Is a vapour barrier needed under a screed?

A plastic film is almost always laid under a screed. However, depending on the context, this film does not serve the same purpose at all, and this is the most common source of confusion on the subject.

‍

The separation film: the general case

Under a floating screed, the polyethylene film is primarily used to prevent laitance—water laden with cement—from seeping between the insulation panels. Without it, the mortar would flow into the joints, create rigid bridges between the screed and the substrate, and ruin the desired acoustic performance.

It also acts as a slip layer: the screed must be able to expand freely without rubbing against the insulation. It is the logical complement to the perimeter strip, which performs the same function along the edges. In this role, the film is not a vapor barrier: it is a separator.

‍

When a true moisture barrier is needed

A vapor barrier or a waterproofing membrane becomes essential as soon as moisture can come from below:

• a slab poured on grade without original waterproofing, a common occurrence in older Belgian buildings;
• a floor above a crawl space poorly ventilated crawl space or a damp cellar;
• a substrate showing signs of rising damp, a situation detailed in our guide on pouring a screed on a damp substrate.

Above a heated and dry room, however, adding a waterproof membrane is useless and can even trap residual moisture from the screed.

‍

Installation matters as much as the choice

A poorly installed film protects against nothing. The sheets should overlap by about 20 cm, the overlaps must be taped, and the film should extend up the perimeter above the finished level of the screed before being trimmed once the mortar has hardened. Any holes made for conduits must be sealed. For choosing the insulation that goes on top, our comparison on screed with or without thermal insulation sets out the right criteria.