Screed for heated floors: options, thickness and solutions

Heated floors are often installed as an alternative to radiators. The large heating area and the ability to regulate the temperature allow you to create heat not only in a private home, but also in an apartment. Even if the radiators heat the room well, it is much more pleasant to step on warm tiles or laminate.

The heating system is installed at the stage of preparation and laying of floors. If the customer wants this heating option, then they need to inform them in advance so that the workers can correctly calculate the amount of materials. For private houses, it is preferable to organize a heated water floor, powered by a gas boiler or a boiler for solid fuel. Electric or infrared heating elements are often installed in apartments.

Tasks of screeding heated floors

To figure out how to make a screed for a heated floor, let’s define its tasks more precisely. First of all, remember that ties are fundamentally divided into tied and floating. The floating screed is not connected either to the base of the floor or to the walls of the room. If, relatively speaking, you look at a floating screed, then it is a separate, often concrete slab in the room that performs its tasks.

It is worth noting that a floating screed is the priority type of screed in the construction of a concrete heated floor.

The tasks of a floating heated floor screed include:

Holding pipes or underfloor heating cables;
Be a heat exchanger between the floor heating system and the room, ensuring uniform heating of the floor surface;
Take on the power load of operating the premises.

It is to solve these problems that specific requirements for installing heated floor screed are aimed.

Requirements for a heated floor screed

This is the time to remember about the types of heated floors. Fundamentally, there are two types of underfloor heating: water and electric. In a water heated floor, heat is transferred to the screed from the water circulating through the system. In an electric heated floor, heat is obtained from heating special heating cables laid in the floor.

Note: When we talk about heated floor screed, we mean only water heated floors and cable electric heated floors. Covering electric heated mats with screed is not required. Also, a screed is not required for TP (warm floor) flooring systems used in wooden houses and in houses with floors on joists.

Total screed thickness

So, two tasks of the TP screed, distribution of operating load and uniform distribution of heat, are a priority. It is to accomplish these tasks that all further discussions about the thickness of the screed will be directed.

Two parameters are important in screed thickness. The first is the overall thickness of the screed, which is usually left out of brackets. The second parameter is the thickness of the screed layer above the pipes. First things first.

The total thickness of the heated floor screed, namely the screed, and not the entire heated floor structure, should be as follows:

If the heated floor is installed above an unheated room, basement, or ground, the minimum thickness of the screed is 85 mm. This standard value is very controversial (more on this below);
If the TP is done on a concrete slab, the total minimum thickness is the sum of 10 mm of screed under the pipe (cable), the diameter of the pipe (cable) and the permissible technological thickness of the screed above the pipe.
It is recommended NOT to make the maximum thickness of the screed more than 100 mm, due to the large inertia of the system. Thick screeds will take a long time to heat up and spend heat not on warming up the room, but on heating the screed itself.

Note: if you need to raise the overall floor level, you do not need to do this with a heated floor screed. You need to first make a leveling screed, and then install a warm floor, and not try to solve all the problems in one screed.

Maximum

Covering with a thick layer of fill makes the pipes well protected. This layer retains heat for a long time and distributes heat well. But at the same time, its device is more expensive, and its operation may require unjustified energy costs

It is important not to overdo it and make a layer of moderate height

For ordinary apartments and houses above a heated floor, a maximum height of the protective layer of screed covering the pipe is allowed to be 60 mm. This thickness is justified if a regular, not very strong mixture of cement and sand is used. This will make the surface sufficiently durable and maintain the possibility of acceptable heat transfer.

Attention A thick floor has high thermal inertia and heating it up requires a long time and possibly an increased temperature of the coolant.

Screed thickness over pipes

This is an important parameter on which, first of all, the uniform heating of the floor depends, and secondly, the load perception (strength).

I’ll tell you right away about durability. The following rule works here: the thicker the underlying insulation in the TP structure, the greater the thickness of the screed above the pipes should be. For a central climate, a sufficient insulation thickness is 2, maximum 3 cm.

To answer the question about the relationship between the thickness of the screed over the pipes and uniform heating, let’s look at the thermal diagram of the heated floor.

As you can see, the heat from the pipes rises along the screed along peculiar cones. Optimal heating of the floor will occur if these cones “end” on the surface of the screed. If the screed is made thinner, then not so much the screed will heat up as the finishing coating, which is bad. If the screed above the pipes (cable) is made thicker, then the heat will not reach the surface of the screed. On the surface we get the so-called strip, uncomfortable heating of the floor, otherwise “zebra”. Walking along the floor you will feel the change of warm and cold stripes. This is also bad.

As you can see, it is the thickness of the screed above the pipes that is the most important technological parameter of the TP screed. The load on the floor in residential premises is moderate and the thickness parameter is not affected by the load on the floor.

So, the thickness of the screed over the pipes. Without further ado, let’s look at the recommendations on this parameter from the main manufacturers of water heated floor elements, namely Oventop, Uponor, Valtac, Thermotech, KAN. Summarizing their recommendations, I can draw the following conclusion:

When installing a heated floor on a concrete base, with a layer of insulation no more than 20 mm, the thickness of the screed above the pipes should be:

At least 30 mm for a wet solution with the addition of plasticizer and fiber;
At least 50 mm for wet mortar (concrete or cementitious mixture);
At least 45 mm for machine-made semi-dry mortar.

Thermal insulation of the base

Before installing the heating system, it is necessary to insulate the base. The thermal insulation layer will protect the water circuit from heat loss going into the basement or foundation of the house. The following is used as insulation:

Styrofoam;
extruded polystyrene foam;
foil-coated backing.

The listed materials are characterized by high strength to mechanical load, resistance to moisture and temperature changes. The thickness of the insulation is 30-50 mm, it is laid on a layer of waterproofing polyethylene film. The canvas is laid overlapping on a clean, dry base; it will protect the insulation from getting wet. Thermal insulation boards are laid end to end, the joints are coated with glue or blown in with foam, the excess of which is cut off.

Penofol or a similar sheet of reflective insulation is placed on top. A layer of aluminum foil is directed upward, this will reflect the bulk of the heat back towards the room. The joints of the foil insulation are taped with special tape. Penofol will protect the insulation from the aggressive effects of concrete. A metal reinforcing mesh is laid over the thermal insulation. This element evenly distributes the high load from the water circuit system and the concrete structure.

Option 1. Concrete screed for heated floors

Wet concrete screed for heated floors is made using floating floor technology, based on a solution of cement, sand, crushed stone and water. To make your own solution, you need to make a B22.5 (M300) solution with the obligatory (!) addition of crushed stone or gravel of 5-15 mm fractions to the solution.

The classic concrete mortar M300 (concrete B22.5) is made on the basis of M400 cement. Concrete proportions (Cement: Sand: Crushed stone) - 1: 1.9: 3.7.

It is important to note that this brand of concrete is heavy concrete and is difficult to install. The load of such a screed per 1 m² of flooring will be 125 kg, with a screed thickness of 50 mm. And this does not take into account the weight of the heated floor “pie”.

All this reveals the disadvantages of the “concrete” option:

When laying concrete, damage to the pipes (cables) of the heated floor is possible;
Not possible to compact, may lead to the formation of air bubbles;
The difficulty of leveling concrete will require an additional layer of leveling screed.

The floating concrete screed (6) is necessarily reinforced with a mesh (2), welded in knots, with cells of 10 by 10 cm. The reinforcing mesh not only holds the screed together, preventing it from cracking when the floor dries and is in use, but also serves as a base for fastening ( 4) pipes (5) water floor and electric floor cable. It is important (!) to ensure that the mesh (3) rises 10 mm from the insulation (film) (1).

The concrete screed must be insulated from the walls with a damper. This is a special tape or any dense insulation no thicker than 1 cm.

Possible errors during installation

Newbies at work or simply men who prefer to do home repairs themselves often make mistakes. They boil down to the following factors:

incorrect and inaccurate setting of the level and beacons - leads to the formation of waves on the surface;
failure to comply with proportions increases the risk of cracks;
thin layer - leads to cracking of the finish coating;
no expansion joints - recommended when working in a room larger than 20 square meters. m, otherwise the finishing coating will be destroyed;
the surface is not reinforced - it is better to reinforce the screed layer to prevent cracks;
improper care (before the cement layer dries) also leads to the formation of cracks.

Which screed for a water floor is better, everyone can determine for themselves. When choosing, you should be guided by the advantages and disadvantages, technology features and precautions. Otherwise, the topcoat will not withstand constant use.

Option 2. DSP

Cement-sand screed for heated floors is made using floating floor technology, based on a solution of cement, sand and water with the addition of plasticizer and fiber, or based on ready-made dry mixtures.

Important! Plasticizer and/or fiber are mandatory in underfloor heating screed.

If you are doing TP at home, M200 grade cement is sufficient to prepare the DSP solution. The sand needs to be clean. Sand/cement proportions, 3/1 (three sand - one cement).
If the heated floor is made in a garage, then the grade of cement is taken higher from M300 to M500, optimally M400.

An obligatory element in the DSP TP solution is reinforcing fiber and plasticizer fibers. Fiber fiber is added in a volume of 900 g. per cube of solution. Fiber plays a reinforcing role.

Important! A plasticizer (this is not fiber) is added to any type of DSP underfloor heating. It (plasticizer) compensates for the thermal expansion of the heated floor, protecting the central fiber board from cracking.

The TsPS TP screed is separated from the base of the floor. For this purpose, a layer of polyethylene with a thickness of 200 microns is laid on the base. For better heat transfer, a layer of heat insulation with a thickness of 20 mm or more is placed under the screed. Important! The foil backing is not insulation.

Important! For trouble-free operation of the heated floor, the heated floor screed must be laid only on a flat, solid base. Unevenness of the base can lead to the formation of air cavities, which can shrink under load. If the base of the floor is uneven, under the heated floor screed, you need to make an additional leveling bonded screed.

The DSP screed will be separated from the walls of the room, for which a damper tape or strips of any solid insulating material 5-10 mm thick are fixed around the perimeter of the room.

A floating DSP screed must be reinforced with a mesh with 10 by 10 cm cells. The reinforcing mesh not only holds the screed together, preventing it from cracking when the floor dries and the heated floor is in use, but also serves as a base for fastening water floor pipes and electric floor cables. It is important to ensure that the mesh rises 10 mm from the film.

The thickness of the screed layer, in the version of a cement-sand screed without fiber with a reinforcing mesh, cannot be less than 10 cm. It is this layer that will ensure the creation of a durable floating slab. In this case, the thickness of the screed must ensure that the pipes (cables) are covered with a layer of at least 30 mm, otherwise there will be strip heating of the floor.

Ready-made mixtures will simplify the work

There are a lot of manufacturers producing ready-made mixtures (levelers), including those suitable for installing heated floor screeds. A solution from this mixture is made by adding water in the proportion indicated on the package. The leveler is laid with or without beacons, depending on the brand and manufacturer.

The use of ready-made mixtures is the best option for installing screeds for heated floors yourself.

Minimum indicators

In accordance with generally accepted standards, the minimum acceptable screed thicknesses are as follows:

2 cm – for ready-made self-leveling mixtures;
4 cm – for cement screed without reinforcement.

With a smaller layer thickness, it simply will not be possible to achieve the required wear resistance and strength.

Thickness indicators. Choosing the best option

Filling a thin screed is only possible if:

there is already a subfloor;
a rough screed was installed, through which significant unevenness was leveled;
There are no reinforcing elements in the “pie” design.

In addition, a 2-4 cm screed cannot be used in rooms with high load rates. These include: toilets, bathrooms, corridors and kitchens, various technical rooms.

Video - About installing a warm water floor

Option 3. Semi-dry screed on heat-reflecting boards

To simplify the installation of heated floors, companies began to produce special slabs of remarkable designs. These slabs, on the one hand, are ready-made channels for laying cables or underfloor heating pipes, on the other hand, they take on part of the operational load and create a layer of heat insulation.

In this version of underfloor heating, a semi-dry screed is usually made, based on cement-binding mixtures with the addition of plasticizers. The best quality of such a screed can be achieved by using ready-made mixtures of levelers for heated floors or by purchasing ready-made semi-dry factory-made mixtures.

Pros and features of semi-dry screed

A semi-dry screed does not require reinforcement and dries much faster than wet screeds.
The semi-dry screed is laid with a damper tape.
The thickness of the screed should ensure that the pipes (cables) of the heated floor are covered with a layer of 40-60 mm.

Minuses

However, a semi-dry screed, porous and in the construction of a heated floor, will significantly increase its inertia;
In addition, semi-dry screed requires serious professional skills, which makes it difficult to recommend it for independent use.

Sequence of work

Before the screed, waterproofing, insulation and water-bearing pipes are laid. Be sure to reinforce the surface with metal mesh

It is important that each twig is at least 3 mm. The fittings should be at the bottom, and pipes are attached to it

But there are experts who prefer to place the mesh above the water pipes for optimal load distribution. This will allow enough heat to escape from the concrete layer.

The waterproofing layer is applied over the rough screed using roll or liquid waterproofing. Next, they begin to install the insulation layer. Typically, dense polystyrene is chosen. The slabs are stacked compactly due to the presence of locking devices on the sides. The screed for heated floors is poured with concrete of such a consistency that it is easier to level. The solution is mixed by hand or with a concrete mixer. It uses 1 part cement and 4 parts sand, which is pre-sifted. Also add enough water to knead the dough-like mixture and a plasticizer for elasticity.

To make the screed strong, it is recommended to add fiber to the solution, which consists of synthetic fibrous materials.

Pouring the floor with mortar Source couo.ru

It is important to install it at the same time. You can’t fill one area and then do the rest a few days later.

If you do this, cracks will appear in the future.

You should check the condition of the pipes; when the concrete hardens, there will be no access to them. Test the system by connecting the operating water pressure and check for leaks during the day. If smudges appear, they must be eliminated and only then proceed with filling.

conclusions

The article discusses several options for screeding heated floors:

TP screed in a high-load room. It is made with B22.5 concrete, possibly on soil with a thickness of 85 mm or more, with obligatory mesh reinforcement. Concrete underfloor heating system.
TP screed in a house (apartment) is made with a cement-sand mixture reinforced with mesh or plastic fiber on a 2-3 cm insulating substrate with the obligatory addition of a plasticizer.
The semi-dry screed technology is NOT recommended for independent use , excluding small rooms (bathroom, toilet) due to the complexity of the technological processes in the production of the solution and its installation.

Nuances and secrets when styling

Craftsmen can share their experience gained over years of work and pouring screeds under water-heated floors. The following recommendations will help novice professionals or home owners:

The presence of cracks or chips on the dried surface prohibits laying out the next layer. It is necessary to repair any defects that have arisen. This is done by pouring additional mixture.
Waterproofing is always required in the bathroom and kitchen. The finishing layer is laid on top of the waterproofing layer.
When using the dry variety, it is better to additionally use thermal insulation. It is also installed in a private home, which provides additional comfort for the family.
The screed should dry under protection from sunlight. Therefore, you should hang thick curtains on the windows; it is better to use blinds.
The surface must be clean and dry before pouring the mixture. This will prevent cracks from forming.
It is recommended to coat the surface with a primer. It forms a film, therefore providing a water-repellent effect and making the mixture adhere to the surface more firmly.

Compliance with the presented nuances will help you avoid mistakes when working independently. It is recommended to take technology into account and use expert advice.

Regulations

SNiP 2.03.13-88 paragraphs 2.144-1/88 (nodes 63, 69, 75, 81, 87), 2-244-1 (nodes 140,147,149, 160, 161).
Design of heated floors: DIN 18560 and SNiP 41-01-2003.

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