Vapor Retarder and Grade Beams

[KBVT]

I have a structural slab-on-grade system with grade beams and girders. I am trying to determine how to run the vapor retarder below the slab. Is it typical to run the vapor retarder continuous below the grade beams or can the vapor barrier be turned down at the face of the grade beam? I have always considered a retarder system (or even a barrier system) to be continuous below the slab and providing a joint or seam at each grade beam is inviting vapor/moisture propogation at these locations.

Additionally, the grade beams are supported by pile caps located below. If the vapor retarder is to be continuous, how do you handle the pile cap locations?

 

[archeng59]

The vapor retarder/barrier is under the slab only, not under grade beams or footings.

 

[structuresguy]

If the slab and grade beams are cast integral with each other, then run the vapor barrier under the beams as well. If the slab just sits on top of the grade beams, then you could run it continuously over the top of the beams. You would need to seal any rebar penetrations through the vapor barrier for it to be fully effective. THe better vapor barrier manufacturers, like Stegowrap, have a mastic product which makes sealing penetrations very easy.

 

[KBVT]

The grade beams are designed to act composite with the concrete slab. Running the vapor retarder through the construction joint at this location would impact the horizontal shear transfer at this location.

 

[apsix]

I hope you realise now that good design entails more than just sizing structural elements.
I would run the barrier under the beams as well, presumably you can cast the beams and slab together.
The risk of problems due to the break in the barrier at the pile caps is small, due to the depth of concrete between cap and top of slab.

 

[RHTPE]

If the grade beams & girders are cast monolithically with the slab, then the vapor barrier should run below the beams & girders. BUT - be realistic about the cross-sectional shape of those beams & girders - depending upon what you've specified for base material under the barrier, it will be impossible to achieve vertical sides on the beams & girders. Further, you will create compaction difficulties for that base material near the beams & girders.


Ralph
Structures Consulting
Northeast USA

 

[archeng59]

Stego Industries has several details showing how to install their product under the slab in several different conditions.

http://www.stegoindustries.com/cad_drawings/stego_wrap_between_exterior_concrete_footer.html