Thread: Vapor barrier -yes or no

I have seen this discussion here before but cannot find anything now. A client has gotten conflicting advice from 2 professionals on this. I suggested he contact a third. He said that was why I was there. So I guess I had better come up with an answer. This is a house with poured concrete walls in SE Wisconsin. Finished basement has 2X4 drywall and stud walls with batt insulation and no vapor barrier. Due to flood damage, the client had to cut open the walls and wants to know if he should take them down and put in a vapor barrier, and where would it go.

I am thinking basement walls do not necessarily act like those above ground. However, I assume even with temp swings from -20 to 90, the exterior of the foundation is likely to always be colder, so a vapor barrier just under the drywall might make sense. However that would put insulation unprotected against the concrete, unless one created an air space. At the same time, it seems to have done just fine for 6 years without any vapor barrier.

A factor in this may be that the exterior of the foundation is also insulated. This is a second question. What is this stuff? Anyone know a brand name? (see photo). It is a white foam board material with a covering that seems to be permanently bonded to it. It looks like a burlap material or maybe woven plastic that has paint or some other liquid applied to it. I assume the material is to protect the foamboard from UV rays. But it seems to shrink at the joints and expose the foam. What would be a reasonable repair?

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I have seen other guys forget their attachments and figured I was too smart for that. Then I signed out and saw my camera sitting here and thought, O shoot!. So now there should be a photo.

Hi Rodger. I inspect in Calif. and I dont like a vapor barrier. As I see the problems that come from them. Excessive moisture to the soil. mold, fungus conndition. And then we get standing water on top and under the plactic. Unless you have a complete air tite sub-structure and control all exterior water condition its no good. Now If this is done then i can see one working. But i like to see a well ventilated sub-structure with lots of air to move all moisture out of the sub-structure. This also provides access to the soil/foundation for inspection as the years pass. One other problem is rats, cats, dogs, and other criters that make a mess on the plastic. Then the only way to clean that up is to take out all the plastic and install all new.

Please keep in mind im in Calif.
A lot dryer condition.

Best

Ron

Scroll down to insulating basements. From Building Science.
BSD-103: Understanding Basements — buildingscience.com

Hello Roger –

It is not surprising to find conflicting statements on vapor barriers, and it is not surprising to find that the conflicting opinions may both be correct under the specifics being discussed by each expert.

Before, I address the VB on walls, as Ron Bibler points out, vapor barriers in crawlspaces can INCREASE the moisture in a crawlspace and in other circumstances, a VB can decrease the moisture. The specific situation will dictate the results.

When folks speak of controlling moisture, we often assume that “moisture control� is synonymous with “moisture reduction.� However, the control action we take may increase the moisture and control the moisture to an higher level than before. Vapor barriers are a good example.

Some years ago, I was asked to comment on altering building codes to address the “growing� problem of indoor mould in new construction (no pun intended). The municipality wanted to require the automatic inclusion of VBs and active ventilation in the crawlers; I said it was a bad idea. They ignored my arguments and made the changes, and the net result was that the incident of mould in houses in one area went up – not down - as a direct result of the VBs plus the crawlspace fans.

The problem with VBs is that they restrict water vapor migration. Unfortunately this not only means that they prevent water vapor from entering a building system, they also prevent water vapor from LEAVING. Like anything else, until one understands the specific situation, blindly requiring a “fix� may not fix anything at all.

The assumption was VBs reduce %RH in crawlspaces, and therefore, control (reduce) mould growth. But, for a start, the relationship between mould growth and %RH is very complex and not at all simple. In one house I looked at, a swimming pool size of 20 inches of free standing water had been present in the crawlspace for three years. The %RH in the remaining 12 inches of head space was almost always 80-90%. Yet there was NO mould growth in the crawler – Why? Answer: Why would there be?

There is no correlation between relative humidity and mould growth outside the context of dew point temperatures and especially dew point temperatures on surfaces. Let’s look at a simple example of the temperature and moisture complexities –

Posit: A nice summer day (in Colorado) with a relative humidity of only 35% and an outdoor temperature of 98°F, the outdoor air will contain approximately 16 grams of water per cubic meter of air. Crawlspace vents are closed and %RH in the crawlspace is also 35% relative humidity. However, the temperature in the crawler is 65°F and, therefore, the air in the crawler only contains about 5.6 g/m3.

The minimum temperature needed to precipitate the water from the outdoor air is only 66°F (the dew-point). That means that if the air temperature or surface temperatures in the crawlspace are less than or equal to 66°F, the %RH in the crawlspace will skyrocket to 100% RH and the moisture from the outdoor air drawn into the crawlspace will precipitate onto those surfaces (even though the actual moisture content in the air has not changed). The condensed moisture could provide a water source for mould to grow if that water does not migrate somewhere; such as the soil, provided there is no VB. If the effect is prolonged, and wood may remain damp, and the probability is that moulds will proliferate. In this way, fans and vapor barriers meant to “dehumidify� crawlspaces actually defeating their own purposes by increasing the potential humidity in the crawlspace.

Where the soils are a source of moisture, the vapor barriers may be a good idea, maybe not; where the soils are dry, the installation of a vapor barrier may or may not be a good idea. My point is that there should always be an articulable reason to do something.

Regarding VBs on/in walls – there was an excellent article by Joe Lstiburek which addressed this issue in a very succinct manner. I believe it is copy-righted material, and so I can’t supply it here, but it was in the February 2002 ASHRAE Journal (pp. 36-41) as part of the “Moisture Control Series.� It is one of the best discussions on the matter I have seen.

Dr. Lstiburek breaks down the country into climatological regions, and in the context of those regions explains when a VB is appropriate, and explains where the VB should be installed.

Hope that helps.

Cheers,
CaoimhÃ*n P. Connell
Forensic Industrial Hygienist
Forensic Industrial Hygiene

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG

Originally Posted by Roger Kautz

I have seen this discussion here before but cannot find anything now. A client has gotten conflicting advice from 2 professionals on this. I suggested he contact a third. He said that was why I was there. So I guess I had better come up with an answer. This is a house with poured concrete walls in SE Wisconsin. Finished basement has 2X4 drywall and stud walls with batt insulation and no vapor barrier. Due to flood damage, the client had to cut open the walls and wants to know if he should take them down and put in a vapor barrier, and where would it go.

I am thinking basement walls do not necessarily act like those above ground. However, I assume even with temp swings from -20 to 90, the exterior of the foundation is likely to always be colder, so a vapor barrier just under the drywall might make sense. However that would put insulation unprotected against the concrete, unless one created an air space. At the same time, it seems to have done just fine for 6 years without any vapor barrier.

A factor in this may be that the exterior of the foundation is also insulated. This is a second question. What is this stuff? Anyone know a brand name? (see photo). It is a white foam board material with a covering that seems to be permanently bonded to it. It looks like a burlap material or maybe woven plastic that has paint or some other liquid applied to it. I assume the material is to protect the foamboard from UV rays. But it seems to shrink at the joints and expose the foam. What would be a reasonable repair?

Does the basement have any type of HVAC system? Was the basement wall waterproofed before it was insulated and then backfilled? If so and if the prior insulation and drywall was OK before the flood, I would not change anyting.

Putting a vapor barrier on the inside of the home will only cause additional problems.

With a basement like you are describing, you are not going to stop the migration of moisture from the outside by installing a vapor barrier on the inside.

Personal preference, Frame the wall an inch or two from the concrete. Provide one or two vents at the bottom and tops of the exterior wall to allow air flow. You can insulate the framed wall if you like but keep it off the concrete. Do not add any other vapor barrier.
This provides air flow behind the wall. If the concrete already weeps or leaks put a perimeter drain in.

Regarding VBs on/in walls – there was an excellent article by Joe Lstiburek which addressed this issue in a very succinct manner. I believe it is copy-righted material, and so I can’t supply it here, but it was in the February 2002 ASHRAE Journal (pp. 36-41) as part of the “Moisture Control Series.� It is one of the best discussions on the matter I have seen.

Dr. Lstiburek breaks down the country into climatological regions, and in the context of those regions explains when a VB is appropriate, and explains where the VB should be installed.


My post above references Dr. Lstiburek. Lots of good info on that web site.

This question to anyone with radon credentials; is the sub-slab ventilation somthing that would prevent radon migration? All other sources aside, migration from the soil is what my question addresses.

Hello Steve-

You may be referring to sub-slab depressurization, instead of sub-slab ventilation. You may find my discussion on it interesting: Radon: Truth vs. myth

Yes, the technique is a proven method for reducing radon.

Not that it matters much.

Cheers!
CaoimhÃ*n P. Connell
Forensic Industrial Hygienist
Forensic Industrial Hygiene

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG

Caoimhin, thank you. Seems pretty ironic that these "threats to health" may only exist if you choose to chase them. Schroedinger's cat lives.

Being as I am just a small town boy with no colege edumucation, I did a Google search on "Schroedinger's cat lives" ...

... very interesting information.

I say, don't open the box for 3 weeks , the evidence of whether or not the cat was alive or dead would be overpowering.

Yeah, I read about the environment being "the observer" and thus causing the death of the cat - which was both alive and dead at the same time.

LOL, Jerry.

In response to Rick's post a couple of weeks ago, how would adding vents to the exterior walls help dry out the interior space? We know that venting crawl spaces is bad but I'm not sure I am following your suggestion or am misinterpreting it. Can you elaborate please...we do both agree on no vapor barrier though.

Ross

Venting crawl spaces is bad? I disagree. I have inspected crawl spaces under 10 year old homes that I doubted the vents were ever opened (based on everything else I saw). Most of the underside will be rotted and moldy in southwest Missouri. Here the wind blows mostly from southwest to northeast. I try to leave one vent open on the north and south side of my house right on thru the winter unless temperatures in the teens or below are forecasted.

Edited

Without knowing or inspecting all of the crawlspace itself, it is commonly known that venting a crawl increases the relative humidity within that space. Sealing off the vents AND applying at least a 6 mil, hopefully more, thick vapor retarder that is sealed will help mitigate some of the moisture and "pollultants". There are several well known articles and authors, however here are two I've found.

RR-0401: Conditioned Crawl Space Construction, Performance and Codes — buildingscience.com

http://www.advancedenergy.org/buildi...0To%20Vent.pdf

One thing which has not been addressed in all these discussions regarding sealing crawlspaces:

Flood Elevation.

*IF* the crawlspace floor is at or below flood elevation, then the crawlspace must not be sealed, it *must* be allowed to be vented, either open or with automatic flood vents (which open automatically during a flood) with the vents placed at such height that the difference in height for outside to inside is no greater than 12", at which point the flood waters flow through the vents into the crawlspace, filling the crawlspace to the level of the water outside the crawlspace.

If that is not allowed to happen, the hydrostatic pressure on the outside the crawlspace walls could well cause the crawlspace walls to fail, leading to structural failure.

If you are not in a SFHA (Special Flood Hazard Area) where floor insurance is required/recommended, then the above may not be a problem.

I have read so many articles I am sick.


My opinion, vent crawl spaces.

I have never seen an unvented crawl space that I liked.

Sewage leak under the home (what home owner ever goes down there) Water line minor leak. ground moisture.

Every sealed crawl I have been in, let me think now, yup, every unvented crawl space I have been in I did not like usually for multiple concerns.

I will never buy the argument (unless you could create a perfect world) that seal all (or any) crawls is a good idea.

I advise a large portion of clients to to install hunidity controlled fans for when the moisture in the crawl exceeds x amount

Even in crawls that did not appear to have a water problem the moisture content in the joists were in the middle to upper teens,

Go ahead, say something (I like it when you do) This is from way to much experience in crawl spaces. Way to much can be wrong in almost any crawl, whether it be from ground water, sewage line leaks, water line leaks.

Just my opinionated opinion.

Damn, another day off

Ted, you must be having as slow a week as me.

Crawl spaces were fine and needed ventilation before the advent of air conditioning.
Here in the hot/humid south, venting a crawl space with hot humid air to try and dry a cool crawl space just does not work.
It turns into a rain forest, crawl spaces are quite often below the dew point causing water vapor to condense on surfaces. One reason I like slabs in my part of the world.
I have not seen a properly sealed and conditioned crawl space, but I have seen bunches of older homes that have vented crawls and have been converted to central A/C that now have damp crawls.

In theory, I like the sealed crawl space, but I don't think I would recommend trying to retrofit an existing house.
This seems like something that would need to be designed into the structure from the beginning.

Well thank you Jim

First off. If I am not working for a day I am crying. But when tomorrow afternoon comes and it will be only my second inspection this week, I can be down right hateful.

Second. You said a whole lot more than I did about sealing crawls but I have to admit, you are right to an extent.

There are so many damp crawls and people do not take care of the situation, such as gutters, grading, drainage, plumbing leak of all kinds.

You did notice I hope, that I said controlled by a humidistat for only when there becomes to high a moisture content. I also tell folks that they should stick their heads down there now and then or have a plumber do an inspection now and then. I also tell them that if the fan appears to be running a lot it is time to check things out. If I suggest to someone that they need a fan and better exterior vents or more this is always to correct the moisture problem but fix the concerns I have noted, grading, drainage, plumbing leaks etc.

If a crawl is designed to be sealed and is a new build then great. Not to many of those have I seen that should have been sealed.

As so many have said in the past with many different threads and posts 'Do you really believe the average home owner is going to keep items corrected or proper maintenance and checking for plumbing leaks, keeping the drainage situation under control.

I guess what I am saying is "We" home inspectors might be perfect in every way but the typical home owner is not and cannot be counted on to keep their crawl water free.

Originally Posted by Jim Luttrall

Crawl spaces were fine and needed ventilation before the advent of air conditioning.
Here in the hot/humid south, venting a crawl space with hot humid air to try and dry a cool crawl space just does not work.

I will *assume* that South Florida fits into the "hot/humid south" (or in this instance it is still considered New York South, or Havana North? ).

I have seen many vented crawlspaces (all, every single one was vented - confining that to 'wood floor system crawlspaces) and none had any problems relating to humidity.

Now, switching over to sealed (no vents) crawlspaces with concrete walls and concrete floor slabs, they were all sweating like a pig (do pigs actually sweat?, I would think not, that is why they wallow in the mud - to cool off).

In theory, I like the sealed crawl space,

In theory, one could build a structure tall enough such that the centrifugal forces at the outer end in outer space would offset the gravitational load on earth, making it such that the load would need to be secured down, not supported up, but in practice ...

I do agree with the rest of what you said:

but I don't think I would recommend trying to retrofit an existing house.
This seems like something that would need to be designed into the structure from the beginning.

While I may not have the experience in examining as many crawls as a lot of you below Chicago, I have to ask you gents- how are these crawls insulated and what prevents any contaminants from leaching underground into the living space above? Have you evidence that above crawl IAQ is acceptable for these homes? Is there an established air/pressure boundary at the ceiling of the floor above? I am trying understand the science behind crawlspaces and the PhDs, PEs and others who call building science their livelihood think differently upon this. Just because there is a long standing belief that "its been working" doesn't mean that there isn't a better way to do it. Am I wrong?

Nope, you are not wrong. Past building practices succeeded in part from intelligence and doing what worked from a historical perspective and succeeded in part from dumb luck because there was enough leakiness (is that a word?) built into the structure so there was a "forgiveness" factor built into the structure. With todays tighter structures, it is not a given that old practices will work satisfactorily, especially with new unproven materials and technology. JMO