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Vapor barrier with void form

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Let me add some comments to the discussion -

Not all void forms work by degrading. There are several products available that are made from polystyrene foam that are designed to reduce the pressure on the underside of the concrete by either elastic deformation or by actual failure of the section when the soil heaves. Data is available from the manufacturers that will tell you how much force is transmitted to the underside of the concrete.

Even if the void form is a degradable one, it works by crushing when the soil heaves under the concrete. These forces are so large that even an intact void form will provide the protection that you need. Again, check with the manufacturer for recommendations and for the load transfer values.

Using Styrofoam or any other insulation is an expensive solution that also introduces other problems. These products are designed to be strong enough to resist deformation, so you can't rely on them to reduce the load on the concrete. They insulate the ground, which prevents or reduces frost heave by protecting frost-sensitive soils below the insulation, but you need to be careful how you install them. If you are in an area where the temperatures can drop below freezing, you need to make sure that the insulation is not in direct contact with the concrete. The reason for this is that the insulation will actually prevent the ground heat from getting back into the concrete, with the result that frost can form on the surface of the concrete. The insulation manufacturers usually recommend that a granular layer be placed between the insulation and the concrete to act as a heat sink and reduce the possibility of frosting. I expect that this will raise other questions, but that might be another topic for discussion.

Darryl Baron, P. Eng.
Engineering Solutions
Dow Chemical Canada Inc.

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