The most common question asked about the suitability of underfloor heating (UFH) in a building conversion is ‘how can we minimise the floor build up?’ This is an important area to master within a project such as an office to domestic conversion, particularly when there is an acoustic and UFH requirement. Andrew Grimsley, technical director of Nu-Heat UK Ltd, discusses the low-height options for conversion projects
Underfloor heating can either be installed on top of an existing deck, within an existing suspended timber floor structure or can become part of the floor structure. In many conversion projects the specifier often prefers to avoid pulling up existing floors by fitting the UFH over the top, which will inevitably add height to the deck profile especially in an acoustic floor. The main impact of this can be seen at the top of stairs, at thresholds, and on doors, which will all have to be adjusted – and the bigger the increase, the more expensive and time-consuming this becomes. Low ceiling heights may also have an impact on marketability, either making the property more difficult to sell or affecting the purchase price.
The other point to consider is whether the conversion needs to meet the acoustic requirements of Part E of the Building Regulations. There are two ways of ensuring compliance with Part E: either install an acoustic floor recommended by the supplier, which is then tested by an independent consultant, or use a Robust Detail (RD) floor construction.
Correct heating system design is especially important in older buildings. The building’s construction, U-values, ceiling heights, existing floor construction, heat source and floor covering must all be factored in. From this information the designer can calculate heat losses and the output required from the UFH, produce a detailed system specification with associated CAD pipe layouts, flow temperatures, and system schematics to illustrate how the various components should be configured. This level of detail, backed up with a comprehensive manual, makes for a straightforward installation process and also allows a good supplier to guarantee their system design.
The choice of floor covering is important as the conductivity of the material has a bearing on the effectiveness of the UFH system. Natural stone or ceramic tiles conduct heat extremely well and can be installed directly onto the pre-routed gypsum board or castellated panel systems with just a thin decoupling layer. Laminates or engineered timber can be ‘floated’ directly over the floor. Carpet with underlay is the least conductive option but can be used where the combined tog value is no greater than 2.5.
The options for low-height floor constructions are fairly limited, but those available can be very effective. For instance, a system that uses pre-routed gypsum panels together with 10mm UFH pipe gives a total height build-up of just 15mm, making it one of the slimmest profiles available. With a flow temperature of 50˚C heat output can be up to 80W/m2.
This type of system is also available in acoustic floor variants which have been independently proven to exceed the required standards. The panels can be used over a structural concrete deck or a suspended timber sub-floor.
An alternative new and innovative low-height option for retrofit UFH consists of a castellated panel to hold the UFH pipe in place, which is then covered with a specialist self-levelling compound with an average depth of 22mm.
This system produces a very high heat output: at 50˚C flow it is in excess of 120W/m2. Its fast response times allow it to be used as a direct replacement for a radiator system, making it ideal in a retrofit situation. The finished floor construction offers a perfectly level finish on which to lay ceramic tiles or natural stone products.
This system can be used over a concrete deck or suspended timber sub-floor and, like the pre-routed panel system, can be used in the sort of irregularly shaped room found in many retrofit projects.
Another retrofit option would be specialist metal heat diffuser plate systems, which are ideal for use in suspended timber floors. The plates, which hold the UFH in place, can be fitted from above on top of joists where the existing floor is being lifted or between the joists from below if ceilings are being replaced. There is no additional height build-up because the plates are just 1mm thick and sit within the floor structure. Heat output is up to 85W/m2 when used in conjunction with a specialist deck and acoustic variants are available.
The choice of heat source will affect the performance of the heat emitter and vice-versa. For instance, the low flow temperatures of UFH allow a condensing boiler to work in efficient condensing mode for more of its operating time, which keeps running costs down. Heat pumps linked to UFH are suitable for buildings with U-values equivalent to current Building Regulations. In this situation, flow temperatures of 35-45˚C are typical, and will ensure good performance from both heat pump and UFH.
UFH in retrofit projects allows every available inch of room space to be utilised for furniture and fittings, it is less prone to damage and is an energy-efficient option for tenants and owners. Specialist UFH design-and-supply companies should be happy to support architects and specifiers in choosing the right option for the individual project.