Vapor barrier: Yes or no?

[seedtime]

It seems to me that a vapor barrier would always be recommended as a inexpensive precaution. My builder told me it was not needed, only in the south. I was willing to pay the extra for it, but I got the impression it would do more harm than good.

The floor in my other attached garage that is insulated but not heated, gets dark on humid days. I wanted to avoid that. Did I make the right decision?


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[TommyK]

Insist on the vapor barrier.

 

[ConCretin]

There is no downside to a vapor barrier. It's purpose is to keep water vapor from the soils below your slab from making their way into interior spaces. This is always desirable but especially important if you might be applying epoxy or a floor covering with a moisture sensitive adhesive.

A vapor barrier has no bearing on condensation issues, which result from a cool concrete surface and warm humid air in the space.

It should be fairly inexpensive and you only get one chance to do it.

 

[LegacyIndustrial]

Should be code, imo.


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[Armorpoxy]

Use it!

 

[like2wheel]

It seems to me that a vapor barrier would always be recommended as a inexpensive precaution. My builder told me it was not needed, only in the south. I was willing to pay the extra for it, but I got the impression it would do more harm than good.

The floor in my other attached garage that is insulated but not heated, gets dark on humid days. I wanted to avoid that. Did I make the right decision?


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It bet he's trying to talk you out of it to make his job easier.

 

[James-W]

It bet he's trying to talk you out of it to make his job easier.

I don't understand, please explain. How hard could it be to put down a vapor barrier before pouring the concrete?

 

[Favguy]

Well, usually he'd have to lay and compact a layer of sand before putting in the liner to avoid the risk of puncture, which will take a little more time and a little more money...

Better to have one and not need it than the other way round, it's not like you can add it later!

 

[ConCretin]

I don't understand, please explain. How hard could it be to put down a vapor barrier before pouring the concrete?

It isn't terribly costly but if you didn't specify a vapor barrier before you obtained bids, you will likely have to pay a little more. Most contractors won't throw one in for free.

There are a few costs involved other than the material and labor to spread it out. Ideally vapor barrier should be turned up and secured at the edges and all seams and penetrations must be taped. The vapor barrier must also be protected from punctures, which involves some care and attention including to the base underneath.

Another consideration is the product used for the vapor barrier. You could use a 6 or 10 mil poly, which is inexpensive and reasonably effective but prone to damage. The other option is a 15 mil product such as Stegowrap, which is more effective at blocking water vapor and much more resistant to damage. It comes with a higher cost however.

Another reason for your contractors reluctance could be that finishers are not fond of them. As concrete sets, excess mix water works its way out of the concrete. Vapor barriers keep this water from going down into the soil resulting in slower set times and more bleed water at the surface. This can easily be mitigated by keeping slumps low or by using a mid range water reducer - both of which are highly desirable for unrelated reasons.

It might not be quite as simple as we made it sound but it is still worth doing and shouldn't break the bank. Sorry you asked? lol

 

[James-W]

It isn't terribly costly but if you didn't specify a vapor barrier before you obtained bids, you will likely have to pay a little more. Most contractors won't throw one in for free.

There are a few costs involved other than the material and labor to spread it out. Ideally vapor barrier should be turned up and secured at the edges and all seams and penetrations must be taped. The vapor barrier must also be protected from punctures, which involves some care and attention including to the base underneath.

Another consideration is the product used for the vapor barrier. You could use a 6 or 10 mil poly, which is inexpensive and reasonably effective but prone to damage. The other option is a 15 mil product such as Stegowrap, which is more effective at blocking water vapor and much more resistant to damage. It comes with a higher cost however.

Another reason for your contractors reluctance could be that finishers are not fond of them. As concrete sets, excess mix water works its way out of the concrete. Vapor barriers keep this water from going down into the soil resulting in slower set times and more bleed water at the surface. This can easily be mitigated by keeping slumps low or by using a mid range water reducer - both of which are highly desirable for unrelated reasons.

It might not be quite as simple as we made it sound but it is still worth doing and shouldn't break the bank. Sorry you asked?

No, not sorry I asked, now I know more about it.

Basically, you are saying that putting down a vapor barrier would take a little more time and there may be a little extra prep cost, but overall it is a fairly simple thing to do and is well worth it.

 

[Jaguar Fan]

I got the impression it would do more harm than good.

Uh, what harm did you get the impression it could possibly do?

 

[seedtime]

Uh, what harm did you get the impression it could possibly do?

I was told it could interfere with the curing process by trapping excess water under the slab. Now that I think about it, it may only prolong the cure time (big deal). I was never in a hurry, I was more concerned with getting a high quality building. On the bright side, at least I could use Race-deck, since my use will be light duty.


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[75gmck25]

The other consideration may be what type of flooring or coating you want to put on the floor. Concrete allows moisture to go through it forever, and without a vapor barrier, you will always have a fairly significant amount of ground moisture coming up through the floor. If you want to use certain types of paint or floor coatings (epoxy, etc.) it will not work well on a floor that always has moisture coming through it.

Bruce