Need Under-Slab Vapor Retarder AND Under-Slab Void Forms?

There’s not a lot of information out there about this topic (what to do when you need both an under-slab vapor retarder AND an under-slab void), probably because this situation is a regional condition.  Not every part of the United States has the expansive soils that parts of Colorado have. 

The first part – the voids: 

Expansive soils on project sites often prompt geotechnical engineers to recommend under-slab voids, which are created by placing concrete for slabs on top of void forms, also known as carton forms.  Yes, we actually call for wet concrete to be placed on top of cardboard boxes… but not directly on top of the cardboard boxes!

The cardboard void forms, which are in contact with the soil, absorb moisture from the soil and degrade over time, while the concrete slab stays in place, so a void space is created between the bottom of the slab and the top of the earth.  This void allows expansive soils to expand, or swell, without impacting the slab.

The second part – the vapor retarder:

We specify under-slab vapor retarders to prevent water vapor from migrating through the floor slab-on-grade and damaging moisture-sensitive floor coverings or moisture-sensitive equipment.  (We do this everywhere, not just in Colorado.)  Without a vapor retarder between the slab and the soil, water in the soil can travel through the slab in vapor form, and then it can condense and form into liquid water once inside the building, where it can cause damage. 

Now – put the voids and the vapor retarder together:

The requirement for an under-slab void on a project calls for slightly different vapor retarder requirements than we normally specify.  My article on how to deal with this situation is published on the Construction Specifications Institute Denver Chapter website.  Click the link below for the article.

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