Vapor Barrier Under SOG

wurdan freo

Wondering if you had any comments on your experience on the impact of vapor barriers under slab on grade. When do you remember starting to specify vapor barriers beneath the slab and has that resulted in less flooring failures? Do you specify vapor barriers in residential design or just commercial?

The number one issue that comes to mind is moisture migration and my understanding is that the "vapor barrier" keeps the water and water vapor from being absorbed from the soil/rock into the concrete. The concrete then emits the moisture through the surface causing mold, delamination and /or rotting of finish materials. Any insight here? 

Sep 19, 12 12:54 pm

Moisture migration is definitely an issue. I've seen two cases where wood floors laid over SOG buckled and were ruined because the slab didn't have a vapor barrier under it. Concrete is basically a sponge.

Sep 19, 12 1:13 pm

Only use a vapor barrier under the slab if you have uninterrupted sub-slab drainage with a gravel bed or something. Else you're looking for a waterproof membrane, especially with concrete. The new membranes create a molecular bond between the concrete and the membrane, so no water can get in at all.

Hope that helps.

Sep 19, 12 1:42 pm
wurdan freo

Wow. That sounds awesome. Are you saying the concrete "sticks" to the membrane when you pour it? Or is the membrane in a liquid or paste form or integral with the concrete? What happens at the footing?

Sep 19, 12 2:36 pm

always at least use vapor barrier. Otherwise the slab technically never cures. You may not need a full waterproofing layer for slab on grade especially if you are far from any water tables. The waterproofing that you want is called blind side waterproofing and Grace pretty much dominates the field

Sep 19, 12 8:01 pm

That Preprufe stuff Rusty mentions is absolutely dynamite if you are doing new construction. If you find yourself to have inherited an existing slab with  vapor migration issues, you can use top-side applied moisture vapor emission reduction coatings. Koester VAP and Aquafin are both good... 

Sep 19, 12 10:55 pm

I second the Grace waterproofing Preprufe recommendation. It's a thick mat with a 'sticky' coating on one side, and the concrete is poured on it. It sticks and bonds due to the weight of the concrete. 

With existing slabs, you can use what Janosh recommended, or you can inject this material called BBZ through minute drilled holes in the slab, and it expands and hardens and forms a waterproof membrane on the other side.

I would recommend talking to your local Grace rep.

Sep 24, 12 11:22 am

my experience with slabs from the 60s that DON'T have vapor barriers: ugh. 

we now have to put a moisture-barrier coating on top of the slab at a lot of mid-60s schools before the finish floor can be installed. it's a sneaky issue. asbestos floor tile and mastic were surprisingly good moisture barriers. abate the floor and you might end up with a permanently damp slab surface! 

we experienced some floor finish failures before we figured out what was happening. 

Sep 24, 12 1:02 pm

Rusty owned the thread.  Unless your really high and dry there should be some sort of sub-slab waterproofing membrane or mat applied.  Even in high rises in New York wet facades can mean moist concrete slabs, especially at edges, and I've seen moist slabs cause flooring damage even in new construction up here.

Sep 24, 12 7:33 pm

apurimac, NYC has always been a steel-town (stronghold by mobs, unions, etc...) and reinforced cast -in-place concrete is still a bit of a mystery to most. We are making mistakes that were popular in the '60 in other cities. Exposed slab edges? Christ. I see that in new construction all the time.

Back to vapor. It took me years to fully understand vapor retarders and their potential. Great in wall assemblies as well. Henry makes these great vapor-permiable, water/air tight membranes that allow you to have a separate vapor membrane. This frees a designer to do all kinds of design gymnastics that look great and perform at a top level. 

I wish archinect had a stronger technical section. technical education is something that benefits anyone involved in this profession. Except for them UX fucks. Make that button more round son!

Sep 24, 12 8:59 pm

cast-in-place is a mystery for the entire northeast. 


and to expand on anarchytecture's first comment - you can get some crazy heaving if you don't have sub-slab drainage (especially if you have lousy grading around the building) - also - if you apply the spray-on stuff after the fact you might have to drill holes in the slab and put in a sump otherwise your building is going to move.


also - agreed on the technical/building science section.

Sep 24, 12 9:35 pm
wurdan freo

I've used preproof before, but it was always in the most detrimental areas. For example.

elevator pit in the water table  - preproof

slab on grade - 6 mil poly

to take it a step further

Walls  at the elevator pit - preproof

foundation walls at habitable areas, above water table - spray applied or damp proofing

foundation walls with no habitable space - nothing.

Sep 25, 12 2:50 pm

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