Floating screed: definition and principle

What is a floating screed?

A floating screed is one that does not rest on any structural element. It is poured over an intermediate compressible layer, such as thermal insulation or a resilient underlay, and is separated from the walls around its entire perimeter. There is no rigid contact between it and the structure, neither at the base nor along the sides.

The term "floating" describes a position, not a composition. A floating screed can be traditional, liquid, anhydrite, or lightweight. What defines it is its discontinuity from its surroundings.

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How is it constructed?

The system is built from the bottom up. First, the substrate: a concrete slab, floor joists, or an existing floor. Next, a moisture barrier, if needed to block rising damp. Then, the compressible layer: insulation panels or spray foam for thermal purposes, or a flexible underlay for acoustics, sometimes with both layered together. An edge strip runs along the entire perimeter, including around columns, thresholds, and pipe penetrations. Finally, the screed is poured on top, without ever touching a vertical wall.

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What is a floating screed used for?

There are three main reasons for its use, often combined.

• Acoustic insulation. This is the principle behind a floating floor, which breaks the transmission of impact noise to the levels below.
• Thermal insulation. It allows for a continuous layer of insulation to be placed between the cold floor and the room, without any gaps or thermal bridges.
• Underfloor heating. The insulation under the screed directs heat upwards instead of letting it escape into the slab.

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What thickness should you plan for?

A floating screed must be thicker than a bonded screed, precisely because it is not supported. It flexes across its entire surface, like a slab resting on a mattress. In residential settings, a traditional cement screed over insulation is generally at least 6 cm thick. A liquid screed achieves the same performance with less thickness, which is important for renovations.

The exact thickness depends on three parameters: the type of screed, the compressibility of the insulation underneath, and the expected loads. It is a calculation, not a default value.

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Floating screed or bonded screed?

The bonded screed is integrated with the substrate. It can be much thinner, supports heavy loads, and dries faster, but it does not allow for intermediate insulation or acoustic decoupling.

The choice is settled by a simple question: is there anything to be placed between the screed and the substrate? If yes, use a floating screed. If not, use a bonded screed. As soon as there is a need for thermal or acoustic comfort, the answer is already known. Our acoustic solutions are all based on this principle.