Guidance Note on Underfloor Heating DRAFT FOR COMMENT Overview Underfloor heating (UFH) is now an established and appealing and potentially cost effective source of heating, hence its increased popularity within the UK market. Specifiers and Contractors should take advice and recommendations from the flooring manufacturer during the design stage. All manufacturers of floorings will have had experience with the application of their products over UFH and can give good advice - it is important to get the correct advice from the manufacturers of the floor covering before installing over UFH. Similarly manufacturers of screeds and underlayment s can also give advice and recommendations on the use of their products over and incorporating UFH. It is important to establish at the design stage that all the products that make up the thickness of the overall floor are suitable and compatible with UFH. Most products will act differently when installed over an UFH system than when installed on a standard unheated subfloor. The type of floor covering installation needs to be taken into consideration when designing the UFH. Insulation factors of floor coverings will obviously have an effect on the performance of the UFH whilst some flooring products, adhesives and screeds may be affected by high subfloor temperatures and large fluctuations in subfloor temperatures. Subsequently those in charge of operating the underfloor heating should be aware that sudden changes in subfloor temperatures must be avoided. They should also be aware of the maximum temperatures at the flow and surface interfaces and that some types of floor coverings can contend without them being affected in some way. The following information is intended to assist the specifier to select the appropriate combination of underfloor heating and flooring installation. Types UFH can be broadly categorised into two types, namely warm water piped systems and electrical systems. The majority of modern UFH systems in new build projects, especially commercial projects are hot water systems where the water is circulated from a heating manifold through pipes. These systems are normally encapsulated in a floor screed but may sometimes be incorporated in boards or insulation or even between joists or battened sub-constructions. They may also incorporate metal heat distribution trays Electrical systems require the installation of a flexible heating element beneath the floor covering or underlayment. The systems are supplied in the form of cables, mats or films. These are normally encapsulated in a floor smoothing underlayment that is capable of withstanding the changes in temperature in the subfloor although they can be utilised with some types of flooring screed. 1
In these cases we recommend that a floor Thermostat should be incorporated to prevent the screed overheating At this point it should be noted that electrical mat systems are not always compatible with all types of floor finishes if they are installed without having been encapsulated within a floor smoothing underlayment and are directly beneath floor coverings. Specific advice should be taken here in conjunction with the flooring manufacturer with regard to this point. Design Considerations Whilst most floor coverings can be used over UFH this should not be taken for granted and the manufacturer of the floor covering should be consulted. From experience and discussions the Contract Flooring Association recommends that maximum design temperature should be 27 C at the adhesive bond-line beneath the flooring. Moisture content of screeds Before commencing the installation of a floor covering the cement based or Calcium Sulphate sub floor should be checked for dryness in accordance with the recommended methods as shown in BS8204 Parts 1 and 7, and to ensure that it is adequately protected from passage of moisture from below (rising damp) by the presence of an effective damp-proof membrane. The cement based sub-floor must be dry i.e. less than 65% Relative Humidity (RH) for timber floor coverings and 75% RH for resilient and textile coverings when tested using a calibrated hygrometer. To achieve these figures it is essential that the ambient RH of the air in the area is less that the target to be attained within the base. The Calcium Sulphate sub-base must be dry to 0.5%w/w or less when tested using a CM Tester (carbide bomb calorimeter) or oven dried sample, additional advice can be sought from the suppliers of these screed types. Floor Temperature Interpretation The UFH industry designs to BS EN 1264-2: Floor Heating Systems and components Determination of Thermal Output; which contains parameters for allowable floor surface temperatures. This Standard allows for maximum floor surface temperatures of 29 C in occupied areas and 35 C for a strip 600-800mm wide under windows and along external walls, furthermore some thicker floor coverings, especially those with foamed backing layers, have a higher thermal resistance and this can result in excessively high interface temperatures. In areas where the use of these higher temperatures is unavoidable, then agreement from the adhesive and flooring manufacturers should be obtained. The British Standards for contract flooring contain differing advice; for example the operating surface temperature of a heated levelling screed is about 27 C; however, some locations operate at higher temperatures, e.g. 35 C. BS 8425 Code of Practice for Installation of Laminate Floor Coverings and the Code of Practice issued by the Association of European Producers of Laminate Flooring for instance state that the surface temperature should not exceed 28 C. BS 8203 Code of Practice for the installation of resilient floor coverings states: When used with many flooring materials underfloor heating can cause problems if the temperature at the interface between the subfloor and flooring exceeds 27 C, or is subject to rapid fluctuations in temperature. In the majority of installations this temperature will not need to be exceeded if the building insulation meets the requirements of Part L of the Building Regulations. Opinions differ between the floor covering and UFH industries as to where temperatures should be measured; the progressive improvement in Building Regulations Approved Document L has alleviated the situation. Prior to its introduction, the 27 C interface limit restricted the choice of floor coverings that could be used over underfloor heating. However the resulting reduction in the amount of heating power required by the successive improvements to Part L now allows systems to work at temperatures below those of concern. 2
For example BS 8201 Code of Practice for the installation of Flooring of Wood and Wood Based Panels states hardwood floor finishes are installed with a maximum surface temperature of 27 C on the top surface of the hardwood flooring. As a result of the confusion arising from the wording in these Standards the CFA and the United Kingdom Resilient Flooring Association issued a joint guidance note in September 2003 stating that for resilient floorings the temperature should never exceed the agreed maximum of 27 C at the underside of the floor covering (the adhesive line). Note: UFH designers may refer to this as the interface temperature. Whilst there are minor differences in actual temperature level recommendations, specifiers should be made aware of them and it is essential when installing floors over UFH to consult the flooring and adhesive manufacturer s for advice. The UFH must be controllable and fully regulated to ensure that the temperature on the surface does not exceed the recommendations. It is important to generate an even heat across the floor. Hot spots will affect the floor finish. Rugs, etc. laid on the final floor finish will act as an insulator and so will have an adverse effect on the flooring. It is important to ensure that all parties are consulted to ensure that problems do not occur at the manifold area, as the concentration of pipes in a small space can cause problems if not addressed at the design stage. Floor Screeds - Floor screeds that incorporate warm water UFH systems are commonly referred to as heated screeds. Heated screeds are usually installed as floating screeds and installed at an appropriate thickness on a slip membrane over thermal insulation. Although most types of screeds may be used with UFH it is vital that their selection and design is checked. If the UFH system is an electrical wire system contained within a screed then this is generally treated in the same way as a hot water system Wire Heating Systems Whilst these are mainly cables or mats there are some systems based on film/foils (speak to manufacturer of the film/foil systems for installation instructions) which are more commonly found in domestic and DIY installations. The latter are deemed to be particularly suitable for use with hard ceramic or natural stone tiling but may be used with textile, resilient and timber floor coverings (consult flooring manufactures for specific advice). Sometimes the elements are applied over an insulating mat/tile backer board or plywood, cement based board or terracotta base. It is important to ensure that they are of a suitable quality and capable of supporting the chosen floor finish, this is to prevent movement which will affect the bond and the finished floor surface. Certain insulation material such as expanded polystyrene may not be suitable to receive a floor smoothing underlayment. Tile backer boards reinforced with a nylon mesh encapsulated in polymer modified cement slurry are the most common. In every case the manufacturer of the floor smoothing underlayment, and the floor covering manufacturer should be consulted to ensure that the insulation material is suitable to receive an underlayment in terms of strength, resistance to compression, etc. All boards must be securely fixed to the substrate and may require priming before an underlayment is applied. Fixing is normally carried out using suitable flexible ceramic tile adhesive and/or mechanical fixings. 3
Plywood should be a minimum 18mm thick and conform to EN314-2 Class 3 exterior and securely fixed at regular centres using screws. This is important to prevent shrinkage and movement in the plywood which will affect the finished floor covering. It is extremely important to ensure that there is a sufficient depth of floor smoothing underlayment applied to cover the wires as there may be a tendency for the underlayment to crack directly over the wires themselves or to affect the floor covering if the element is too close to the surface this should be a minimum thickness of 3mm. In some instances it is prudent to apply the underlayment in two layers the first to cover the wires the second to provide adequate cover over the heating elements. However whilst ensuring adequate cover is maintained, it is important not to apply too thick a layer as this will introduce additional stress in the system The wire elements or heating mat are sometimes fixed to the insulation board with self adhesive tape; excessive use of the tape will reduce the potential contact area of the floor smoothing underlayment and may cause failure of the floor covering. Where possible consideration should be given to the use of mechanical fixings, some systems for instance provide staples or clips for this purpose. Systems fitted between joists or battens When either an electrical or hot water system is used here and when installing between joists on a ground floor, it is essential that the void underneath is very well ventilated and dry in accordance with current building regulations. When installed between battens over a slab, a vapour check membrane must be installed on the slab and the slab must be dry to BS8204 Part 1. Start Up / Drying Out It is essential that before any floor covering is installed the screed must be sufficiently dry and that the heating system must have been fully commissioned, run and tested correctly as prescribed in BS EN 1264 Part 4 and BS8204 Part 1 and must be dry to the levels previously described. 1. Force drying of the screed is not recommended, but the heating system can be used to help dry the screed as follows. Heating must not begin for 21 days after laying cement and sand screeds, or 7 days for calcium sulphate screeds. Some manufactures of specialist sand cement screeds may allow for this time to be reduced, so therefore it is recommended to consult the manufacturer for guidance if a quicker drying time is required prior to the installation of the screed. 2. Initially ambient water temperature should be maintained for a period of 3 days. After this, water flow temperature may be raised incrementally by a maximum of 5 C per day until the design temperature has been attained, this should be maintained for another 4 days minimum, although this can be extended until the screed is dried. Cooling is a reverse of the above process. 3. With regards to the Calcium Sulphate based screeds the commissioning process is recommended as, initially ambient water temperature should be maintained for a period of 1 day. After this, water flow temperature may be raised incrementally by a maximum of 5 C per day until the maximum design flow temperature has been attained, this should be maintained for another 4 days minimum to ensure drying, although this can be extended until the screed is dried. Cooling is a reverse of the above process. 4. When electrical element systems are installed individual manufacturers should be consulted for advice. As a general rule they should not be switched on for at least 7 days after installation and increased slowly so that the maximum temperature of 27 C is not reached for at least 7 days. 5. When additional levelling compounds are required on the sub-screed surface to make good levels or remedial action, due consideration must be taken to ensure that enough time has elapsed between the application of these systems and the overlaying of final floor finishes. In extreme cases (e.g. poor site conditions) it may be necessary to apply background heating to the sub-screed (as previously described) and to increase ventilation to ensure that adequate drying 4
out has occurred, in all cases the material manufacturer should be consulted to ensure the correct recommendations are applied. Movement Joints Heated Screeds - The most commonly known problems of curling, stress cracking and shrinkage cracking can be exacerbated in a heated screed. Heated screeds expand and contract with temperature changes, recommendations in British Standards are that movement joints should be placed within the screeds at door thresholds, between independently controlled heating zones and at the perimeter of the rooms where heated subfloors abut walls/upstands and where design criteria dictates. This is especially important in larger areas where the screed should be divided up by movement joints into square to rectangular areas not greater than 40 square metres with a length to width ratio not greater than 5:8. with cement and sand screeds and up to 300m2 with a 6:1 aspect ratio for some proprietary calcium sulphate based systems. (Manufactures recommendations must always be followed) Note: Under no circumstances should movement joints be covered, they should be carried through the subfloor to the floor finish and all applied layers terminated either side of the joint. The joint should be filled with suitable flexible sealant and a proprietary cover strip/movement joint profile applied to cover the joint. Heated Wire Systems - Detailing of movement joints should be carried out as mentioned above and must be in accordance with the UFH manufacturers recommendations. Points to Note: Conventional liquid applied waterproof surface membranes are generally not used on heated screeds, as the moisture vapour control rate within the screed is affected which may lead to differential movement issues. Some membranes are available, however they should only be considered when approved by the relevant manufacturer. In all instances the UFH systems must be commissioned and run before any subfloor preparation or installation of decorative floor coverings is carried out. The Underfloor heating should be turned off 48 hours prior to the floorcovering installation and remain off for at least 48 hours after the installation is complete. During this period an alternative heating source should be provided (only normally required during the normal heating season) to ensure that the air temperature within the area of installation is in accordance with the manufacturers and British Standard recommendations, i.e. kept at a constant temperature of between 18 C to 27 C and with an ideal subfloor temperature of 15 C, with a minimum surface temperature of 10 C and a maximum surface temperature of 20 C. Failure to follow these guidelines can result in the floorcovering de-bonding, joints opening and on some occasions discolouring, all of which can occur within a long or short period of time.. 5