Do you like the idea of installing underfloor heating yourself? In this article you will find all the important information on how to go about it.
You will find the complete range in the underfloor heating category – system insulation boards, pipes, manifolds, controls and electric heating mats.
Preparing the building
Before the underfloor heating is actually laid, the building must be prepared so that later work does not damage the floor structure – for example, so that the insulation layer does not catch fire when metal parts are welded, or so that the floor is not deformed or otherwise mechanically or chemically damaged by incorrect handling during installation. This means that plastering must be finished on all structures adjoining the floor being laid, the door frames fitted and the boiler circuit completed.
The manifolds, collectors and the other fittings and control components of the system are installed in the building in such a way that later welding on the metal parts of the system cannot set fire to the insulation layer. This is the first step in installing underfloor heating yourself.
Preparing the floor
Before the underfloor heating is laid, any defects that could adversely affect its quality are removed from the sub-floor. The surface of the load-bearing layer is levelled and cleaned of dirt, bumps and grease stains. If the heated floor is in contact with the ground, a damp-proof layer must be inserted to prevent ground moisture from penetrating its structure. PE or PVC sheeting is used for the damp-proofing. It is laid loose on the base concrete so that it covers the whole area without gaps. The individual edges of the sheeting should overlap by 20 to 30 mm and, at the same time, should be turned up the surrounding vertical structures at least to the thickness of the underfloor heating build-up.
Laying the perimeter insulation strips
The next step in installing underfloor heating yourself: once the sub-floor has been prepared, the perimeter insulation strips are laid. They are fitted as a continuous, unbroken strip up to the height of the floor build-up. Lay the edge insulation strip around the perimeter to the height of the floor layers, making sure it is continuous (including around columns, floor penetrations, door thresholds and so on). Where necessary, divide the floor into expansion bays with a maximum area of 40m2. Expansion joints are also formed using edge insulation strips. Along architecturally more complicated structural features, such as columns, corners and alcoves, we recommend fixing the insulation strips firmly with nails.
Laying the thermal and acoustic insulation
Once the edge insulation strips are in place, a layer of thermal and acoustic insulation is laid on the prepared surface of the base concrete. The thickness is specified by the underfloor heating designer on the basis of a calculation; it is usually 4 to 6 cm. The thermal and acoustic insulation may be supplied as individual boards, or as so-called rol-jet and falt-jet, which allows system rolls to be formed.
The insulation boards are laid against one another so that they form a continuous layer over the whole area. They are butted together as tightly as possible, ideally in two layers so that the joints in the lower layer are covered by the solid part of the upper layer. They can be fixed to one another with metal pins.
The thermal insulation is laid from the edges of the room towards its centre, which allows the edge insulation strip to be held in place by pressing it against the wall.
The insulation boards are laid loose on the sub-floor or they can be bonded. With the second option, care must be taken to choose a suitable adhesive. Polystyrene must not come into contact with organic solvents, otherwise it will degrade and lose its thermal insulation properties. The boards can be bonded to the load-bearing structure with pure bitumen or with water-based dispersion adhesives and sealants.
Laying the damp-proof membrane
An equally important step in installing underfloor heating yourself is laying the damp-proof membrane. A PVC or PE membrane is laid loose on the surface of the insulation boards. The edges of the individual sheets overlap by at least 20 to 30 cm and, around the perimeter of the room, are turned up above the edge insulation strip so that the concrete mix cannot run into the joints. Nowadays a multi-purpose, multi-layer foil is often used instead of the conventional damp-proof membrane; besides acting as a moisture barrier, it also reflects heat.
Where manufacturers use system insulation boards, the membrane is not needed, because it is part of the board and forms its top layer.
Where the underfloor heating borders the outside environment, foreign manufacturers recommend laying thermal insulation both above and below the load-bearing floor. Thermal insulation on the outer side of the cooled structure gives better insulation performance.
Fixing the pipes
The next step in installing underfloor heating yourself is fixing the pipes. Underfloor heating pipes are fixed in the following ways:
• tying to mesh,
• fixing to mesh with plastic clips,
• fixing with plastic clips to clip rails,
• fixing into the insulation board with plastic clips,
• pressing the pipes into a system insulation board,
• joining by means of heat-spreader plates,
• fixing to reinforcement mesh,
• joining into plate cassettes,
• laying modular climate floors.
Tying to mesh
Welded steel mesh of the lightest possible type (wire thickness 4 to 6 mm) is laid on the prepared thermal insulation on spacer rings about 5 mm thick, which allow the concrete to flow under the pipes. Wire or PVC tape can be used as the tying material. The mesh size is chosen to suit the pipe spacing specified by the heating designer, which makes it easier to set the distances when laying and fixing the pipes. A grid for laying the pipes made from flat welded mesh may have a mesh size of 100/100 mm, 150/150 mm or 200/200 mm with a wire thickness of 2 to 5 mm. Some types of cable clip can also be used instead. The individual sheets of mesh within one heating field are joined with steel tying wire.
Fixing to mesh with plastic clips
As in the previous case, the welded mesh is laid first, but the pipes are no longer tied; instead they are held by rows of plastic clips fixed to that mesh. This both simplifies and speeds up the installation as a whole and gives better contact between the pipe and the concrete screed, and therefore better heat transfer from the pipes into the room.
Fixing with plastic clips to clip rails
Moulded plastic rails are fastened to the insulation layer with simple connectors, forming a base for fixing the pipes, which are placed in the pre-formed slots in the rails; their spacing module is determined by calculation.
Fixing into the insulation board with plastic clips
With this method the pipes are fixed into the thermal and acoustic insulation layer either by hand or with a special stapling tool, so that the damp-proof membrane or heat-reflecting foil is pierced.
Pressing the pipes into a system insulation board
The pipes are pressed into a purpose-shaped system insulation board, which may be formed and modulated in various ways by rows of studs, including the upper damp-proof layer. This makes it possible to achieve different pipe spacings as well as changes in spacing (closer spacing) in the edge zones. It speeds up installation considerably, makes the fitters' work easier and ultimately reduces labour costs.
Joining with heat-spreader plates
In essence this again involves laying the pipes in pre-prepared system thermal and acoustic insulation boards, but with precisely shaped channels into which metal heat-spreader plates are inserted. This solution requires a thinner concrete screed and gives a more even distribution of floor surface temperatures, but it is more labour-intensive.
Joining into plate cassettes
In this case the solution using heat-spreader plates is usefully extended by shaping the plates into cassettes. This method is used mainly for sprung floors.
Fixing the pipes to reinforcement mesh
In principle this is similar to fixing the pipes to welded mesh; the only difference is that here the heated floors are exposed to higher loads, so the first priority is to keep the pipes safe should the integrity of the floor structure be compromised by excessive loading.
Laying modular climate floors
This solution represents a completely different approach. The heating pipes are replaced by climate floors, which are integrated hollow heating panels; a heated floor is created by connecting them together.
Laying the heating pipes
When uncoiling the pipes while installing underfloor heating yourself, you can proceed in two ways:
• The coil of pipe is placed in the centre of the room and the pipe is gradually uncoiled and laid in a spiral with a spacing of 2 x 1. When bringing the pipe back, the whole coil must be turned through 180° so that the pipe uncoils smoothly without twisting.
• You can also lay the pipe the other way round – from the manifold to the room in question. In the room, work from the perimeter towards the centre in a spiral with double spacing, 2 x 1. After the central bend, work back along the spiral to the manifold. The permitted bending radius is specified by the pipe manufacturer; for our polypropylene pipes the permitted bend is 20 D, which is 400 mm.
Once the required length has been uncoiled, the pipe is cut with a hand saw. The cuts must be square to the axis of the pipe. The cut ends must be deburred with a utility knife, chamfering the edges at an angle of 45° at the same time, and smoothed.
The end of the pipe is joined to the fitting either by welding or with a mechanical coupling, depending on the type of fitting. When installing underfloor heating yourself, the pipes can be joined:
• by welding, using a socket and a special welding tool,
• with a plastic push-fit coupling,
• with a metal push-fit coupling.
Before the pipe is joined to the fitting by socket fusion welding, the parts to be joined must be cleared of coarse dirt and wiped with a dry cloth.
When installing underfloor heating yourself, the coupling is taken apart and the individual parts are slid onto the pipe end in the prescribed order. The pipe end with the parts fitted is pushed into the body of the coupling or adaptor and secured with the union nut. The nut is tightened to the torque specified by the manufacturer. The torque can be checked by hooking a calibrated spring balance onto the handle of the pipe wrench and reading the pulling force on the balance.
Joining heating pipes within a heating circuit that is cast into the floor structure is not recommended. Near windows or external walls, lay the pipes more closely in a strip 0.5 to 1.0 m wide, at half the normal spacing.
At a bend the pipe needs to be heated, for example with a hot-air gun for welding PVC, then bent and fixed. Once the heated pipe has been bent and tied down, you can carry on working without cooling the bend.
Wherever there is a risk of mechanical damage to the pipes (for example where they cross an expansion joint or leave the floor structure), they must be protected with a flexible sleeve or felt wrapping.
Forming expansion joints
One thing to watch when installing underfloor heating yourself is the forming of expansion joints. Given the destructive effect of the regularly repeated heating and cooling of the floor structure, attention must be paid to the layout of expansion joints in the underfloor heating build-up, in accordance with STN 73 2400 and STN 73 2001. The dividing (expansion) joints run continuously from the thermal insulation up to the surface of the heated floor. When planning them, avoid having joints cross the heating pipes. If this cannot be guaranteed, the pipes must be protected with a sleeve at the crossing point, as described above.
The expansion joints must be made so that they really do allow the floor to expand. The joint should be at least 8 to 10 mm wide. All expansion joints must be filled with a suitable permanently plastic material.
The length of an expansion bay should not exceed 8 m and the ratio of its sides should be no greater than 1 : 2. The layout of the expansion joints also depends on the geometry of the room. L-, T- or Z-shaped areas are divided into rectangular or square fields, and expansion joints are also formed at external door openings.
Laying the concrete screed
Once the pipes have been laid, a pressure test must be carried out before they are concreted in, to detect any faults. During this test the system is vented and filled with water. The concreting itself then takes place with the system in this state. Concreting is one of the final stages in installing underfloor heating yourself.
The thickness of the concrete layer is determined by calculation in the design. The concrete mix is delivered to where it is needed by pump or in wheelbarrows, and the production, transport and inspection of the mix must follow STN 73 2400. If the mix is brought in by wheelbarrow, the pipes must be protected against mechanical damage, for example by covering them with wooden boards.
When spreading the mix, make sure that:
• the edge insulation strip remains continuous and undamaged,
• the fine and liquid components do not run into the thermal insulation layer, let alone beneath it,
• the pipes do not sag or float during concreting and their position remains unchanged,
• the pipes are not mechanically damaged or otherwise impaired.
An important detail to watch when installing underfloor heating yourself is that, while it is setting and in the early stages of hardening, the concrete must be kept at normal temperature and humidity (the room temperature must not fall below +5 °C). Nor may it be exposed to vibration, impact or other harmful effects such as excessive cooling or heating, or excessive drying caused by draughts.
These requirements mean that the concrete has to be kept at least damp for 7 days, either by protecting it against excessive evaporation of water from the mix, or by wetting it, or by a combination of the two.
Excessive evaporation is prevented by covering the surface of the concrete with sheeting or matting. Products designed for curing fresh concrete can also be used. When laying an underfloor heating system, it is essential to make sure the temperature in the working area does not fall below 10 °C.
The water used to wet the concrete must comply with STN 73 2028 and should be no more than 10 °C warmer than the surface temperature of the concrete. Wetting must begin as soon as the concrete has hardened enough not to be washed out. During this process, make sure the room temperature does not fall below + 5 °C.
For the functional test, the floor may be heated once the concrete has reached the strength corresponding to its class, but no sooner than after 28 days. The temperature must be raised smoothly and evenly. The heating water temperature should not be increased by more than 5 °C per day.
Laying the floor finish
The last step in installing underfloor heating yourself is laying the floor finish. Once concreting has been completed by levelling and smoothing the surface, preparations are made for laying the floor finish. The concrete surface usually needs to be levelled to a thickness of about 15 to 25 mm with a cement screed base of fine-grained MC-100 mortar, in accordance with STN 72 2430.
Depending on the material, the floor finish is fixed in one of the following ways:
• laying in a levelling cement bed (tiles, stone, etc.),
• laying in a layer of adhesive (PVC, parquet, carpet).
The floor temperature can be raised gradually over three days until the operating temperature is reached. When laying floor coverings, the further requirements set out in STN 06 0312 must also be taken into account.
References:
Dušan Petráš, Miroslav Kotrbatý: Heating of Large-Space and Hall Buildings
