Floor Screed for Radiant Floor Heating - DIY Repair

What is the best material for floor screed?

Screeds for radiant floor heating can be made using either wet method (cement-sand mixture) or semi-dry method (also cement-based, but with less water content). Dry screed is not suitable for wet radiant floor systems as moderately moisture-resistant GVL sheets fear moisture.

Photo 1 — Electric Heated Floors

The required elasticity of the cement base can be achieved by adding plasticizer to the screed mixture. Thanks to these special emulsions, the density of the screed increases, less air enters the mixture, which itself increases strength and thermal conductivity of the base.

IMPORTANT! Please note: the heating pipe or cable, whether electric or water-based, must be well surrounded by this mixture with room for thermal expansion.

Photo 2 — Water Heated Floors

Thickness of Screed Over Radiant Floor

The thickness of the screed depends on the type of screed and anticipated loads on the floor. For example, in rural areas, the screed layer may not exceed 45 mm provided that the total height of the radiant floor screed, including hidden in the cement communications, is 65 mm.

Liquid screeds are more expensive material, but the price is justified: pouring radiant floor screed takes less time, it is lighter in all respects. Self-leveling floors are stronger than regular ones, so they take up less height: just 25 mm over the heated floor with a total screed height of 45 mm.

IMPORTANT! The screed mixture for radiant floor is prepared in a ratio of 1:4. The composition includes sand with a grain size of 0.8 mm, cement 200–250 kg/m³ of concrete and water.

Thickness of Screed Over Water-Based Radiant Floor

For MV 12 pipes, the correct screed should be approximately 60 mm high, and the total structure height for MV 17 pipes is 65 mm (the water floor without screed is about 20 mm in height).

IMPORTANT! Before making a wet screed for radiant floor, consult specialists: the weight of the structure may amount to around 150 kg/m²; not all slabs can withstand such additional load. It is also important to choose screed for radiant floor so that the screed material provides good thermal conductivity, ensuring the heating system works in its usual mode.

Photo 3 — Some Possible Additives to Screed for Radiant Floor

Proper Screed Installation: Expansion Joints

Since the floor base will expand due to constant heat exposure, it is necessary to prevent cracking, scratches on the screed, and deformation of the heating elements. For this purpose, the screed must be made mobile: compensating strips are laid along the walls, expansion joints are installed.

The mentioned compensating strip is a piece of soft polyurethane foam applied to foil. The foil side of the strip is placed on the thermal insulation as a kind of sealant. The strip is mounted along all sides of the room.

Regarding expansion joints, they are installed:

around entrance doors;
when the floor area exceeds 40 m²;
if one of the walls is longer than 8 meters;
when the ratio of length to width of the room is 1:2;
in rooms with complex shapes.

Insulation Under Screed

If you plan to heat the ceilings of neighbors or warm external walls, take care of an insulating layer placed under the screed. Thermal insulation screed or additional insulating material will help retain up to 25% more heat in the room.

Photo 4 — Pouring Screed by Standard Method Using Rails

Screed for Water-Based and Electric Radiant Floor: After Installation Works

After laying the radiant floor system and pouring the screed, once renovation is almost complete, it is important to properly and gradually warm up the floor. The floor base should be heated after the established technology, regulatory period for setting of the screed before laying the finishing floor covering.

The temperature of the heating pipes should be raised gradually. Throughout the warming period, it is necessary to isolate the screed from direct sunlight and drafts. When warming up by yourself rather than ordering master services, be extremely careful: any mistake at this stage can damage the system.

Radiant Floor Screed: VIDEO

$ Radiant Floor Screed: Cost

in Moscow − from 195 rubles/m²;
in Kiev − from 45 UAH/m².