To Vent or not to vent : crawlspaces

[spwest]

Hello,

Investigating the issue of crawlspace vapor barriers I came across a controversial stand on venting the crawlspace.

There is this school of thought that venting actually is a bad idea when for warm months, because warm humid air comes in from the outside, hits the cold in the crawlspace and you get condensation. They seem to recommend completely sealing off crawlspaces.

Has anyone heard of these guys ?

http://www.basementsystems.com/csmold.html they are selling a really thick vapor barrier installation, no idea how expensive they are...

Any idea what this vent/no vent issue would mean for the Pacific northwest area ?

Thanks

SP

[kurt]

That's pretty much the party line now. No vents, vapor retarders, and condition the crawl meaning heat or cool it like you would the house.

Personally, I think there are still instances & locations where venting is necessary or preferrable, but for the most part vents are now out of the picture.

[Amn]

Ah, a great opportunity to place my neck on the block!

Some fundamental points:

1. The grade in a crawlspace is lower than the exterior surrounding grade, therefore a crawlspace is essentially a large sump, and moisture will naturally migrate from the surrounding grade (planting beds, etc) into the crawlspace.

2. Moisture will migrate from wetter to dryer environments, eg. vapor drive.

3. Moisture is going to migrate into a crawlspace with or without venting.

With a lack of venting, when moisture migrates into a crawlspace, the moisture-rich air is stuck there.

The benefit of venting is that moisture has a way out. When the moisture content (MC) inside the crawlspace exceeds the exterior MC, it will migrate to the dryer air outside via the crawlspace vents. Additionally, with ventilation the crawlspace air will have the ability to change over when a pressure differential occurs from wind on the windward side of the house (high pressure) and a resultant lower pressure (suction) occurs on the leeward side.

Sure vents will allow some moisture laden air in, but moisture will migrate into the crawlspace with or without the vents. You want a way for it to get out.

Vapor barriers/retarders, etc help to prevent moisture drive but cannot eliminate it.

Moisture migration into the crawlspace is obviously exacerbated by poor grading, poor drainage, and irrigation systems that are set to overwater the areas adjacent to the perimeter.

I'm shocked to read in Kurt's post that people are actually conditioning the air in crawlspaces. Did I read that right?

A

[kurt]

Yup.

Basically, insulate, vapor retard, dry, & condition the crawl. Personally, I like your (Amn's) analysis; it makes the most sense to me. But, it is at odds w/current theory per Joe L. and similar experts.

The issue seems to be one that effects the SE states the most; the studies show that warm moist air entering crawlspaces condenses on the cooler members & promotes mold or elevated moisture. Eliminating vents supposedly stops this. Installing vapor barriers, sumps, drain tile, grading, or whatever to keep water out of the crawl is, of course, an important component of the process.

[Amn]

At my former job in Dallas (Forensic Engineering company) our Cert. Industrial Hygienist (CIH) recommended motorized vents on timers to evacuate the crawlspace because of high moisture from the sump effect.

What made that house unique was that we actually measured a dead air area where a bunch of large ducts bordered an area and essentially blocked nearly all air movement. Of course, that was where the mold found a happy home.

One nice item to mention though. In this area of town homes are VERY nice and all the crawlspaces are at least 30" high with plastic vapor retarders from the grade beam in to 10 ft. and the entire floor is covered in pea gravel; much nicer than the clay dirt typical in Dallas.

A

[tombijak]

Aug-04 edition of the "Journal of Light Construction" has a good how-to artical on sealing up/conditioning a wet crawl space. I've not done one myself, but I am intrigued by the idea.

[Thomas O. Maynard]

Combining the above post:

(1) If one would eliminate the path of moisture movement to the space in question, there should be problem. (Air sealing, vapor retarders, proper site drainage & crawl space elevation in relationship to the exterior grade)

(2.) Condition the space in question. (Mechanically control the temp.relative humidity and ventilation)there should be no problem.

No hole - No moisture - No problem.

Crawl space vents might be alright if they were controlled mechanically.

Opened or closed depending upon the specific conditions of the temp. and relative humidity of both the outside air and the inside air of the crawl space.

Practical maybe not.

I like the condition crawl space theory myself.

That is until one of can convince me other wise.

[Jim Katen]

Originally posted by spwest

Hello,

Investigating the issue of crawlspace vapor barriers I came across a controversial stand on venting the crawlspace.

There is this school of thought that venting actually is a bad idea when for warm months, because warm humid air comes in from the outside, hits the cold in the crawlspace and you get condensation. They seem to recommend completely sealing off crawlspaces.

Has anyone heard of these guys ?

http://www.basementsystems.com/csmold.html they are selling a really thick vapor barrier installation, no idea how expensive they are...

Any idea what this vent/no vent issue would mean for the Pacific northwest area ?

Thanks

SP

SP,

I'm in the Pacific Northwest. Just west of Portland, Ore. I've very familiar with the current concept of unvented crawlspaces. As long as the vapor barrier is very good, it works fine.

However, venting crawlspaces traditionally, according to the code requirements, works well in our climate. We don't have the very humid summers that cause the problems that led to the development of the unvented crawlspace concept.

Sooooo. . . I'm inclined to leave well enough alone and continue recommending traditional venting. I've *never* seen a moisture problem caused by proper venting in our climate.

- Jim Katen

[hausdok]

Hi,

I have to agree with Jim. I'm in Seattle and sometimes it feels like I literally live in crawlspaces because I've done so many of them. When properly ventilated, even in our wet climate and with water in them, the number with insect, rot or mold issues is very small compared to those that are fine.

I'm also not unfamiliar with the sealed crawlspace concept. I've done less than a dozen of those over the years, but all except one were doing great. One done that way was actually 50 years old. The one that wasn't doing so well had some water intrusion through crack in the sidewall and it was like a big petri dish and not doing great. Out here, they haven't been using Jeff Toolie's method of installig low-speed blowers in the crawls to exchange air with the house. They're simply vented up through the floors with metal registers and they allow gravity and natural convection to move the air. It works fine.

ONE TEAM - ONE FIGHT!!!

Mike

[kurt]

I agree w/ O'Handley; he probably did live in a crawlspace...errr, wait, wrong topic.

I agree w/ O'Handley & Katen; venting of crawls works pretty good around here. Actually, so does not venting. I think the things noted in Amn's observations (about 6 posts up) are accurate descriptions of how it works most of the time. I think it's somewhat custom; you have to look @ the conditions & make the best plan of action for each crawl.

[Bruce Thomas]

Consider this guys,

A sub-membrane ventilation system for that "problem" crawl space. Just like you would install if you had a high radon level. Seal 6mill plastic to the walls and around any penetrations like plumbing or a pier. Install a plastic pipe, 3" or larger, rout it out side and attach a low vacuum high volume fan. It costs as much as a 60 watt light bulb to operate and it takes all the water vapor and puts it outside where it belongs. The plastic separates the ground from the framing and it makes everyone happy, especially the installer.

Just a thought,

Bruce

[hausdok]

Hi Bruce,

Yeah, that would probably work alright too, except for one small detail - builders around here haven't a clue what a radon mitigation system looks like. They tell us that radon out here on the sound is practically nil and that people here are in more danger of getting sick from second hand smoke, car exhaust and polen than they are from radon.

Here, it would be a matter of educating them to understand first what the system looks like and then why they need to have it. Since they've been getting away with open vents and poly for decades upon decade, they'll be pretty hard to convince that it's even worth the trouble to seal them.

ONE TEAM - ONE FIGHT!!!

Mike