Vapor barrier or no?

[WAKman]

I'm building my dream garage in Seattle, where the weather is moderate and temperate. My builder, for whom I have a lot of respect, recommends against a vapor barrier under the slab. He says that in cases like ours where the slab will be sealed with epoxy or otherwise finished, a vapor barrier creates moisture imbalances that cause the floor finish to peel off the slab. Curious what people think of this rationale. This guy is good at what he does.

 

[mike1956]

Sounds like he may just not wanna bother. A vapor barrier keeps moisture from the ground from coming up thru the concrete. Moisture coming up thru the concrete is the very thing that will make any floor coating fail. Just tell him you respect his opinion but you would really like the barrier.

 

[mike93lx]

Creates moisture imbalances? Interesting take...

I would put one in. Keeping the slab dry can't hurt

 

[pattenp]

You need to post this question over in Flooring forum. I think you'll get an overwhelming put in a vapor barrier.

 

[ConCretin]

With all due respect to your builder, he is simply wrong on this one. The primary purpose of an under slab vapor barrier is to block water vapor migratIon that can interfere with the bond of adhered floor coverings such as epoxy. Unless you are in an arid region, a vapor barrier is a good way to block moisture intrusion into your structure and a must if you are going to apply any type of adhered footing. I’d insist on it.

The only potential downside to a vapor barrier other than cost is that it increases bleed water on the surface of the slab during finishing. As the fines settle in freshly placed concrete, mix water is displaced and since the vapor barrier keeps any of it from going down, it all comes to the surface. This isn’t a problem as long as your finisher is aware and mix water is controlled to minimize bleeding in the first place.

Give my Guide to Floor Slabs in the link below a read for some additional thoughts. Good luck with your project.

 

[Bretny]

You need a vapor barrier. Concrete is not waterproof no matter what coating. Treating the top may keep the top dry but the bottom can be wet. Wet concrete actually rots.

My basement has no vapor barrier, I get wet spots and the concrete is falling apart like a salted sidewalk.

 

[spudley]

My neighbors garage, less than 75' from mine, has no vapor barrier and is damp most of the spring/summer.

I have a vapor barrier and a dry slab.

Most concrete guys like to finish and move on, vapor barriers require patience.

 

[paul.nmedia.net]

Epoxy coatings are water proof not vapor proof. It can still breath just not as well. Ask builder for documentation which he can’t produce.

 

[WAKman]

Thanks for all the input guys. I plan on using Hellfire to finish the floor, and I have reached out to them for recommendations with respect to their product. I suspect their advice will be in accordance with what you guys are saying.

I have experience with this builder, and he does not do things simply because they are cheap or because he doesn't want to be bothered. He does a lot of things above and beyond, so it's not that, thank goodness. That said, he'll do what I ask, since I'm writing some pretty big checks.

 

[redneckcharlie]

I’d be curious to know what his thinking and logic is. Not to be an ****, just curious what his experience with it is. It’s good to hear you have a good contractor. That doesn’t happen much with this crowd.

Thanks for all the input guys. I plan on using Hellfire to finish the floor, and I have reached out to them for recommendations with respect to their product. I suspect their advice will be in accordance with what you guys are saying.

I have experience with this builder, and he does not do things simply because they are cheap or because he doesn't want to be bothered. He does a lot of things above and beyond, so it's not that, thank goodness. That said, he'll do what I ask, since I'm writing some pretty big checks.

 

[LB-1911]

Thanks for all the input guys. I plan on using Hellfire to finish the floor, and I have reached out to them for recommendations with respect to their product. I suspect their advice will be in accordance with what you guys are saying.

I have experience with this builder, and he does not do things simply because they are cheap or because he doesn't want to be bothered. He does a lot of things above and beyond, so it's not that, thank goodness. That said, he'll do what I ask, since I'm writing some pretty big checks.

Previous thread from Flooring

GJ Gang:
Happy to introduce our newest product...HellFire Concrete Coating!
This is a single component, low prep, fast cure, easy to use concrete coating that has extreme resistance to ALL the chemicals you can muster.
Available in any color you wish as long as it's Saber Gray!
Link: http://www.legacyindustrial.net/products/hellfire-coating/hellfire-concrete-coating.html

 

[ConCretin]

I have experience with this builder, and he does not do things simply because they are cheap or because he doesn't want to be bothered. He does a lot of things above and beyond, so it's not that, thank goodness. That said, he'll do what I ask, since I'm writing some pretty big checks.

No-one can be an expert in all the trades and we all keep learning as we go. Concrete was widely used for the first time by the Romans over 2000 years ago and we are still arguing about how to build with it. There has been a tremendous amount of debate on the subject of vapor barriers so it's not surprising that your builder has his own opinion.

 

[LegacyIndustrial]

I'm building my dream garage in Seattle, where the weather is moderate and temperate. My builder, for whom I have a lot of respect, recommends against a vapor barrier under the slab. He says that in cases like ours where the slab will be sealed with epoxy or otherwise finished, a vapor barrier creates moisture imbalances that cause the floor finish to peel off the slab. Curious what people think of this rationale. This guy is good at what he does.

I think I sent you a message regarding this but it's worth mentioning here too.
Everything the posters have said is true...

You want a vapor barrier. If you end-up having moisture issues because you did not install one from the get-go, it will cost much much more than the barrier that can be placed when they pour. Good luck, sounds like a great project!

 

[clamhammer]

Must have a vapor barrier. Im having a building built for my business right now, specified in the plans that it needed a vapor barrier. Well guess what? They evidently missed that part, they already had 22,000 sq. ft. of wire laid and tied. Made em pull it up and put it down. I wasnt around when they were told, but im guessing they werent happy with the GC.

 

[ConCretin]

There is one additional matter of debate regarding the use of a vapor barrier, which is whether or not they cause slab curling. While curling is almost exclusively a problem with large commercial/industrial slabs, some of the larger garages we see on the GJ probably qualify. If I sound jealous, its only because I am.

Curling occurs when the top of the slab 'drys out' faster than the bottom and thus experiences more shrinkage. The slab edges curl upwards as the top contracts more than the bottom. It's not usually a problem until you start running forklifts over the joints and they start deteriorating. Traditionally vapor barriers have been thought to increase curling because they block mix water from escaping downward maintaining the bottom of the slab in a moist condition.

As it happens, it's not that simple. We do a lot of work for a large grocery chain that does it's own design work. They decided that vapor barriers caused curling and started spec-ing slabs without them. Their floors were mostly polished concrete and they were more concerned about curling than the adhesion of floor coverings.

As it turns out, their floors experienced more curling than ever. It seems that water vapor from the soil is much more of a problem when it comes to differential moisture than retained mix water. Mix water works it way out of the slab over time while water vapor from the soil doesn't go away.

Curling is more of an environmental issue than a construction one. Slabs often curl early in construction and seasonally thereafter with changes in humidity and HVAC but settle down as moisture in the slab section equalizes. Turns out that the lack or presence of a vapor barrier isn't much of a negative after all.

As with so many elements of good concrete design and construction, we are still trying to figure it all out. At the end of the day, most of the arguments against vapor barriers have been discredited. Control your mix water and there is really no downside and many advantages to using one.

Probably more info than anyone really wants but there you have it. My wife calls me a concrete nerd. She's probably right.

 

[bdk1976]

I live in the same region as OP. Put 15mil under my slab before pouring last summer. After reading thread after thread on this subject over the years, there was no way I was going without one. I may be putting Hellfire on mine as well (still debating between HF and a polyurea but need to pull the trigger soon).

 

[Fatboy148]

Probably more info than anyone really wants but there you have it.

My wife calls me a concrete nerd. She's probably right.

Not really. It's never too much information when it is good information and presented in a way that the normal person can understand. Personally, I always look forward to your input. I think it's nice to know some of the science and reasoning behind why we do what we do when placing and finishing concrete properly.

You are fortunate on charge #2.

 

[WAKman]

I greatly appreciate all the concrete and other knowledge here. I told my builder today that we are going with a 15 mil vapor barrier because the Hellfire vendor recommends it. Now I guess I'll have to buy the product!