Tip

VAPOR RETARDERS – Where & When?

Floors that will receive any form of floor covering should be built with a vapor retarder below the slab. A vapor retarder may also be needed in areas with a high water table or other sources of subsurface moisture.

There is disagreement on the need for a granular or blotter layer between the vapor retarder and the concrete. A blotter layer can offer these advantages:

• Puncture protection of the vapor retarder
• Reduced finishing delays because the bleeding period is shorter
• Less chance of blistering or sheet scaling
• Less chance of settlement cracking over reinforcing steel
• Less curling of the floor when it dries
• Reduced likelihood of plastic shrinkage cracking
• Possible increased concrete strength

To provide these benefits, use a 75-mm to 100-mm (3 in. to 4 in.) layer of trimmable, compactable, self-draining granular fill for the blotter layer. Concrete sand is not recommended for this purpose because it does not provide a stable work platform.

Concrete placed directly on the vapor retarder can offer these advantages:

• Reduced costs-less excavation, no need for additional granular material
• Better curing of the slab bottom
• Less chance of moisture problems with floor covering materials caused by water trapped in the granular layer
• Low friction between the concrete and vapor retarder-reduced cracking

When placing concrete directly on a vapor retarder, it is recommended to use a low-water-content concrete and water-reducing admixtures to reduce bleeding, shrinkage, and curling of concrete.

 

 

This information was obtained from Concrete Floors on Ground, a 148-page book that provides detailed information on constructing slabs on ground, from subgrade preparation to toppings and finishes.