurate. The end mixed results are what stick in my mind. It was a 1970's? project to develop new technology and construction methods in order to build more energy efficient homes, primarily dealing with the design of the building envelope. The raised heel (or "Arkansas Truss" was one of the real positive things that came out of it, of course there were many, but on the negative side, the super sealing of the wall cavities led to dry rot inside the wall cavity. If one were to go back and research it properly , which I can't, I can't find anything when I do a web search; this was most likely the start of the problems we are having with trapped moisture inside the building envelope leading to unhealthy indoor environments, the remediation of which promises to be the next big thing in building technology.

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This is an article that I wrote for a local website. YES it is area specific!


Every day that I wake up & head out to inspect someone’s home that has a crawlspace, I could probably fill some of my inspection forms ahead of time. It is an unfortunate fact is that most crawlspaces have moisture problems. As we all know, there are plenty of horror stories out there about crawlspaces & moisture problems so why don’t we talk about the cause and cure of these problems.

There are two different types of crawlspace setups, vented and non-vented.

Vented: There are vents that provide adequate cross ventilation, the sub floor of the home is insulated along with the pipes in the crawlspace to prevent freezing, there is a vapor barrier on the ground and the vents get closed in the winter time. A small heater or heat tape is used help keep the pipes from freezing in addition to the insulation on the pipes.

Non-vented: The crawlspace foundation walls are insulated, the sub-floor is not. The vents are blocked off with insulation all year long, a vapor barrier is in place on the crawlspace ground and the air is conditioned with a dehumidifier. A small heater and pipe insulation are still recommended.

Causes & Cures:

1) No rain gutters. Although controversial, we all need gutters to keep the rainwater away from our homes. The average roof sheds 1,000 gallons of water during 1 inch of rain. This water will fall along the foundation and find the crawlspace very quickly. The best rain gutter covers are solid and cover the gutter opening, allowing water to come in by curling under the cap via gravity. Mesh and screen covers just get clogged up or collapse.

2) Downspouts terminate along foundation. The downspouts need to shed water away from the home at least 6 feet or more or to underground pipes that lead away from the home.

3) Terrain slopes toward foundation. Make sure that the landscaping does not do this. All surface water should be directed away from the home. A swale may be necessary if your home is built on a slope. The foundation can have significant structural damage caused by water along the foundation that freezes in the winter collapsing the foundation inward.

4) Inadequate ventilation. Don’t forget to open those vents that you closed for the winter. A crawlspace needs a minimum of 1 square foot of ventilation for every 150 square feet of crawlspace floor area and should be within 3 feet of the corners allowing for cross ventilation. Low decks, other types of construction above or against vents along with shrubs can block the vents making them useless. Your home needs to breathe.

5) Insulation installed upside down. Depending on how you have your crawlspace set up, either vented or non-vented, you may have insulation in the flooring. The paper side of the fiberglass insulation should always face the heated living space and sit against the sub flooring. When it faces downward towards the earth, it has a tendency to trap moisture between the sub floor and the paper hiding moisture damage. If your crawlspace is set up properly as a non vented crawlspace, no insulation is necessary in your floor.

6) Damaged or loose vapor barrier. You should have a 6 mil thick poly vapor barrier on the ground of your crawlspace, overlapped and sealed around columns and the walls. This will help to keep the ground moisture vapors from rising up into your framing. Most homeowners will add gravel on the top for protection.

7) No or non functioning sump pump. If you have a sump pump installed, make sure it is at the lowest point in the crawlspace and all area easily drain to it. The pump should be inspected regularly and function automatically. If you get standing water in your crawlspace even with items 1 through 6 in place, you need a sump pump. Nothing can take the place of a properly installed perimeter foundation drainage system by the builder but a sump pump is the next best thing. The pump should be in a plastic cylinder in the ground and eject water outside, away from the building and not into the septic or sewer system.

8) A non vented crawlspace with no dehumidifier. If you are going to set up your crawlspace as non-vented, you must condition the air with a dehumidifier and have the water collected piped to the sump pump or install a condensate pump. If you don’t pipe it to drain automatically, you will be emptying the collection device daily. Buy a quality dehumidifier with an adjustable, automatic setting so it does not run all of the time.

Please remember that a crawlspace is an integral part of your home and should be kept neat, clean and easily accessible. Inspect if often looking for signs of moisture or mildew and consult the local code official for any repairs. Remember the old saying “An ounce of prevention is worth a pound of cureâ€

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Back in the 70's my parents had formaldyhide foam insulation pumped in to their walls because they didn't have insulation. (it was the greatest thing since sliced bread THEN) Right after that the paint came off of the redwood siding in sheets. Moisture was defeating the bond.

Getting back to crawl spaces. I guess I'm hearing "make provision for the moisture or else damage will occure" just like in every other situation.

In the north venting is ok if they are closed in the winter and the vapor barrier is in tact.

I still like the sealed conditioned space approach but how can you write up a crawl that was built to yester year's standards. It's like the 150 year old log home I inspected with hand cut 4 by something rafters spaced however. If a buillder did that today I would write pages but how can you argue with 150 years of life.

If it's dry and in good condition say so? If not write it up.

Thanks for all of your input,


Fixed a couple of typos

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