Negative pressure drying uses suction to draw moist air out of a building cavity or assembly. It differs from positive pressure drying, which pushes air into the space. The method describes airflow direction; it does not by itself establish that a material can be saved.
Where it may be considered
A restoration assessment may consider cavity drying where moisture has reached an enclosed wall or another area that ordinary room airflow cannot readily reach. The assembly, access, material condition and any contamination concerns must be evaluated first. Some damaged materials still need removal rather than drying in place.
What to ask about the proposed work
- Which cavity or material is being treated, and what findings support that choice?
- Are access openings needed, and who will restore the affected finish?
- How will airflow and moisture readings be checked during the work?
- What findings would change the plan from drying to removal or replacement?
How completion is evaluated
Equipment running for a set number of days is not, by itself, evidence that the assembly is dry. Ask for the recorded observations, measurement locations and completion criteria. Keep the drying record with the repair scope, particularly when walls or flooring will be closed up afterward.
The equipment manufacturer's description of cavity drying illustrates the distinction between drawing air out and forcing it in. This is a description of the technique, not a promise that a particular device or method will be used on your property.
Related guidance: drying, moisture content and water damage restoration.