Stephen Boulton, technical service manager at F. Ball and Co. Ltd., provides a guide to guarding against excess subfloor humidity.

Unmanaged subfloor moisture, whether residual construction moisture or rising damp, is one of the leading causes of floor failure.

Before proceeding with any flooring installation, F. Ball advises contractors to take the time to thoroughly assess the composition and condition of the subfloor to ensure a suitably sound, smooth and dry base for the receipt of new floorcoverings. This often means removing any old adhesive residues in the case of a refurbishment project or grinding off laitance when working over a newly installed screed.

In all situations, contractors should take the time to assess levels of subfloor moisture. If not properly managed, excess subfloor moisture levels can lead to costly remedial work. It is one of the leading causes of floor failure, causing adhesives to de-bond, carpet underlays to rot, wooden floors to warp, and resilient floorcoverings to blister. It may also promote the growth of mould, which may stain and risk further damage to the floorcovering. If subfloor relative humidity (RH) levels are 75% or above (65% if wood floorcoverings are to be installed), a moisture management solution will be required to impede the passage of rising damp or residual construction moisture.

TESTING, TESTING Following initial preparation of the subfloor, a moisture measurement test should be carried out to determine whether the subfloor is dry enough to proceed with the installation of new floorcoverings. A quick, non-intrusive way to identify the presence of subfloor moisture is to use a handheld radio frequency moisture meter. If the device indicates the presence of moisture in the subfloor, further testing will be required to determine subfloor relative humidity levels and the requirement for a moisture management solution.

The only method of measuring subfloor relative humidity levels with certainty, and the method advocated by British Standards, is to use a calibrated hygrometer. Affixed to the subfloor using butyl tape to create an airtight seal around the base of the instrument, these devices measure the relative humidity of a small volume of confined air in equilibrium with the subfloor, taking into account the ambient temperature.

When using a hygrometer, it’s important to allow sufficient time to allow entrapped air to reach moisture equilibrium with the screed or base before the unit is switched on. Equilibrium can be assumed either when consecutive readings, taken at four hourly intervals, show no change, or if the instrument is left in position overnight.

NEXT STEPS There are a number of options available to contractors for creating a barrier between floorcovering and subfloor where excess levels of subfloor moisture are detected.

The application of a waterproof surface membrane is the usual solution for effectively controlling damp. Advanced


products are now available, such as F. Ball’s Stopgap F77, which will isolate excess subfloor moisture where relative humidity values are up to 98%, with a single coat application, which will fully cure in as little as three hours.

F. Ball’s Stopgap F78 provides an even quicker, two-coat system. The first coat cures in 15-20 minutes, and a further 30 minutes curing time is required for the second coat. This means an effective barrier against residual construction moisture where relative humidity values are up to 95% can be created in less than two hours.

ALTERNATIVE SOLUTIONS An alternative solution for dealing with damp is available where a waterproof surface membrane cannot be applied. This may be the case where screeds are contaminated with oil, other chemicals or old adhesive residues, as well as in historical settings where the subfloor must be preserved.

In these situations, a loose-lay isolator membrane, such as F. Ball’s Stopgap Isolator Membrane, can be laid directly onto the subfloor, without the requirement for an adhesive, to provide a barrier to stop excess subfloor moisture causing floor failure.

The membrane has nodules on the underside, creating an airspace to allow water vapour to travel to the edge of a room, into a dry wall or ventilated area, where it can safely escape. A wide range of floorcoverings can be adhered to its upper surface.

By acting as a base to receive resilient floorcoverings and carpet tiles, loose-lay isolator membranes allow durable new floors to be installed and easily removed at a later date, allowing buildings to be returned to their original state, offering a solution for temporary flooring installations.

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