Polystyrene sheets and vapour barriers: beware!

A widespread “insulation solution” involves placing polystyrene sheets and a vapor barrier under the basement slab.

However, this combination poses several problems, particularly regarding radon infiltration:

• Ensuring seamless, continuous insulation proves to be very difficult.
• It is precarious, and prone to shifting when the concrete slab is poured.
• It is very difficult to correctly fasten the polystyrene sheets to the foundation walls – under the concrete slab – and seal the whole thing off.
• The same difficulty arises when the time comes to seal the joint between the foundation walls and the end joist.

This explains why this type of insulation is ineffective, especially against radon infiltration.

According to Isolation Majeau, only one type of insulation effectively attaches and seamlessly seals the concrete slab to the end joist: sprayed polyurethane foam.