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Carpet tiles, moisture and condensation

Richard Renouf recently inspected new office blocks where carpet tiles were lifting
around the edges. Here’s how he got to the bottom of the mystery.

A GOOD moisture meter will have an often-overlooked feature which may be marked TdoC, DP or dew point This is the temperature at which condensation – known as dew when it’s found outdoors – will start to form.

In my office right now it’s 19.1deg C and the air has a relative humidity of 59.8%. This means the air is holding almost 60% of the maximum amount of water vapour it could hold. If the temperature drops, air can hold less water, so the relative humidity will rise and the dew point is the temperature at which the Rh reaches 100% and so the air cannot hold more moisture, so below this temperature the excess will form condensation.

My moisture meters are indicating that that the dew point is 11.4deg C, and I’d hope the temperature won’t drop this low even overnight.

But I recently went to look at some new office blocks where carpet tiles were lifting around the edges and this affected the appearance of the tiles on all floors of the two blocks.

The subfloors were steel decks with concrete screed and in the office areas a raised access flooring system had been installed. The concrete was still not fully dry, but this shouldn’t have affected the tiles which had been stuck down with a tackifier to the steel floor panels with an air gap underneath of 140mm.

The site foreman gave me some of the history. The building work had started early in 2022 and the laying of the tiles began in July of that year. However, there was then a problem with the entire development site and after just a week of floorlaying, no work of any kind was carried out onsite until the following April. Then the original tiles were found to be lifting so these were replaced and more tiles were laid.

A further pause in the works followed. On return to the site more problems were found.
Throughout all this time the buildings had been unheated. The heating in one of the blocks had been turned on the week before my visit as occupation was imminent. Typical temperatures in Block A were in the region of 19deg C (it wasn’t a cold day) and in Block B they were about 23deg C.

Of course, most contractors know a building should be at ‘normal operating conditions’ before floorlaying begins. This will ensure the environment is stable and that there’ll be no big changes to temperature, humidity, or other factors that could affect the flooring. This would have avoided the issues in this case, but in the real world everyone is put under pressure to get the job done regardless.

It’s worth noting that the CFA produce a simple credit-card sized temperature sensor that can be very useful when trying to explain why ‘no’ should be the answer in such circumstances, and it’s free to CFA members. In this case the pressure was enough to overrule best practice.

Simple measurements showed what had happened. In the unheated block the ambient temperature was about 19deg C and the dewpoint was 13.8deg C.

The surface temperature of the raised access flooring panels was 13.7deg C and so condensation was indicated by the meter. By mid-morning, the panels had still not warmed up to ambient temperature after a cold night. Although the ambient conditions didn’t appear to be much different to those in my office as I write, the site conditions were enabling condensation to form and affect the tackifier and allow the tiles to lift at the edges.

In the now-heated block, the dew point had risen to 16.4deg C but the surface temperature of the raised access floor panels was now over 24deg C (it was actually higher than the air temperature), so condensation was no longer a possibility as long as the heating was maintained.

Some years ago I was called to a site in London where builders were having problems laying some LVT. The property was being refurbished and although two big windows weren’t yet installed, the flooring ‘had to go down’.

The builders told me they had acclimatised the flooring diligently and took me to the room where the flooring was stacked to above waist height. They were taking the boxes and opening them in the room being fitted, but then the tiles were not bonding to the adhesive and they believed this to be a product fault.

I lifted enough boxes off the stack to reach the middle. I then opened a box and took a temperature reading of one of the tiles. It was 4.8deg C. It was February, after all, and without the two windows the site was cold. Of course, the builders had portable heaters in the area where they were working and so, as the tiles were below the dew point, condensation was inevitable.
Richard Renouf is an independent flooring consultant

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