No, a vapour barrier and its corresponding primer should not be applied to a concrete surface that is not air-dry (i.e. if visible moisture is still present).
For the correct installation of a bituminous waterproofing system, it is essential that the substrate is dry — both on the surface and internally.
Why is a dry surface important for a vapour barrier?
Applying a vapour barrier or primer to a damp or still-curing concrete surface (such as freshly poured concrete or foamed concrete) can lead to several issues:
- Poor or failed adhesion of the primer and vapour barrier
- Blistering or delamination due to trapped residual moisture
That’s why it’s crucial that the concrete is fully air-dry before installing a vapour barrier.
How can you check if the concrete surface is still damp?
A visual inspection is a simple first step. Concrete or foamed concrete that is still drying often shows clearly visible dark patches. These areas indicate residual moisture or parts of the slab that are still curing.
A uniform light grey colour, without any dark spots or patches, generally indicates that the substrate is sufficiently dry.
Note: The commonly referenced theoretical drying time for fully cured concrete is approximately 28 days.
Example of foamed concrete with a partially wet and dry surface.
For more technical guidelines on substrate dryness for roofing works, refer to Technical Note TV 280 by Buildwise.