Vapor Barrier Under Outdoor Patio

[FootNMouth]

Had a client who was installing an outdoor patio adjacent to their house. The previously had issues with the concrete "sweating" so they wanted to add a vapor barrier. I saw no problem with this but the contractor is adamant that it is bad to install a vapor barrier under an outdoor slab. He did not have a good reason except that this was common knowledge not to do this. Can anyone provide we some insight into whether or not this is a bad idea?

 

[Ron]

OK ddudley88....let's think about this.

The reason you put a vapor barrier under an interior slab is to prevent moisture from migrating from the ground into and through the slab. If the vapor barrier were not there, additional moisture would collect in the slab and cause a variety of moisture related issues, including "sweating".

For an exterior slab, if you put a vapor barrier beneath the slab, it will prevent moisture from coming up from the ground, but will also prevent water from rain or other surface sources on the slab from going through the slab. In short, it will trap water in the slab. Is that an issue? Well, it can be in that it might allow algae growth at the surface, thus increasing the "slipperiness" of the slab. Not good.

There is no compelling reason to add a vapor barrier under an exterior slab. Leave it out. It won't prevent "sweating" and will likely increase that potential. This is one instance (a rare one) that I will side with the contractor.

 

[oldestguy]

If you are in a northern climate area where there gets deicing salt, then that will later cause "sweating" on a cold slab during damp periods, attracted to the salt. Thus, the vapor barrier has nothing to do with it. In summer when the slab is warmer, that does not occur.

 

[JAE]

No reason to use a vapor barrier below an exterior slab.

Reason not to use one: water from the sky drops - soaks slab - migrates to just above the VP and sits there - if cold climates then freezing and all sorts of problems - if not a cold climate then algae/moist conditions that do not good to an exterior slab.

Ask yourself, for what reason is the vapor barrier trying to prohibit moisture migration from lower to upper or upper to lower areas? None.



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[FootNMouth]

Thanks for the posts. Your advice makes sense. Do you think that the rain water would cause an issue in the patio even if an adequate slope was provided to drain the water?

 

[dhengr]

Ddudley88:
Put a well draining granular base under the slab, and slope it to drain too. Don’t put it in a shallow dug bath tub which does not drain. If you are feeling really adventurous, consider pervious concrete or paving.