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Adhesive Can Help To Control Moisture Moves

Peter Kaczmar, flooring specialist at BM TRADA, explains its research project to develop an underfloor heating test rig for wood flooring:
WHEN it comes to flooring, nothing quite matches the performance and beauty of a wooden floor. Whether it’s engineered, multi-layered or solid, a wood floor can lend a different dimension to a space, with an aesthetic that other flooring finds hard to match. This is why timber continues to grow in popularity as a flooring material in both the residential and commercial sectors.
Timber has a natural warmth to it, but, with developments in underfloor heating (UFH) systems, its use alongside these developments has, on occasions, caused problems. Wood is a dynamic material and can be prone to movement. Even without an UFH system, a wood floor is designed to accommodate shrinkage and swelling movements: add heat into the equation and those movements can be amplified.
Following initial feasibility studies, TRADA commissioned a BM TRADA research project to develop an underfloor heating test rig for wood flooring. The construction of the rig was designed to enable new materials and designs of flooring to be tested (such as bamboo, modified wood, etc) as well as being able to assess the suitability of different multi-layered floor systems.
There’s no doubt that wood flooring manufacturers are increasingly pushing the boundaries of accepted practice in their approaches to aspects of flooring design and installation.
Floor systems intended for use with UFH are increasingly being installed in widths traditionally considered as oversized for such installations. This is not to say that such flooring should not be used with UFH systems, but that their suitability should be verified by means of appropriate testing.
In its research work, BM TRADA found that some composite floor materials, with a hardwood veneer on a plywood base may be prone to splitting because of dissimilar movement characteristics when exposed to conditions that promote drying whereas others do not. This behaviour is by no means certain, and can be very difficult to predict.
The increase in popularity of UFH systems has been associated with a perceived rise in the incidence of in-service shrinkage, cracking and distortion failures; as a result, there is a need to test new flooring systems with UFH to verify manufacturers’ claims.
BM TRADA had increasingly been called in to deal with legal disputes over wood flooring failure – and this was the main driver behind the development of the test rig. The company’s research comprised a number of trials that have provided technical insight into the manner in which certain types of wood flooring behave when used over UFH systems under certain installation and operating conditions.
n The results: The research makes a convincing case for concluding that the performance of a wood flooring, when installed over UFH as a stick-down system, can be significantly improved by ensuring that the adhesive layer bonding the flooring is applied as a continuous layer.
It was found that if the adhesive application is continuous and uniform, it can function, to a degree, as an effective barrier against the upward passage of latent moisture still present within the screed subsequent to its casting.
This is not to suggest that the adhesive can or should be used as a substitute for an effective dpm or vapour membrane, but rather as an added precaution against latent screed moisture.
This is a key factor in the success of an installation particularly considering that the adhesive layer is rarely applied as a continuous film in practice – something BM TRADA has seen in many flooring investigations.
The results of this investigation were even more significant when looking at a floor made from acetylated timber (the process used in the manufacture of modified wood products like Accoya). Here, the specific use of modified wood was found to constitute an added insurance against a ‘wet’ screed, if bonded with a continuous layer of adhesive.
In such cases, the adhesive acts as a measure helping to control the rate of moisture movement from the screed to the backs of the boards, if applied as a continuous layer helping the wood covering to remain stable.
n Why is this important?: The presence of an effective moisture barrier in a stick-down wood flooring installation is particularly important when used in conjunction with UFH systems. This is particularly important at the start-up phase where the likelihood of latent screed moisture still being present is a real risk. In such a scenario, heating the screed at high heat regimes will drive excess moisture upwards through the screed to the backs of the boards causing them to deform and the floor to fail.
Tests showed that a phased start-up regime involving initial intermittent heating over a period of time is much less likely to cause disruption of a wood flooring by latent screed moisture than continuous heating.
n Final words: These are examples of the types of factors that can be researched and which are likely to be of importance in the development of new product lines. In this context the floor test rig constitutes an invaluable means of verifying performance claims, or otherwise, may be used in conjunction with manufacturers’ in evaluating new product developments.
• For more details on the research project, visit

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