Crawspace vapor barrier question

[Tim H]

I got a call today from a termite inspector (more often than not, they are the guys that install vapor barriers around here), about a recently installed vapor barrier that I had called as defective on a 48 yr old house. I used this statement:

The vapor barrier in the crawlspace is inadequate, as it doesn't cover 100% of the soil under the home. I recommend that this be corrected immediately. This is important because the barrier limits the amount of moisture that can evaporate out of the soil into the crawlspace.

A proper vapor barrier consists of black, 6-mil polyethylene sheeting and must be applied from wall to wall of the foundation. It should cap the footer and lay tightly against the sidewalls, overlapping at least a foot at all adjoining sheets and be tightly sealed around the base of support piers. Ideally, the barrier is secured to the foundation walls, and overlaps at adjoining sheets are sealed with waterproof tape. Additionally, the barrier should be applied with some slack so that it can float free without separating and still inhibit evaporation if minor flooding occurs.

The termite guy says that he left a 1' gap around the perimeter of the crawl on purpose. He said he had to do it that way or the sudden lack of moisture would dry the framing so quickly that the floors would buckle.

This is the second time I have heard this (not completely covering on purpose). The first time was about a month ago when a termite guy from a very big national company said his company training literature instructed that vapors barriers should be installed leaving room for the ground to 'breathe'. No kidding (at least he didnt claim that the floors would buckle).

Sounds like folklore to me. Anyone ever heard this? Is my statement accurate?

Thanks,

Tim

[randynavarro]

I think your statement is accurate; at least if it were in my area where crawlspaces are notorious for lots o' moisture.

You being in Alabama, its a different climate. Sealing crawlspaces entirely is preferred now in the hotter, higher humidity climates.

[Brian G]

I read in an article somewhere, and I really want to say it was either in The Old House Journal or Preservation, that if you're dealing with a historic property and hardwood floors you should not cover all the ground. The flooring dries to a point never before experienced and shrinks enough to open all kinds of cracks and gaps in the flooring. I don't recall a thing about the framing, but if you buy the first premise the second isn't much of a leap. I've been telling folks in that situation to talk to someone with lots of experience in that arena before going ahead on the vapor barrier.

I hope I'm not promoting folklore.

Brian G.

Les Made Me Do It [:-cyclops

[hausdok]

Hi,

I think it's folklore; I've never heard it...ever.

ONE TEAM - ONE FIGHT!!!

Mike

[Brandon Whitmore]

Brian,

I was taught the same thing a few years back at a training seminar.

[Richard Moore]

Around here they just leave a 10 foot by 15 foot gap in the center of the crawl.

NOT!!! Just kidding, but that is the equivilant of a one foot gap all the way around a 1250 sq ft crawl. No way I wouldn't say that is wrong. But that's here and our climate is way different from Alabama.

"...the sudden lack of moisture would dry the framing so quickly that the floors would buckle" is a little hard to believe. It's still a crawl. What would be so "sudden" unless you deliberately put a dehumidifier down there?

[Darren]

This is coming from a guy who sees very little crawlspaces so take it with a grain of salt:

I read somewhere (maybe on another BB) that leaving a gap at the perimeter of the crawlspace will allow any water that enters the crawlspace to perc down, not run on top of the barrier.

[Brandon Chew]

Originally posted by Brian G

I read in an article somewhere, and I really want to say it was either in The Old House Journal or Preservation, that if you're dealing with a historic property and hardwood floors you should not cover all the ground. The flooring dries to a point never before experienced and shrinks enough to open all kinds of cracks and gaps in the flooring. I don't recall a thing about the framing, but if you buy the first premise the second isn't much of a leap. I've been telling folks in that situation to talk to someone with lots of experience in that arena before going ahead on the vapor barrier.

I hope I'm not promoting folklore.

Brian G.

Les Made Me Do It [:-cyclops

I would believe that for flooring but not for framing. In response to changes in moisture content, wood swells and shrinks the most in a direction that runs across the grain, and very little in the direction running with the grain.

He said he had to do it that way or the sudden lack of moisture would dry the framing so quickly that the floors would buckle.

I think he has it backwards. Drying out the crawl could cause the flooring to open gaps between the boards. Getting flooring too wet causes it to buckle.

[AHI in AR]

Originally posted by Tim H

The termite guy says that he left a 1' gap around the perimeter of the crawl on purpose. He said he had to do it that way or the sudden lack of moisture would dry the framing so quickly that the floors would buckle.

Putting it politely, that defies logic, doesn't it? He says the floors will BUCKLE from a DECREASE in moisture?

Any change in dimension form a decrease in moisture levels will result in wood shrinking, not expanding. Furthermore, any change will be gradual enough that I doubt you'll see more shrinkage than the normal wintertime amount when everything dries out a little anyway. That wood won't suddenly shrink. What shrinkage occurs will be gradual.

Personally, I like to see the foundation walls lapped. A lot of surface water enters through the foundation. With the purpose of the vapor barrier being to keep the soil moisture IN the soil, not in the home, why would you deliberately leave a gap around the perimeter? Any percolation through the soil can take place with or without the vapor barrier. If your soil contains as much clay as ours does here in Arkansas, there's not a rapid perc rate. Most moisture is lost to evaporation...and a lot of that moisture will find its way into the structure.

I say cover the footing and lap it up the side walls at least a foot. That way, you have a better chance that any moisture migrating through the base of the foundation will enter under the plastic, not on top of it. So long as your joints are lapped consistently with the lower elevation sections on top of the upper ones, any water will not run out on top of the plastic.

Dang! Brandon posted while I was being long winded!

[hausdok]

Originally posted by Darren

This is coming from a guy who sees very little crawlspaces so take it with a grain of salt:

I read somewhere (maybe on another BB) that leaving a gap at the perimeter of the crawlspace will allow any water that enters the crawlspace to perc down, not run on top of the barrier.

Hi Darren,

That would work if:

a. The center of the crawlspace were higher than the perimeter and everything was graded toward the footers - which is very, very rarely the case.

b. The house were on a raised section of land with everything around it much lower so that groundwater can't surface under the center of the crawl. - also rarely the case and even those will get water on the barrier if the downspouts are oriented wrong and are dumping water at the base of the foundation wall.

Tim did good. He described the ideal installation, as he should. That's all we can really do and then hope that someone somewhere who's supposed to correct such things will give a frack about what we think and will fix it. The bug guy is just trying to baffle him with bullsh*t.

Tim, look at it this way, if you've got 95% of the soil covered, the barrier is being 95% efficient. You're a heck of a lot better off than if you're dealing with no barrier at all. Make them understand the bug guy is off his nut, but don't get all wrapped around the axle with it unless conditions are such that you feel it's likely that they're going to end up with a lot of water under there.

ONE TEAM - ONE FIGHT!!!

Mike

[randynavarro]

. . . who's supposed to correct such things will give a frack about what we think. . .

Too much Battlestar Galactica?

[Brian G]

I'm still surfing around for something dead on point, but I ran across this article at the The Old House Journal.com

http://www.oldhousejournal.com/npsbriefs2/brief39.shtml

In the third paragraph under Vapor Diffusion it says:

"In some instances, using vapor barriers, such as heavy plastic sheeting laid over damp crawl spaces, can have remarkable success in stopping vapor diffusion from damp ground into buildings. Yet, knowledgeable experts in the field differ regarding the appropriateness of vapor barriers and when and where to use them, as well as the best way to handle natural diffusion in insulated walls."

If "knowledgeable experts in the field differ", a hard & firm answer may not be out there.

Brian G.

The No Answer Answer [8]

[hausdok]

Hi Brian,

The knowledgeable experts in the fields that most of these articles refer to are builders. If you check out Dr. Joe's site and the US Government laboratory programs, you'll find that they pretty much agree on how buildings behave and where vapor barriers should be installed.

The use of vapor barriers in walls and ceilings is problematic because where it's used is dependent on where the home is built.

If a builder doesn't do his homework, or does his homework and then installs the barrier in the right location but does so sloppily, you can have problems. In cold climates, the barrier needs to be just behind the drywall on the face of the studs. In warm climates, it needs to be on the other side of the studs. Plenty of folks don't understand vapor diffusion and why this needs to be done and that's where the problems start.

A vapor barrier over dirt in a crawlspace is good because it limits the amount of moisture that can escape from the soil under the home. However, in very humid southern states when humid summer air enters these crawls it tends to condense on every cold surface present, including the barrier material and in the underfloor insulation. That's why I said that a barrier that covers 95% of the floor is 95% efficient. Which would we rather have, a combination of 11 gallons per 24 hours for every 1000 square feet of soil evaporating into that crawl and combining with moisture-laden exterior air that's migrating into the crawlspace through vapor diffusion, or 1 or 2 gallons of moisture every 24 hours combining with the moisture in that exterior air that's migrating in there? Naturally, we want the latter.

What folks are struggling with these days is the idea that you don't have to vent crawlspaces to the exterior at all and they can be safely and completely sealed off to the exterior and can become part of the conditioned space of the home. However, I think that's a topic for another discussion on a completely different thread. Bottom line here is that the bug guy was selling Tim wolf tickets because he doesn't want to return to the house and adjust that barrier.

ONE TEAM - ONE FIGHT!!!

Mike

[Richard Moore]

"Bottom line here is that the bug guy was selling Tim wolf tickets because he doesn't want to return to the house and adjust that barrier."

Once again, driven to Wikpedia...

The phrase wolf ticket is a corruption of woof ticket, an African American slang expression for the practice of verbal intimidation, "sellin' (or passin' out) wolf tickets," that was misinterpreted. Over time, the misnomer has become accepted terminology in some quarters.

Woofing (woofin')," like "signifyin" and "talkin' trash," is part of the African American oral tradition. The term is derived from the onomatopoeic expression of the sound of, for instance, a junkyard dog barking to ward off potential intruders.

"Selling wolf tickets" is the act of engaging in threatening or intimidating verbal aggression, usually without the intent of doing actual physical harm. In West African and African-American cultures, verbal sparring and physical displays traditionally were employed as proxies for physical violence to preserve life and maintain peace and order. Woofin' also can be a means of "calling someone out," of challenging an opponent to a verbal or physical match.

Examples of use:

"You aint sellin' no woof tickets over here Kansas City (Mac)." Bill Cosby in the film Let's Do It Again.

"I don't believe the hype or buy wolf tickets..." Xzibit from the song "Paparazzi"

“Well I pulled on trouble's braids and I hid in the briars out by the quick mud stayin' away from the main roads passin' out wolf tickets...“ Tom Waits, from the song "Trouble’s Braids"

Dang Mike, who you be hangin' wid?

[Brian G]

Originally posted by hausdok

The knowledgeable experts in the fields that most of these articles refer to are builders.

I don't think that's true of this particular article. The lady who wrote it writes presevation briefs for the National Park Service (the Heritage Preservation Services Program). I'm pretty sure she didn't consult builders for her article in The Old House Journal.

If you check out Dr. Joe's site and the US Government laboratory programs, you'll find that they pretty much agree on how buildings behave and where vapor barriers should be installed.

But isn't all that really aimed at new construction, much moreso than retrofitting long-existing construction? Does any of that discuss the possibly detrimental effects of a complete, relatively-tight vapor barrier installed many years after construction? I'm not saying I'm ready to buy the theory the guy is trying to sell Tim, I'll even agree it's more likely he's just squirming, but we don't really know that from anything turned up so far.

Brian G.

Still Seeking the Bullseye [:-magnify

[Brian G]

Well, at least one respectable fellow who's well-familiar with old houses recommends a complete vapor barrier...

http://www.oldhouseweb.com/stories/Detailed/15125.shtml

"These soil coverings need to be properly installed to be effective. The seams should overlap at least a foot and be sealed with appropriate tape. It needs to completely cover the soil and be sealed to the foundation."

Of course, that article is about controlling moisture in crawl spaces. If he has any comments specific to possible resulting wood shrinkage, I'm sure we'll hear about soon enough.

I've run a bunch of searches, and I can't find a single dang article or brief that says anything about wood shrinkage after a vapor barrier is installed; framing or flooring, true, false, or otherwise; nothing.

Brian G.

Done For Now [:-crazy]

[hausdok]

Originally posted by Brian G

I've run a bunch of searches, and I can't find a single dang article or brief that says anything about wood shrinkage after a vapor barrier is installed; framing or flooring, true, false, or otherwise; nothing.

Brian G.

Done For Now [:-crazy]

That's because it's a self-serving myth created by someone without any basis in fact.

Let's just assume that moisture in the crawlspace caused the wood to expand. Well if the framing expands with all that weight on it it's going to want to sag downward. Remove the moisture (and it would have to be a lot of moisture to have caused the joists to sag appreciably) and the wood will just shrink again and maybe flatten out a little, maybe not.

I can't think of one bad thing that could happen to joists as a result of removing moisture from a crawl but I can think of lots of bad things that can happen by adding moisture to a crawl.

ONE TEAM - ONE FIGHT!!!

Mike

[Richard Moore]

I doubt that anyone, even the pest guy, would argue that a complete moisture barrier would be preferred, all other things being equal. So, even if there were a "scientific" reason for doing this in some old houses, surely that would involve a thorough evaluation of present conditions, including existing structural moisture content, crawl ventilation (probably minimal in an older home), and seasonal soil conditions. You think the pest guy takes all that into consideration? Me neither! Maybe, it would actually be beneficial in one old house out of ten. Who knows? But, it seems that doing this in every old house is going to leave a lot of them less protected than they could be. As Mike pointed out, unless the grading in the crawl happens to be perfect, there's a very good chance that water will get on top of the barrier, sit and slowly evaporate there, and might be worse than no barrier at all.

Hell, as an inspector, I wouldn't even know if the new "skimpy" barrier was simply a replacement for a complete, but tatty old one.

I think Tim's original statement is just fine...

The vapor barrier in the crawlspace is inadequate, as it doesn't cover 100% of the soil under the home. I recommend that this be corrected immediately. This is important because the barrier limits the amount of moisture that can evaporate out of the soil into the crawlspace.

A proper vapor barrier consists of black, 6-mil polyethylene sheeting and must be applied from wall to wall of the foundation. It should cap the footer and lay tightly against the sidewalls, overlapping at least a foot at all adjoining sheets and be tightly sealed around the base of support piers. Ideally, the barrier is secured to the foundation walls, and overlaps at adjoining sheets are sealed with waterproof tape. Additionally, the barrier should be applied with some slack so that it can float free without separating and still inhibit evaporation if minor flooding occurs.

Maybe he could add...

And if anyone tells you any different, have them provide the documentation supporting their claim along with the warranty that it won't cause future moisture related damage.

[inspect4u]

Around here, there are no vapor barriers installed in the crawl spaces.

Mike M

[Brian G]

Since I've already cast myself as the devil's advocate on this, I'll see if I can reasonably stay with it.

Originally posted by hausdok

That's because it's a self-serving myth created by someone without any basis in fact.

Likely correct, but not yet proven. It could also be that I just haven't found it. If Brandon heard the same thing I read about older hardwood floors, at least that part should be discussed somewhere (even if only to debunk it).

Let's just assume that moisture in the crawlspace caused the wood to expand. Well if the framing expands with all that weight on it it's going to want to sag downward.

Not necessarily. If the wood in a frame expands enough, it's going to create pressure; pressure that wants the wood to give. If it can only do that by bending, it will, but unless there's an awful lot of weight on top it'll bend whichever way the grain wants it to bend. Most smart carpenters, like many used to be, turn the crown up (the natural, grain-determined bow in a board).

I think all that's unlikely though, unless the framing was bone-dry when nailed together and exposed to very high moisture levels later. As Brandon notes, wood expands much more across the grain than up and down it. Problems caused by that shrinkage is much easier to imagine (like gaps between the boards of a hardwood floor).

I can't think of one bad thing that could happen to joists as a result of removing moisture from a crawl but I can think of lots of bad things that can happen by adding moisture to a crawl.

There's no question which way is the safer bet. I won't even try that one.

I will say I'm not convinced that anything less than a 100% coverage vapor barrier is "inadequate" for an older house. A 1" gap around the perimeter is a big deal? Really? It may turn out to be silly and unfounded, but a horror it isn't. That expert with the National Park Service was speaking specifically about historic properties and the preservation field when she said...

Yet, knowledgeable experts in the field differ regarding the appropriateness of vapor barriers....

After reading her article I'm willing to give her more credit than me, you, or the termite guy on this subject. So for me, in those kinds of cases, it isn't cut and dried at this point.

Brian G.

Waiting For More Facts [:-tophat]

[Tim the Termite Guy]

Originally posted by Tim H

I got a call today from a termite inspector (more often than not, they are the guys that install vapor barriers around here), about a recently installed vapor barrier that I had called as defective on a 48 yr old house. I used this statement:

The vapor barrier in the crawlspace is inadequate, as it doesn't cover 100% of the soil under the home. I recommend that this be corrected immediately. This is important because the barrier limits the amount of moisture that can evaporate out of the soil into the crawlspace.

A proper vapor barrier consists of black, 6-mil polyethylene sheeting and must be applied from wall to wall of the foundation. It should cap the footer and lay tightly against the sidewalls, overlapping at least a foot at all adjoining sheets and be tightly sealed around the base of support piers. Ideally, the barrier is secured to the foundation walls, and overlaps at adjoining sheets are sealed with waterproof tape. Additionally, the barrier should be applied with some slack so that it can float free without separating and still inhibit evaporation if minor flooding occurs.

The termite guy says that he left a 1' gap around the perimeter of the crawl on purpose. He said he had to do it that way or the sudden lack of moisture would dry the framing so quickly that the floors would buckle.

This is the second time I have heard this (not completely covering on purpose). The first time was about a month ago when a termite guy from a very big national company said his company training literature instructed that vapors barriers should be installed leaving room for the ground to 'breathe'. No kidding (at least he didnt claim that the floors would buckle).

Sounds like folklore to me. Anyone ever heard this? Is my statement accurate?

Thanks,

Tim

California regulations state a vapor barrier has to cover 80% or more of the subarea soil with 4 mil or thicker poly.

I personally like 6 mil black poly. After inspection I usually cover as much of the subarea as I can access leaving about a 4" to 6" gap adjacent the foundation so the soil can breath at least a little. My concern is what might happen over time to a foundation in expansive soil that is always wet on one side and seasonal on the other. I've seen old houses where people over-irrigate causing damage to the foundation, hmmm.... Another concern is the presence of standing water. You don't want to create a permanent pond under your house.

[hausdok]

Hi,

Welcome to TIJ. You might want to re-think that; by leaving that soil uncapped, you never allow that soil to reach equilibrium and its moisture will constantly fluctuate with the seasons. With expansive soils, that's like putting a big wrench on the foundation and tweaking it whenever the weather and temps change. The cap sheet keeps expansive soil underneath it permanently wet, expanded and uniformly stabilized. With a situation like that, you can overwater the lawn all you want and the exterior soil isn't going to be able to torque a stem wall or footer because that soil in the crawl under the cap sheet is already fully wetted and exerting maximum pressure from the inside.

Standing water is easy; install the barrier above the footer and on top of the pier pads and seal the overlaps. Leave some slack in it and it can float free when water perks up from beneath; as the water recedes it will settle back down with it and leave things dry. If you're concerned about water staying on top of the barrier, ensure that everything slopes to a drain or a sump well so that it will drain to the exterior. If you were here, I could take you to a house up in Everett that was built about 37 years ago on a flood plain. It's got a barrier that was very carefully installed and designed to float. This time of the year, it's got a 4-inch thick waterbed beneath it but the underside - insulation, framing, etc. looks as clean as the day it was built.

In your climate do you really want to encourage moisture to accumulate under your floors 24/7/365?

ONE TEAM - ONE FIGHT!!!

Mike

[Chris Bernhardt]

After inspection I usually cover as much of the subarea as I can access leaving about a 4" to 6" gap adjacent the foundation so the soil can breath at least a little. My concern is what might happen over time to a foundation in expansive soil that is always wet on one side and seasonal on the other. I've seen old houses where people over-irrigate causing damage to the foundation

Can you cite any authority that claims that partial coverage of the ground is significantly beneficial vrs full coverage?

Sounds like folklore.

Chris, Oregon

[sirch]

Here's my two cents (for what it's worth), I really don't think leaving a space at the outer edges makes any difference. I have inspected crawls (of all ages) that never had a vapor barrier and could not find any moisture issues. Most damage I find was caused by leaks from kitchens and baths. Heck, down here the humidity in the air is in the 80 and 90 % range most of the year.

[hausdok]

Originally posted by sirch

Here's my two cents (for what it's worth), I really don't think leaving a space at the outer edges makes any difference. I have inspected crawls (of all ages) that never had a vapor barrier and could not find any moisture issues. Most damage I find was caused by leaks from kitchens and baths. Heck, down here the humidity in the air is in the 80 and 90 % range most of the year.

Heck,

Hop on a plane and we'll go on a tour and I'll show you a few hundred.

What works in Alabama won't necessarily work here and what works here won't necessarily work in California. However, pooh, poohing the idea because you've never had an issue with it is kind of short sighted. It might come back to bite you one of these days.

ONE TEAM - ONE FIGHT!!!

Mike