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Moisture – Too Little Is Bad Like Too Much

Sid Bourne on ‘dry cupping’

I WAS recently commissioned to inspect an engineered floor which had cupped and split in several areas of installation. Most people would automatically think that moisture was the cause, mainly because the subfloor was wet causing the floor to dramatically cup. This time, however, the problem was ‘dry cupping’, a term many in the UK may not have heard of before maybe, because it is not common in this country.

The cause is moisture-related, but the difference is that the cupping is caused by an excessively dry environment. In this case it was less than 18% relative humidity.

What actually happens is the wearlayer dries much faster than the supporting core. The wearlayer then tries to pull away from this, causing the corners to lift or curl away.

The other problem you get with dry cupping is that splits and checks can appear. The splits are when the wood opens all the way through the plank from top to bottom. Checks are similar, but smaller and mainly do not go through the entire depth of the wearlayer.

Other problems that can occur are wrinkles or breaks in the finish when it is unable to attain the elasticity beyond what it is designed for. It then ruptures on the surface. In the majority of cases – and because it is not a common problem in the UK – retailers and contractors and, indeed, end-users do not understand what damage a very low humidity can do to a wood floor or even wood based objects.

Wood is no different from how people live. Wood is happy in conditions that we as humans feel comfortable in. In fact on this par ticular site inspection within five minutes of being there my throat became dry and itchy.
I even asked the receptionist if she ever suffered from a dry throat. Yes, she said, all the time. So I rest my case.
Relative humidity changes throughout the year and it will be out of the ‘comfort zone’ for engineered flooring for some of the time. But as long as the change is not long-term, then no problems should occur.

I continued looking around this installation, paying attention to doors and architraves. I found that they were all gapping with some doors slightly warping.

The heating system was provided by air conditioning and the owners admitted that they had no control over it, as it was monitored by an outside company which apparently is not uncommon in large commercial offices.
The contractor was being hounded to replace the floor and was somewhat confused by what was happening. Having the contractor on-site with me I quickly found the cause. That was easy. The difficult part was convincing the owners who may have thought I was making things up.

My first suggestion was that the air conditioning should be attended to ASAP, not only for the wood flooring, but for the workers’ own health. It was not accepted politely, but facts are facts. Wood is hygroscopic and will
readily react to its surroundings. If persistent abuse of a heating system occurs causing low humidity, then expect problems not only with the new wood flooring, but wood products in general.

I think where we in the UK let ourselves down is at the point of sale and after installation. At the point of sale it should be made clear that wood is hygroscopic and reacts to humidity changes and that at certain times of the year the wood will show small gaps. At other times it will expand, closing these gaps.

However, we have a measured humidity that if possible you need to keep the wood floor within. But we never seem to advise the end-user about this, which leads to problems.

And after installation, maintenance guidelines are not usually given, nor are products sold or demonstrated so that end- users get the best out of their wood floor. And this leads to even more problems.

I repeat that I am not tarring everyone with the same brush, as there are a few who do the right things. But I am afraid to say the majority do not.

And I say again that if more installers were to attend wood flooring training courses with the British Wood Flooring Association, I guarantee that there would be far fewer problems, providing they do ever ything we say and demonstrate.

The problem is that most contractors do not want to pay for courses for staff as they think that, when properly trained, they will all run off to other employers. Shame really! The cost of a training course is far less than you’ll have to pay to replace the floor!


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This article has been reproduced from the Contract Flooring Journal. You can find them at