Design Limits BS EN 1264
To limit discomfort and protect the floor finish while still meeting the calculating heat losses, BS EN 1264 states that the ‘physiologically agreed’ maximum floor surface temperature is 9°C above the room temperature.
This results in a maximum floor surface temperature of 29°C in typically occupied areas with a room temperature of 20°C. A 9°C temperature difference will equate to a floor heat output of 100W/m2.
Types of floor finishes
Underfloor heating can be used with essentially all floor finishes and surface coverings provided that the maximum permissible temperature of each component is respected.
The floor covering manufacturer should be consulted to ensure that any special recommendations are followed, e.g. maximum temperature limits, wood conditions, special glues, etc.
Floor coverings with high thermal resistance, such as thick carpet and underlay, should be avoided as they need higher flow temperatures or closer pipe spacing to achieve the required heat output.
General considerations
In general, floor finishes that would normally feel cooler underfoot will enable a UFH system to operate more efficiently and be warmer when combined with UFH.
As a rule of thumb, a UFH system will produce 10W/m² for every 1°C the floor is warmer than the air. Therefore, some floor finishes and adhesives have temperature limits which may limit the heat output of a UFH system.
BS EN 1264 sets a comfort limit for the floor surface at 9°C above the room air temperature. In peripheral areas, considered as a section of floor up to 1m wide against external walls, and in wet rooms, this limit is raised to 15°C. This higher limit allows heat outputs of up to 175W/m² to be produces where floor finish materials permit.
Ceramic tiles
Ceramic tiles may be laid over all types of underfloor heating included hydronic systems embedded in concrete or screed and suspended or floating floors.
Due to their excellent conductivity stone, slate, ceramic & porcelain tiles make ideal floor coverings for underfloor heating.
They transmit heat evenly and are unaffected by changes in temperature or humidity. They are perfect for storing heat and emitting it back out slowly.
The adhesives and grouts used must be sufficiently flexible to accommodate a small amount of thermal expansion and contraction without cracking.
Large areas of flooring may need additional expansion joints to reduce floor surface stresses.
Wood Flooring
All forms of wood flooring can be used over hydronic underfloor heating, but some are suitable for higher heat outputs than others.
The main issue of concern is usually expansion or contraction due to changing moisture levels. Moisture levels within a heated timber floor will typically vary between 7-8% moisture content in the winter and 9-10% in the summer.
This change in moisture content is only marginally greater than would occur naturally due to changes in the seasons. For all timber floors, installed over UFH or not, it is worth remembering the following.
- Floors laid in winter will normally expand in summer when the heating is off
- Floors laid in summer will initially expand and then contract again in winter when the heating is on
Therefore, ensuring the timber floor is properly acclimatised to the room an installed in line with the manufacture’s guidance is essential to achieving a beautiful and stable natural floor.
When laying over screeds, ensure they have been allowed to fully dry, laying timber over a damp screed and then turning the heating on will drive the moisture out of the screed and through the timber, potentially causing irreparable damage.
With all types of wood-based flooring systems, when designing a system be aware of the lower heat output values generally available with these systems. Many timber flooring manufacturers will stipulate a maximum floor surface temperature of 27°C for their products.
In a room heated to 20°C this effectively limits the maximum heat output to 75W/m². In rooms heated to 22°C this limits the heat output to just 50W/m². As a result, the use of timber flooring and any restrictions imposed by the manufacturer should be carefully considered when designing a heating system.
Occasionally, a timber floor manufacturer will stipulate a 27°C limit on the underside of the timber flooring. For such a timber floor, at say 14mm thick, and a thermal resistance of 1 tog, a 27°C underside in a 20°C room would result in a heat output of just 35W/m², 25W/m² in a room heated to 22 W/m².
Laminate flooring
Laminate flooring in general should be treated as natural wood flooring. However, its composite construction can offer improved dimensional stability.
Not all laminates are moisture stable. Wherever practicable it is recommended that moisture stable laminates are used to provide improved dimensional stability.
When selecting an underlay to combine with the laminate flooring, it is recommended that a low tog underlay, designed for use in combination with UFH is used. Such underlays will typically have a thermal resistance of 0.3 tog or less and include a vapour control layer to prevent rising damp from the structural floor below causing damage to the laminate.
Wood floor adhesives
Ensure that any adhesive used for timber floors over UFH are suitably rated. For an 14mm thick timber floor heated to 27°C in a 20°C, the temperature of the underside and the glue is approximately 35°C. Many manufacturers will produce HT “High Temperature” variants of the most popular adhesives for UFH applications as a result.
Vinyl / Linoleum flooring
Vinyl and linoleum are products of various thickness and surface characteristics, they are usually supplied in rolls and glued to the underlying floor surface.
This type of flooring can become softer with increase in temperature and there is increased risk of permanent depressions caused by concentrated loads such as furniture castors.
Since this type of flooring is relatively thin, latex screeds are used as a surface preparation on normal screeds or concrete to reduce the effect of any irregularities that might become visible at the surface of the flooring.
High temperature glues and adhesive suitable for use with underfloor heating must be used for vinyl or linoleum.
It is also important that the surface temperature of the Vinyl/linoleum does not exceed the limits stipulated by the manufacturer. This is typically 27°C and is set to prevent discolouration of the pigments. Some products are designed for long terms exposure to higher temperatures, either from UFH or direct sunlight which will hold floor temperatures at 40°C for much of the summer, so where increased heat outputs are required, suitable vinyl products are available.
Carpet and Underlay
Carpets are among the most thermally resistive covering to be fitted over UFH as they were historically designed to reduce heat loss through floors and keep homes warm. Typically, they have a thermal resistance of 0.125 tog/mm.
As a result, finding a carpet and underlay combination that is thermally efficient with UFH may result in a thinner carpet being selected.
A common target is a combined resistance of 1.5 tog or lower when fitting Carpets over UFH, however while this is a hard limit for many electric UFH systems to prevent the heaters from overheating, it is not for hydronic UFH. Using a more thermally resistive floor finish with hydronic UFH will result in the floor surface becoming cooler and its heat output dropping accordingly. It is therefore common to increase the design water temperature to offset this reduction in heat output.
Special low thermal resistance underlays (tog < 0.5) have been developed specifically for underfloor heating applications.
Foam backed carpets are generally less suitable for underfloor heating (unless specifically warranted by the manufacturer) as the foam may degrade.
Commercial carpet and carpet tiles glued directly to the screed are unlikely to cause an issue provided that the adhesive is suitable for use with underfloor heating.
Floor finish Resistance
Calculating thermal resistance
Resistance, tog = thickness in mm / ( 100 x thermal conductivity)
So for a 18mm Hardwood floor, its thermal resistance would be:
tog = 18 / (100 x 0.16) = 18/ 16 = 1.125 tog
Heat output = temp difference between water and air x kH