Moisture in subfloors unpacking the gremlins

Proper subfloor preparation plays an integral role in the final result when it comes to residential, commercial and industrial flooring. The calibre of subfloor preparation and the manner in which it is carried out can influence many things, such as the longevity of the floor covering, as well as its overall aesthetics. While there are many aspects of preparation that an installer needs to consider, none are quite as important as keeping moisture to a minimum.

Why excess/prevailing moisture is so dangerous

If an impervious and bonded floorcovering is installed on top of a subfloor that contains high levels of moisture, the tell-tale signs will become obvious almost immediately. Ultimately, high moisture levels within old and new substrates can lead to thermoplastic coverings starting to lift and come away from the underlayment or preparation compound shortly after installation. The development of blisters or bubbles (the higher the moisture content, the larger the blisters and/or bubbles are), the softening or general degradation of adhesives, unsightly indentations in the floor finish, uneven gaps between tiles and the growth of mould and mildew can all become apparent over time. These signs of high moisture content will often become progressively worse until the client is forced to have the floors removed and re-installed, which can have significant cost implications. Besides having an unsatisfied client, it can tarnish the reputation of the flooring contractor and even open them up to a lawsuit.

Conducting moisture tests

There is an array of different tests which can be done to measure the moisture levels before preparing a subfloor. The relative humidity (RH) test and H2O test methods are the most popular. Another worthwhile test is the ACC test, which measures the unique water vapour emission rate by applying absorbent calcium chloride onto the surface of the subfloor. As the calcium chloride absorbs the water, its weight changes. This change in weight allows the water vapour emission to be estimated more accurately.

While moisture prerequisites will vary depending on the type of flooring to be installed, from a general perspective, installers should aim for below 75% relative humidity (RH) and 3% (H2O) total moisture when using the gravimetric, carbide bomb or speed test methods, before proceeding with the installation.

To ensure a successful installation, it is also important for installers to consider not only what the moisture level measurement looks like in the present moment, but also whether or not the subfloor will be at risk for exposure to excess moisture or rising damp in the future.

Allowing the substrate to dry out naturally

It is very important that the substrate be well cured, and whether concrete or screed, to be allowed to dry out naturally before any flooring systems are installed over it. If deadlines are tight, and moisture values cannot be attained, installers need to apply a vapour barrier or waterproof surface membrane before proceeding with the flooring system. This vapour barrier/membrane will also be necessary if, despite allowing ample drying time, the base RH reading is still greater than 75% RH, or 65% RH in the case of wooden flooring.

Vapour barriers

While they are often suggested as an optional precaution, many flooring manufacturers recommend applying a vapour barrier as an integral part of the system, as it will protect the flooring throughout the years following installation. This is because, even if the RH reading is of an acceptable level at one time, moisture content has the potential to fluctuate based on a variety of outside variable such as seasonal changes when there is plenty of humidity in the air, when there are changes in geographic surface run-off or when structural changes take place in and around buildings.

Furthermore, it is important to understand that there is no such thing as a moisture test that is regarded as 100% accurate, most test instruments provide an indicative result. Think about it – as moisture vapour slowly permeates upward and through the dense top finish and, because moisture is released in small quantities, it evaporates relatively fast and as a result, does not show up significantly with non-destructive surface testing methods. It’s for this reason that many professional installers will opt to drill at least 40% of the depth into the slab in an effort to obtain more accurate results. However, even in these instances, readings are never going to be regarded as completely accurate or reliable. As moisture content is likely to vary throughout the slab from day to day, more than one reading should be taken, specifically on active construction sites. Moisture levels may also be higher towards the centre of concrete substrates than it is towards the slab’s outer edges.

With all of these factors in mind, applying a vapour barrier can help an installer cover all bases and ensure a successful final result.

Moisture and different types of flooring

As mentioned, moisture prerequisites vary from manufacturer to manufacturer of flooring products and should be respected before installation over a subfloor. This is why it is essential for installers to refer to the product manufacturer’s guidelines and to take action to ensure that these guidelines are followed throughout the installation.

Resilient flooring: Moisture levels that are above 75% RH will disqualify a subfloor for installation of resilient flooring.

Wood, laminate & bamboo flooring: Wooden, engineered and laminate flooring are highly susceptible and sensitive to underlying moisture. As such, moisture levels will need to be less than 65% RH in order to ensure successful installation and maximum longevity. In all cases, the subfloor should be prepared with by grinding or sanding, priming, applying an appropriate moisture barrier, levelling compounds or de-coupling systems and cast resins.

Access flooring: Access flooring does not cope well with damp or wet screeds, or any areas open to rain or water.

Ceramic, porcelain, and stone flooring: In the case of ceramic, porcelain and stone flooring, suppliers call for the underlying substrate to have a moisture content of 5% or less before tiling can begin.

Vinyl flooring: The moisture content shouldn’t exceed 3% before, during or after the installation of vinyl floor coverings.

Ultimately, it is imperative that installers go out of their way to ensure that moisture levels are not a danger to the finished product and will not pose any future problems. That way, the final result is certain to bring with it a happy, satisfied client.

To watch a video on this subject, see this amazing video by Quick STep

We give full thanks and acknowledgement to www.abe.co.za, www.applefloors.co.za, www.iteproducts.co.za, www.polyflor.co.za, www.f-ball.com and www.tal.co.za for some of the information in this article.

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