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Underfloor heating goes under interrogation

As underfloor heating becomes less straightforward for floorlayers, Martin Cummins answers all the questions you’re probably asking…

I’VE written before about the collaborative work carried out within the CFA manufacturers’ committee, and how utilising different viewpoints helps to create guidance notes for the contractors. This is amply demonstrated when it comes to putting together such a document on underfloor heating (UFH) where there are constantly evolving and changing systems coming into the market all the time.

If I was writing this article 10-15 years ago it would be very straightforward. UFH simply used to consist of warm water passing through pipes encased within a screed. The ‘rules’ there are that the UFH needs to be fully commissioned, not just pressure tested, and only after that can the flooring installation be carried out. A further rule is that the glue line, ie, that point underneath the floorcoverings, shouldn’t exceed 27deg C. Sorted! But nowadays ‘underfloor heating’ can also mean something very different.

When I get a call from a contractor asking for recommendations from Bostik around UFH, I require a lot of information so I can evaluate which systems can and can’t be used. The following is a typical stream of questions which may be presented to you.

The first question would be:
Is the UFH a) a warm water system OR b) electric radiant heating (eg, heating cables)?

Let’s assume the answer is a) a warm water system. Then I’d ask:
Are the pipes encased within a screed? Or are they visually present – and therefore need material applying over them?

If encased, then the rules mentioned earlier would apply. I’d advise commissioning is essential. One grey area still is whether the heating may be switched on during application of our products. The standards from years back say the UFH must be switched off for a minimum of 48 hours before and only brought back on 48 hours after installing flooring.

That’s okay if there is the opportunity for other heating to be introduced and maintained in the room, but this is often either impractical or impossible. We manufacture and test our products in lab conditions, nominally at 20-23deg C, so we know they’ll perform even if the heating is on. We advise that it can be left on at cutback temperatures to give room temperatures of 15-20degC. However, floorcovering manufacturers generally believe that having heating on is problematic, which presents a quandary. Only by having joint meetings and discussions can we explore this to give a cohesive recommendation to the market.

Next question:
Are the pipes routed in screed boards or with a plastic tray system?
The major challenge here is whether smoothing compounds can be applied over the pipes, and, if so, which type and at what depth in order to provide a stable floor. Furthermore, it would be necessary to know what the curing and drying times are and whether the UFH then needs to be fully commissioned as per the systems encased in screeds. The response to this will depend on the manufacturer and the compounds being recommended, so make sure the full details of the UFH are made available as there’ll not be a blanket recommendation.

If the response to the first question was option b) electric radiant heating, meaning mats or cables are present, then I’d need further information:
What are the cables laid or fixed onto and can you apply a smoothing compound system over them?

The substrate below the cables needs to be suitable for the application of smoothing compounds and floorcoverings. If it’s a cementitious faced insulation board or an older screed or smoothing compound, then we know we can apply suitable smoothing compounds. However, if the base is a compressible backer board or insulation mat, then we would not be looking to apply smoothing compounds and resilient or textile flooring. Tiled flooring could potentially be used though and, in my opinion, this is what the electric systems are really designed for.

In addition, if it’s a substrate (such as timber) that can be affected dimensionally by thermal variations, then we would have to consider whether it can be successfully smoothed.
Assuming the project is suitable for smoothing over then I must consider:
What products should be used and how should they be applied?

This is again a can of worms. The major problem is the substrate should be primed to enable smoothing compounds to be applied but priming over a cable or mesh system is very difficult. Attaining a thin, continuous non-pooling primer is not easy to do. Ideally, the floor should be primed, the electrical system installed and then the floor smoothed over, but different trades are doing the different layers, so getting this done is nigh on impossible.

If the floor can be effectively primed, then the cables must be covered with a sufficient depth of smoothing compound. I always quote a coverage depth over the cables and it will differ depending on whether we are selecting fibre-reinforced, bag and bottle, or standard water mix compounds. The depth needed may also depend on what flooring is being installed. I’m finding contractors much prefer to apply the smoothing compound in two hits. T

he first to flood in between the cables and the next to apply over the top. It’s essential to prime between layers.

A further problem I come across is how the installer of the heating has fixed the cables down. If, as on a recent installation, they’ve used gaffer or duct tape to hold it down then we cannot guarantee any systems. This is because the adhesion of the smoothing compound to the tape will not be very strong, and the integrity of the whole system relies on how well the tape sticks to the floor.

Once the right recommendation of primers and smoothing compounds have been applied, I’ll advise a curing/drying time for the product, prior to testing the UFH.

This drying time is essential to enable the product to reach a sufficient strength to prevent dehydrating and cracking when the heating is tested. The heating needs to be tested to make sure it is still functioning and that no damage has occurred. Discovering damage after the floorcoverings are installed is not what anybody would want as it would require a full system replacement.

Finally, so I can recommend the ideal adhesives, I would want to know:
What floorcoverings are being used?
At the moment the British standards for installation of flooring still limits the glue line temperature to 27deg C, so, in theory, any adhesive presently recommended could be used.

However, I am fully aware that the heating is out of our control once the clients occupy the buildings, so I would always suggest upgrading to a high temperature grade adhesive, eg, Bostik STIX A550 POWER ELASTIC for LVT. I’d also instruct that the adhesive must be allowed to achieve a strong bond before bringing the heating online. Typically, this would be seven days.

The above clearly demonstrates that the term ‘underfloor heating’ isn’t enough on its own to enable anyone to make product recommendations. If you have a project with UFH present, be sure you fully understand what this actually consists of before seeking flooring advice.

By the time this article is published, we at the CFA manufacturers’ sub-committee will have had further meetings with the aim of getting a concise guidance document. By liaising with other trade associations such as BEAMA, the CFA can get the most recent and relevant information to ensure its members are guided not by historical thoughts, but by up-to-date knowledge and understanding.
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