BUILD RIGHT Embedded floor heating Embedded floor heating systems incorporate heating within a floor slab and use thermal mass to provide low-temperature, uniform heat over the entire floor area. By Alide Elkink, BRANZ Technical Writer

here are two types of embedded the greater the heat flow. When the room H1 increase the minimum R-value from floor heating systems – hydronic temperature is less than that of the floor, R1.7 to R1.9. (using heated ) and electric. heat will move from a heated floor slab to Calculating the R-value for a floor slab T A hydronic consists materials within the room. As the materials takes into account the thermal resistance of of heated water pumped through continuous become warmer, the rate will the slab, the of the plastic pipework laid in the floor slab (see slow and eventually stop when a temperature beneath the slab, the length and width of the Figure 1). Water temperatures are generally equilibrium is reached. At this point there is slab, and the width of the external walls of between 40 and 60°C and the heating source no heat transfer from the slab, so no heat the building (which defines the length of least may be electricity (including heat pumps and input is required. thermal resistance between the interior and heat exchangers), gas, oil, solid fuel, solar or This process of ‘self-regulation’ depends exterior environments). a combination of these. upon high levels of insulation in the floor slab Rigid polystyrene insulation is typically An electric floor heating system consists of and to ensure minimal used for underslab insulation. This is available heating elements or cables embedded in the heat loss from the building. in two forms – extruded (XPS) and expanded floor slab (see Figure 2). (EPS) polystyrene. Both pipework and heating elements are Slab needs to be well insulated Perimeter insulation or a thermal break embedded 30–50 mm in the slab. Water A building envelope must be designed to must be incorporated to limit the heat loss pipes are spaced at 150–300 mm and meet the requirements of the New Zealand between the slab and foundation, and to electric cables at 50–350 mm. Both systems Building Code Clause H1 Energy efficiency. If maximise the insulation performance of the are laid over, and tied to, the steel reinforcing NZS 4218: 1996 Energy efficiency – Housing slab. BRANZ recommends the incorporation of mesh. Floor temperatures may range between and small building envelope is used as the a thermal break using H3.2, 140 × 45 mm 20 and 29°C. means of compliance to NZBC Clause H1, a timber inserts at the edge of the slab (see Many benefits heated floor slab must have a minimum R- Figure 2). Wall construction using 140 mm value of R1.7 in all climate zones. However, wide bottom plates and framing, rather than Benefits of embedded floor heating systems the changes of 31 October 2007 to Clause 90 mm, and insulation, improves the thermal include: ❚ a ‘self-regulation’ of the heat transmission into the room pump ❚ no air movement or noise from air moving through ducts or fans supply and return manifolds ❚ no space in the room is taken by a heating system ❚ limited vertical temperature stratification, as low temperature radiant heating does water not warm air heater ❚ less likelihood of condensation and mildew.

Self-regulation of heat underfloor circulation pipework Heat transfer occurs from areas of higher temperature to areas of lower temperature. The greater the temperature difference, Figure 1: A hydronic floor heating system.

24 BUILD October/November 2007 performance even further. This was described Retrofitting is often not a practical option in Build 100 (June/July 2007, pages 32–33 and can only be done where a lightweight and 103–105). topping slab can be placed over an existing slab. The existing structure must Energy efficient be checked for: systems are low ❚ structural adequacy of the slab temperature systems. They are efficient heat ❚ adequacy of existing insulation or, generators, requiring less heat input than if required, whether it is possible to other heating systems. Hydronic systems increase the insulation are particularly suited for underfloor heating. The water temperature is generally controlled ❚ adequate floor-to-ceiling height after by , but solar-heated water will the overpour and additional insulation require the water flow to be controlled once is added. the maximum temperature is reached. Electric floor heating systems may provide Additional efficiency can be achieved by a good heating solution for building designing the building to maximise direct additions where extending an existing, or solar gain onto the floor. installing a new, hydronic system would be impractical. Floor coverings have an impact Floor coverings affect and reduce the rate of Other flooring, other systems efficiency of a floor heating system according Floor heating systems are available for to their thermal resistance. A concrete finish other floor types, such as timber floors. and ceramic or slate tiles provide little thermal However, these are not embedded floor resistance (insulation) to the floor, but cover- heating systems and do not provide the ings such as carpet, cork, linoleum, vinyl and thermal mass heat storage of embedded timber will all reduce the heat output rate. systems covered in this article.

Retrofitting sometimes possible For more information see the recently Embedded floor heating systems are most released BRANZ Bulletin 491 Embedded commonly installed in new concrete floors. floor heating.

insulated wall framing wall-mounted flush box

sensor

cold tail connection

sensor into slab 600 mm

H3.2, 140 × 45 mm timber insert

foundation

reinforced concrete electric cables tied to floor slab reinforcing mesh

insulation over dpc

Figure 2: Electric floor heating system with embedded temperature sensor.

BUILD October/November 2007 25