Vapor barrier under perimeter footing

[roberto222]

I always show vapor barrier under slab only. Should i extend it under perimeter footing as well? Talking about residential wood framed building with interior slab on grade and perimeter shallow footings.
Why yes or why not?

Also if you say yes, what about drain rock under the plastic under perimeter footing? Wouldn't it bridge the water into my underslab from outside?

Thanks.

 

[1503-44]

Put the barrier around and under any concrete. Normal Concrete will wick up water quite easily to 3ft above moist ground level.

You should still put the barrier under the slab too. In fact, wrap the whole fdtn with it. You probably have compacted sand/gravel below the slab anyway. It will drain water, but that won't keep the water vapor out of the house. The water vapors move relatively freely through the sand void space and the slab and will rise up into the house, if there is no barrier.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[bimr]

No, typically at the perimeter, the vapor retarder/barrier should be turned up and sealed to the wall; and is not installed under the footing.

See the link:

Vapor Barrier

 

[1503-44]

Right. I thought he was talking about a turned down slab forming a "continuous footing" perimeter.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[roberto222]

Thanks for your answers. I was discussing with geotechnical engineers and they say that vapor barrier would compromise the friction between concrete and soil. Concrete footing around perimeter needs to "lock" into the soil.

But what about if you have basement L-footing retaining wall. This footing could easily be 5ft wide. Looks like too large area of concrete on soil not protected from moisture.

 

[bimr]

Don't understand the 2nd sentence, would you clarify?

Note that the newest energy code that has been adopted in some jurisdictions is now requiring several inches of closed cell foam under the concrete.

 

[roberto222]

To explain basement retaining wall, lets assume that we cut our house/room with slab into the hill.
Then the back of this room needs to be retaining wall which stem-footing will be L-shape.
So under this footing we should not have vapor barrier to not compromise friction, but this footing could be basically covering the entire room if this room is cut into the hill.

Probably the only option would be vapor barrier on top of the slab?

 

[bimr]

Vapor barriers on the interior of basement walls is probably the biggest mistake that is regularly made in the home construction industry. It will lead to mold.

If the wall is facing the hillside, put a membrane on the exterior side. Without being separated from the ground with a membrane the concrete wall will always be wet and that should not be a problem as long as you take steps (like the rigid foam or poly) to prevent the interior walls from being saturated with moisture.