What is the Best Screed for Underfloor Heating? Types and Technology

What is the Best Screed for Underfloor Heating? Types and Technology

A screed for underfloor is a simple cement grout. It is called rough and is made to level the surface, prevent heat loss, pipe deflections. In addition, it affects the durability of the finishing layer, which is poured on top of the heating system. It can be laid in three ways. Let’s talk about them first.

Types of finishing screeds

Dry

It consists of expanded clay , foamed vermiculite, expanded polystyrene or perlite. The granules are covered with sheets of moisture resistant drywall or chipboard. All this is quite simple to arrange – any person can do the job.

Pros

  • High laying speed. Even a large room can be done in a day.
  • You can install a finished floor covering the very next day.
  • The load on the floors is three times less than in the case of wet pouring.
  • Simple installation.
  • Excellent sound, thermal insulation.

Minuses

  • If water gets inside the floor, mold will appear , you will have to disassemble the entire structure. Requires a waterproof finish.
  • Chipboard may not withstand additional heavy partitions.
  • Some time after the completion of the repair, rustling sounds will be heard when walking.

Screed

Wet

Usually it is a cement-sand mortar or concrete mixture. Doing everything yourself will be a little more difficult than in the case of dry backfill, but it is quite possible. Ideal for floors with ceramic, stone finishes.

Pros

  • Strength. Will withstand new, heavy partitions.
  • The dried layer can be left as a final finish by simply painting it over.
  • Excellent sound, thermal insulation.

Minuses

  • Time consuming process. Mortars are thin, but pouring and leveling without skill can be difficult.
  • Great weight. If the overlap is weak, this fill will not work.
  • Long curing time. It will take several weeks before final finishing.
  • You will need mixing equipment – a construction mixer or a drill with a nozzle.

Wet

Semi-dry

Such a coating is difficult to make with your own hands – this requires a pneumatic blower, mixing, and also a grinder. In a small area, you can try to do everything without equipment, but it will be more difficult. It is not easy to level the material on your own, as it is a solid sand-cement mixture. Plasticizers and a small amount of water are added to it.

Pros

  • Tiling can be started after two to three days. For other coatings, the terms are longer, since moisture will be released within a month.
  • Low weight.
  • Good sound and thermal insulation.
  • Relatively clean working conditions.
  • There is practically no shrinkage.
  • Moisture resistance.

Minus

  • The need for equipment. It is rented from a construction company, but this is an additional cost.

Semi-dry

Which is better to choose

It is difficult to answer unequivocally which screed is better for a water-heated floor. Based on weight, dry mixes are the best option. They are much lighter, the load on the floors is not so strong. In addition, they can come out cheaper if repairs are done in a small apartment. They outperform the other two options in terms of laying speed. You can do everything in a couple of days. But this floor is not recommended for use in the bathroom.

For large-scale work, when there is a margin of time before settling in housing and a large batch of material is needed, it makes sense to purchase a wet concrete or cement-sand screed. Also, this option is suitable for those who need a durable coating that is not afraid of moisture.

A compromise could be a semi-dry mixture – it is durable, dries faster when wet. But it requires equipment or the workflow will be laborious.

Screed requirements

First of all, we note that the coating must be even and horizontal. A drop of 5 mm is allowed. Otherwise, the top trim may crack, sag and heat will not be distributed evenly. Let’s tell you more about other requirements.

What material to choose

For warm water systems in residential buildings, a concrete grade M150 or self-leveling compounds for creating warm floors is recommended . For a mortar from a cement-sand mixture, you will need cement of the M500 or M400 brand and sand with a fraction of no more than 3 mm. If you need to make the layer thinner than the recommended values, plasticizers, fiberglass or glue are added to the solution or concrete composition.

Recommended particle size in dry backfill is 5 mm. As for the filler, foamed vermiculite, expanded polystyrene or perlite is considered a more preferable option. They are lighter than expanded clay.

What material to choose

Optimal screed thickness for warm water floor

The indicator is important for any coverage. Not only the quality of heating depends on it, but also the strength of the system as a whole. The thickness is chosen depending on several factors.

  • Ceiling height.
  • Pipe diameter.
  • Type of laying mixture.
  • Overlap strength.

The minimum height for living quarters is 3-3.5 cm. The maximum is 5-10 cm. Too high layers, especially the top one, prevent the room from quickly warming up and eat up the distance to the ceiling. In some apartments this will be very noticeable. If the coating is made too thin, the structure will become less strong and retain less heat. On average, the best option is considered to be a finishing layer thickness of 7-8 cm, where 4-5 cm falls on the space above the pipes. For a rough fill, 3-3.5 cm is enough.

The thickness of the dry coating should be such that there is no empty space between the backfill and the gypsum board sheets, and the heating circuits are at a depth of at least 2 cm from the surface.

The video contains more detailed recommendations for choosing the maximum and minimum thickness.

Rough screed technology

You will need a mixing container, a construction mixer or drill with a nozzle, beacons, a level, usually a spatula, cement, sand and water. Before pouring the draft layer, you need to complete the installation of all communications, window slopes.

Step-by-step instruction

  • Pre plaster walls and the ceiling, to a solution which did not fall.
  • Close the drain holes, fix the cables in a semi-movable state, glue all fittings that may come into contact with the cement .
  • Remove all debris from the surface, determine the level differences in height, make a marking.
  • Install metal beacons, secure them with alabaster (it dries quickly) at a distance equal to the length of the rule.
  • Combine cement and sand in a 3: 1 ratio, add water and a plasticizer (or a liter of glue). Too much water will crack, so don’t make the mixture too thin.
  • Fill the finished solution immediately between the beacons, while tamping it. This is to release air bubbles.
  • Align the composition with the rule. Move it in a wave-like motion towards yourself.
  • Protect the room from sunlight and drafts.
  • On the second day, cover the cement with a film and leave it under it until the surface is completely dry.

This technology minimizes the risk of cracking. Even if you have chosen the minimum screed thickness for a water-heated floor, the coating will be durable. It can take several weeks to dry (3-4). Then the heating system can be laid.

Methods for laying a finishing screed

There are two general rules for all stacking options. Before pouring or filling the top layer, builders recommend sketching the location of all elements of the system, and also checking its performance. There are two ways to test it.

Testing underfloor heating elements

  • With working temperature. Start bringing the system to operating temperature from 20 ° C. Raise 5 ° C after three to four hours. Drain the coolant if leaks appear and fix them. Then start up again. Leave it on for 2-3 days. If all is well, you can do a fine finish by pre cooling the system.
  • With increased pressure. Run the coolant into the system and create a pressure 2-3 times higher than the operating pressure. Leave it for a day. If the pressure drop is not more than 1.5 bar, proceed to filling.

During the pouring of the screed, the pipes must contain a heat carrier heated to 25 ° C.

For pressure testing, it is recommended to additionally secure the pipes. After a successful test run, you can proceed further. We will give you step-by-step instructions for each installation method.

How to fill a wet screed

Pouring in one room should be done in one day, and breaks in work should be minimal. An exception is a surface divided by a damper tape. In this case, the areas can be filled with a break.

  • Fasten damper tape around the perimeter of the room. It is needed so that the frozen floor does not crack during temperature shifts.
  • In a large room, fix it every 20 m 2 , between adjacent contours. At the same time, the pipe passes through it and is enclosed in a corrugation at the place of passage. You can use T-bars instead of tape.
  • Protect the laid water floor hinges from mechanical damage by boards.
  • Insulate the pipes supplying water to the collector. If they are plastic, hide them in the grooves made in the floor insulation.
  • Place a metal mesh with 10×10 cm cells on the heating system.
  • Using a level, install metal beacons with a height equal to the thickness of the layer. Fix the first one at a distance of 20-30 cm from the wall. The following – with a step less than the length of the rule by 20 cm.
  • Secure them with alabaster, wait until he grabs.
  • Prepare a sand-cement mortar in a 3: 1 ratio or dilute the finished composition with water.
  • Spread the material over the surface while tamping it down to remove air bubbles.
  • Smooth out the mixture with a rule.
  • Cover it with plastic wrap the next day.

Drying usually takes 20-28 days. During this period, other finishes must not be installed on top.

The damper tape must not cross the thermal circuits themselves. It is always located between them.

How to install a dry screed

All components of the composition and the surface on which they will be poured must be perfectly dry.

  • Attach damper tape around the perimeter of the room and between heating circuits to compensate for material expansion.
  • Using the level, install beacons. They should be equal to the thickness of the mixture.
  • Lay the two guide rails parallel to each other on top. The extreme one is at a distance of 25 cm from the walls. The gap between the slats should be less than the length of the rule.
  • Place the rule on the guides and start aligning from the far wall towards the exit.
  • Rail can be left or removed. In the second case, the furrows from them are leveled with a wide spatula.
  • In the area where the work was completed, secure the first sheet of drywall or chipboard. The recommended gap between it and the wall is 10-15 mm.
  • Spread the edges of the sheet with glue, carefully lay the next sheet.
  • Secure them with self-tapping screws. Sew the entire floor in this way.
  • Fill the gaps near the wall with polyurethane foam.

The final finish can be mounted the next day – when the glue dries.

How to lay a semi-dry screed

  • Install damper tape around the perimeter of the room and between heating circuits. 
  • Lay a reinforcing mesh with 10×10 cm cells on top of the thermal system.
  • Mix cement and sand with a shovel in a ratio of 1: 3. You can add fiberglass, plasticizer or glue.
  • Place some of the composition next to it and pour some water over it.
  • Repeat this with the entire slide, leave it for half an hour.
  • Then stir the material two to three times with a shovel.
  • Check its readiness. The finished material easily sticks together into a lump, remains wet, but water does not flow out of it.
  • Arrange the beacons and pour the mixture in small portions.
  • Raise the mesh to distribute the composition evenly within the thermal loops.
  • Tamp the material and level the floor surface with a rule.
  • Seal the furrows from the lighthouse the next day in the same way, but add a little more water.
  • If you do not have time to fill the whole room in one day, cut the edge at a right angle.
  • Cover the floor with plastic wrap for two weeks to keep the water from evaporating. Air must not enter under the film.
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