Underfloor heating installation procedure

All the important information for a perfect underfloor heating installation procedure, a guide, and the range of accessories and complete systems – all on our website.

You will find everything for the installation in the underfloor heating category – system insulation boards, polystyrene, pipes, manifolds and electric heating mats.

The build-up of a heated floor consists of:

- base concrete

- thermal and acoustic insulation

- perimeter thermal insulation and expansion strip

- damp-proof membrane

- heat-reflecting foil

- heating pipes

- concrete screed

- floor covering.

The most important element of the build-up is the pipes. They are an essential part of the underfloor heating installation procedure.

BASE CONCRETE

The base concrete is the load-bearing part that forms the substrate for the layers of the heated floor itself. When assessing it, bear in mind that once the pipes are cast into a 4 to 6 cm layer of concrete it will carry a load of 80 to 100 kg.m-2, so it must be load-bearing. The surface for the heated floor must be flat. The provisions of STN 73 0225 apply to the limit deviations from overall and local flatness of horizontal surfaces. If the floor is laid on a substrate in an environment where moisture could penetrate its structure (e.g. on natural ground), the surface of the load-bearing floor is protected with a damp-proof membrane 0.2 mm thick.

The composition of the base concrete must comply with the relevant standards. In refurbishment work, deviations from the standards are eliminated with a levelling layer. Any unevenness must be removed from the floor without fail; it must be absolutely clean and free of any sharp objects. This is an important step in the underfloor heating installation procedure.

THERMAL AND ACOUSTIC INSULATION

In addition to its insulating properties, this layer must meet further requirements arising from the safe and reliable operation of the heated floor – above all sufficient strength and low compressibility of the material. It is made mainly from polystyrene sheets covered with a damp-proof membrane to stop the screed running into the joints. Depending on the location of the floor, the recommended insulation thickness is 20 to 60 mm.

Manufacturers also supply the acoustic and thermal insulation as the base board of an underfloor heating system. The outer edges of the board are profiled. When the layer is laid, the boards lock firmly into one another and form a continuous insulation layer without joints. No additional layer (membrane) is therefore needed and no acoustic bridges are created.

Acoustic and thermal insulation is designed in accordance with the standards on the thermal and acoustic protection of buildings. Foamed plastics or fibrous insulation are used as the insulating material:

- PS 20 or 30 SE polystyrene boards.

- PST SE polystyrene boards (acoustic insulation),

- rigid polystyrene boards,

- rigid polyurethane boards

- high-density polyethylene foam as acoustic insulation.

Their density should be 20 kg.m-3 , and compressibility must not exceed 5 mm. Under the thermal protection regulations, heated floors bordering spaces with a substantially lower internal temperature should be insulated so that the heat transfer coefficient meets the requirement

 k= 0,45 W.m-2.K-1.

When laying the thermal and acoustic insulation, it is essential to make sure that thermal insulation made up of several layers contains only one layer of acoustic insulation, because acoustic insulation laid double could, through its resilience (about 3 to 4 mm per layer), cause the heating screed to crack, which may damage the underfloor heating.

The thermal insulation beneath a heated floor should be rigid thermal insulation foam. Sheets of expanded polystyrene or rigid polyurethane are the most practical. Rooms without a basement beneath them must be thoroughly damp-proofed against ground moisture before the thermal insulation is laid. Take care with primers and with coatings based on cyclic compounds and solvents , which destroy polystyrene foam. In this case, rubberised bitumen with glass fabric is suitable as the damp-proofing.

Mats or rigid slabs based on mineral fibres are also suitable as thermal insulation. Here, however, a certain amount of compression during installation (from being walked on) has to be allowed for. This makes it harder to fix the pipes with special clips. During concreting, mixing water seeping through causes the thermal insulation mat to lose its elasticity. This is another reason why the concrete mix should be only damp when it is placed.

Other materials can also be used, however: for example, expanded perlite lightly mixed with lime milk is, once dry, a good substitute for rigid organic foams. In principle, any thermal insulation will do provided that, at an acceptable layer thickness, it meets the requirement of no more than 15% of the heat being transferred downwards.

PERIMETER THERMAL INSULATION AND EXPANSION STRIP

Fibrous insulation boards or strips 10 to 20 mm thick are recommended. They compensate well for the thermal expansion of the floating concrete layer. They are resistant to a variety of influences (aggressive environments, rodents, etc.). If underfloor heating is being installed in buildings made of lightweight prefabricated timber-based components, only fibrous insulation mats should be used.

Edge insulation strips are laid around the whole of the heated room between the floor and the walls. They serve as layers of acoustic and thermal insulation and take up the expansion of the floor. The minimum thickness of the edge insulation strips should be 5 mm. Installation firms usually supply them in a size of 100 x 10 mm.

Fitting this strip is one of the first steps in the underfloor heating installation procedure.

DAMP-PROOF MEMBRANE

The damp-proof membrane is a continuous layer protecting the surface of the thermal insulation against moisture from the concrete screed. Water causes polystyrene to lose its thermal insulation properties.

All rooms whose floor lies directly on the ground must be given a moisture barrier. For this purpose we recommend PVC sheeting 0.8 mm thick or PE sheeting 0.1 to 0.2 mm thick – in very unfavourable cases 2 x 0.2 mm – which can be welded. An overlap of about 8 cm must be kept when welding.

Apart from the function implied by its name, this membrane also acts as a separating layer. PE and PVC sheeting have proved best, thanks to their good adhesion and elasticity. Bitumen felt is unsuitable for this purpose because of its stiffness and relatively high price, and we recommend not using it in the underfloor heating installation procedure. It is difficult to wrap around the expansion strips along the edges of the concrete slab.

HEAT-REFLECTING FOIL

In some underfloor heating systems the damp-proofing function can also be performed by a heat-reflecting foil, which at the same time helps to reduce the heat flow from the heating pipes downwards, as follows from the composition and principle of this foil.

a) Composition of the heat-reflecting foil

Base layer – a solid, very strong carrier film that acts as a moisture barrier.

Bonding layer No. 1 – corrosion protection for the aluminium and a bonding medium.

Reflective layer – a highly reflective and extremely thin layer of pure aluminium.

Infrared-permeable layer – an infrared-transparent plastic layer that allows the reflected infrared radiation to pass.

Bonding layer No. 2 – a bonding medium, corrosion protection for the Al.

Top layer – mechanical protection against the heating screed, printed with an 8 cm laying grid in both directions.

b) The principle of heat reflection

Part of the heat given off by the heating pipes and radiated downwards through the heating screed passes through the transparent layer and is reflected by the micro-thin aluminium layer beneath it back into the heating screed. As a result, using heat-reflecting foil can save up to about 9 % of energy.

c) Laying the heat-reflecting foil :

An overlap of about 5 to 6 cm is recommended, spot-fixed with adhesive tape. At the walls the heat-reflecting foil is turned up about 1 cm above the expansion strip. The foil can also be hot-welded with a welding tool, or joined cold with a special rubber-bitumen adhesive tape (wet rooms).

HEATING PIPES

The heating pipes are the most important part of a heated floor, so below we describe in more detail:

- pipes for heating,

- pipes for underfloor heating.

The following pipe materials are most commonly used in central heating today:

- metal pipes,

- plastic pipes,

- composite pipes.

The following are particularly suitable for underfloor heating:

- copper,

- plastics,

- composite pipes.

CONCRETE SCREED

The concrete screed used with underfloor heating is the same as commonly used concrete screeds. Admixtures make the mix easier to work. They improve its plasticity, which allows the pipes to be encased better. Admixtures do not, however, improve the thermal properties of the floor.

The minimum thickness of the concrete layer above the pipes is 45 mm. The wet method requires the thickness to be increased to 65 to 70 mm. In rooms where the concrete bed is expected to carry extreme loads, reinforcement (e.g. structural steel mesh 50 x 50 x 2 mm) should be placed between the pipes and the concrete layer.

Temperatures up to 60 °C have no adverse effect on the durability of the concrete. Curing the concrete after it has been laid is particularly important. A freshly laid concrete layer must be protected against excessive drying for at least 10 days. Trial operation of the heating system should not begin sooner than 21 days after concreting is finished. Normal operation must not begin sooner than 28 days after the floor covering has been laid. The required temperature is reached by raising the water temperature gradually, by 5 °C a day, up to the required value. The concrete bed must have expansion joints, made so that they allow the floor to expand. Expansion joints divide the individual layers through the full cross-section, from the thermal insulation to the floor surface. Above certain dimensions, heated floor structures require expansion joints, which are positioned as follows:

a) above existing building expansion joints, in the same place and of the same width,

b) as the boundary of a concrete bay,

c) as edge joints against all adjoining materials.

The joints must be filled with an elastic-plastic compound, or special joint profiles must be built into them. Structural elements in the heating screed (e.g. load-bearing reinforcement) must be interrupted at the expansion joint. The heating pipes are laid according to a previously prepared expansion joint plan so that as few pipes as possible cross an expansion joint. Wherever pipes do cross expansion joints, they must be suitably protected against mechanical damage.

The areas defined by expansion joints should be square wherever possible, should not exceed 40 m2 in size and should have a side ratio of no more than 1.5:1.

Edge expansion joints must be formed at every junction between the floor and building elements (walls, beams, doors, staircases, etc.), using expansion strip without interruption up to the top edge of the floor covering. Mortar bridges are not permitted. Edge expansion joints must allow the heating screed and the floor covering to expand by at least 10 mm on each side.

The concrete screed is made from suitable concrete mixes in accordance with the relevant provisions of STN 73 2400, using cement of the usual composition and quality for ordinary underfloor heating.

To make the mix easier to work and improve its plasticity, plasticisers may be added, provided they do not attack the materials of the heating coil. There is no need to improve the thermal conductivity of the concrete with admixtures.

The thickness of the concrete layer is specified in the design and is around 60 mm, with a screed thickness of at least 40 mm required above the pipes so that they are not mechanically damaged by excessive loading of the floor.

Higher heating water temperatures in the pipe circuit have no adverse effect on the quality of the concrete. Normal operating temperatures of around 60 °C do not change the properties of the cement or damage the screed through its expansion.

Anhydrite floor screeds may only be used if no metal material, e.g. welded mesh, has been used to fix the pipes, since gypsum is corrosive to metal. No additive is needed in an anhydrite screed.

The technique is described in the specialist literature. Latex can be used as a plasticiser (with care). Duvilax is not recommended, because in the event of a local fault caused by the user's carelessness it makes it harder to cut out part of the concrete bed. When the mix is transported by hand it must be handled carefully (especially at lower temperatures) so that the pipes are not damaged.

The concrete mix must not be runny. It is a mistake to think that a good wet mix is better because it will flow more easily between and under the pipes. Good encasement of the pipes in concrete must be ensured by a correctly chosen aggregate grading, a plasticiser and thorough compaction of the damp mix.

Concreting is carried out with the pipes pressurised to the test pressure. Any marked drop in pressure in the system may indicate a damaged pipe. This is an important factor in the underfloor heating installation procedure.

FLOOR COVERING

The floor covering forms the top layer of the heated floor build-up. Laying it is the final stage of the underfloor heating installation procedure. The thermal resistance of this layer, Rkr, must meet the requirement Rkr = 0.15 m2K.W-1. Higher thermal resistances act as a thermal brake. Almost all common floor coverings meet this requirement. Textile carpets with a pile height of more than 10 mm, PVC with a foam backing and softwood parquet are not recommended. Ceramic or stone tiles with a maximum thickness of 6 mm appear to be the most suitable.

For better heat transfer, the covering is not laid loose on the floor surface. We recommend fixing it with adhesive or bedding it in a layer of cement screed or a levelling concrete layer.

The floor covering is laid 28 days after the concrete screed has been placed. The most suitable floor coverings for a heated floor are stone or ceramic tiles. Apart from these, ordinary floor coverings with a thermal resistance of no more than 0.15 m2.K.W-1 can be used.

For better heat transfer, hardwood parquet is chosen. It should be no more than 8 mm thick. Solid wood blocks are less suitable. The moisture content of the wood must not exceed 9 %.

Textile coverings must be heat-stable and antistatic, and should be no more than 5 mm thick. For better heat transfer, carpet and plastic coverings are bonded to the floor – if these conditions are not met, a textile covering may act as a thermal brake, reducing the heat output of the heated surface and making it necessary to increase the output of the heat source.

The type of flooring is taken into account when the floor is sized. It is therefore not acceptable to fit flooring with a different thermal resistance during installation. The type of flooring used usually depends on the specific heat loss of the room. Another deciding factor in the choice of flooring is the client's requirements. From the point of view of moisture, flooring based on natural wood (parquet, floorboards) is not recommended, even though the thermal criteria do not rule it out. The opposing moisture conditions that wooden flooring on a heated surface is exposed to in summer and winter put considerable stress on the material (expansion and contraction, with the joints subsequently having to be filled) and cause it to wear prematurely. This does not apply to mosaic parquet up to 8 mm thick.

PVC-based flooring is likewise not recommended , because it tolerates thermal stress poorly (shrinkage). The chlorinated rubber adhesives available on our market are not intended for bonding flooring to substrates at elevated temperatures.

All the important information for a perfect underfloor heating installation procedure, a guide, and the range of accessories and complete systems – all on our website.

References:

Dušan Petráš: Underfloor Hot-Water Heating