Underfloor heating moves up a level

Ways to install underfloor heating under wooden floors — Bill McConnel.
How do you install underfloor heating underneath a wooden suspended floor? Bill McConnel offers a range of solutions.The rapid growth in the underfloor-heating market in the last decade has led to an array of new products being introduced, enabling the installer to fit UFH more efficiently by cutting installation times. The ground floor in most new builds will be a screeded floor construction with UFH typically installed in the screed. However, for wooden suspended floors, there are many alternatives, and each installation method has its own merits and application. Suspended floors The most traditional method of installing UFH in suspended floors is to use a pug mix. This method is tried and tested with great success throughout Europe and has also been used extensively in the UK. High-quality insulation boards are placed between the joists and held in place with battens, so they are fully supported and the top of the board is about 20 mm below the top of the joist. The UFH pipe is then laid and clipped into place, typically at 200 mm centres onto the insulation board. The joist has to be notched to allow the pipe to pass from one ceiling void to the next. Once the pipe is laid, a dry mix of 8:1 dry sand and cement is smoothed over the pipe work and tamped level with the top of the joist so that when the finished flooring is laid, the sand and cement mix will be in contact with the underside of the flooring.

The is a wide range of approaches for installing underfloor heating in wooden suspended floors. This approach uses radiant reflective foil to reflect heat from the pipes to the underside of the floor and clips over the joists to support the pipes.

This system allows good heat transfer from the UFH pipework to the floor surface, so it is suitable for buildings with higher heat losses and also for use with lower-temperature heat sources such as heat pumps. This, combined with tighter pipe spacings, enables flow temperatures to be kept as low as possible to improve the coefficient of performance (COP) of the heat pump. The weight of the sand/cement mix adds about 28 kg/m2 for joist spacings of 400 mm. Although this weight is acceptable for most constructions, we recommend this is ratified by a structural engineer. This method gives a uniform distribution of heat over the floor and is proving more popular when UFH is combined with renewable heat sources. When notching is not an option Where composite joists such as eco-joists are being used and notching is not permitted, it is advisable to use an ‘over-joist’ construction. This is the same as the ‘between-joist’ construction, except that 20 mm battens are fitted to the top of the joist and insulation is fitted flush with the top of the joist. This means the batten is notched (not the joist) to allow the UFH pipe to run from one joist section into the next. The dry sand/cement mix is added as above, between the battens, and the flooring fitted and fixed to the battens. Heat-diffusion plates for upper floor installation The heat-diffusion plates (also known as spreader plates) method is a simple, lightweight system and is considered an easy method of installation. Although it is the preferred option by many installers, it does not give 100% coverage in the same way as the dry sand/cement method does. This is because the aluminium heat-distribution plates only cover about 85% of the floor area because they only fit into the straight runs and not the returns at the end of each UFH pipe run. Also, frequent grooves have to be cut into the insulation boards to allow for extra pipe runs through the board. Heat diffusion plates for floating floors Another installation method for upper floors is using aluminium heat distribution or spreader plates with insulation boards. These can be applied to a finished floor and will increase the depth of the floor by 30 to 50 mm, depending on the thickness of the insulation. For this type of ‘floating floor’, the spreader plates fit into grooved insulation boards, the insulation boards are laid down and the heat diffusion plates are fitted on top. The UFH pipe is then slotted into the heat-diffusion plates and the floating floor fitted above this. It is advisable to use a layer of builder’s paper between the plates and the floating floor as it reduces expansion noises; aluminium plates expand and contract at a different rate to the floor as the heating comes on and off. Heat-diffusion plates for ‘between the joists’ installation This method can also be used between the joists, with the plates being fitted to bridge the gap between the joists. Insulation is fitted beneath, in combination with battens to support the plates. The pipework is then pushed into the plates, with notching to allow the pipework to go from one joist area to the next. The flooring is then fitted over the top and secured to the joists. Heat diffusion plates for ‘fit-from-below’ If a floor has already been fitted then a ‘fit-from-below’ plate can be used and fixed to the underside of the floor that has already been installed. The pipework is pushed into place, and the insulated boards then fitted from below and secured tight against the ‘fit-from-below’ plate with battens. The SlimFix installation method Another upper floor system is to use SlimFix, a radiant reflective foil that reflects the heat from the UFH pipework back up to the underside of the floor. It is simple to use and can be very quick to install. The reflective foil is unrolled across the joists and allowed to hang down about 50 mm below the top of the joist. Clips are then fitted over the joist so that the pipework is suspended in mid air above the foil. This system is very light and remarkably quick to install. Conclusion With so many methods available for installing underfloor heating into the upper floors of buildings, it is advisable to seek impartial advice and choose an installation method that best suits your project needs. Bill McConnel is the operations director of Continental Underfloor Heating.
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