Impact sound insulation under screed — what to check before installation

Before installation, check the complete floor build-up, whether the insulation is suitable for the specified screed, and the details at walls, pipes and thresholds. Insulation thickness alone, or a decibel figure in a product data sheet, does not guarantee the resulting impact sound insulation between rooms.

This guide covers heavy floating floors with screed on a reinforced concrete slab. It is not a design guide for timber floors, dry floor systems or underlays beneath laminate or vinyl.

1. Distinguish insulation under screed from an underlay beneath the floor finish

Impact noise arises, for example, from walking or dropping an object onto the floor. The mechanical impact makes the structure vibrate, and sound can travel to other rooms.

In a heavy floating floor, the screed rests on a resilient layer and is separated from the surrounding structure. Here, “floating” describes how the screed is supported, not merely the click joints of a laminate floor.

An underlay directly beneath the final floor finish belongs to a different build-up. Its technical figures cannot automatically be used to assess insulation under screed. When choosing a product, ask: Is it intended for this type of screed and this load?

2. Gather information about the complete build-up

Before ordering, list the layers from the load-bearing slab to the finished floor surface:

• type and thickness of the load-bearing slab;
• levelling layers and services;
• exact designation of the impact sound insulation;
• any thermal insulation and underfloor heating system panels;
• screed type, designed thickness and mass per unit area;
• final floor finish;
• use of the rooms above and below the slab.

Take the information from the design and technical documents. Estimating the structural slab thickness, or confusing it with the overall floor thickness, can produce a misleading calculation.

For refurbishment, also consider the total mass of the new build-up. The appropriate designer or structural engineer must assess the slab’s load-bearing capacity; the acoustic calculator does not check it.

3. Do not rely on insulation thickness alone

For insulation under screed, dynamic stiffness, compressibility, suitability for the designed load and conditions of use also matter. A thicker material therefore cannot be described as acoustically better without further information.

Ask the supplier for a current technical data sheet and installation instructions. Check the product’s intended use under screed, installation conditions, compatibility with other layers and any restrictions for underfloor heating.

Do not choose screed thickness from a universal number found online. It depends on the specific system, material, load and design. A condition stated for one tested build-up is not a general recommendation for all floors.

4. Check where acoustic bridges could occur

Even a suitable material can lose part of its benefit if the screed becomes rigidly connected to the surrounding structure. Such a connection creates another path for impact noise transmission.

Before pouring, check:

• perimeter separation at walls, columns and other vertical elements;
• pipe penetrations and their separation from the screed;
• connections at doors and thresholds against the design details;
• continuity of the insulation layer, absence of damage and treatment of joints according to the manufacturer’s instructions;
• protection against screed material entering gaps where it could create a rigid connection.

These details must remain correct when the floor is finished. Skirting boards and connections between the floor finish and walls deserve particular attention. Have the installer confirm the solution against the design and installation documents.

5. Do not confuse insulation improvement with the result for the whole structure

You may encounter ΔLw in a product data sheet. It expresses the improvement in impact sound insulation under specified test conditions. It is not the final impact sound level in your room.

L’n,w describes the weighted normalized impact sound pressure level. For this quantity, a lower value means a better result. For the improvement ΔLw, a higher value is more favourable — always in the context of the conditions under which it was determined.

Do not read one material’s improvement figure as a promise that the same number will be the final result in a particular apartment. The structural slab, complete build-up, flanking transmission paths and installation also matter.

The requirement for specific rooms must be established using the relevant standard and design. One universal limit for every building is not sufficient.

6. Make an indicative calculation before installation

If you are planning a supported build-up, an indicative calculation can help identify potential risk before ordering materials or pouring the screed.

Open the English ImpactCheck calculator

ImpactCheck uses a model of a heavy reinforced concrete slab with screed and the specific insulation product Insulit Bi+9. It is not a universal comparison tool for all insulation products. The result cannot be transferred to another product simply by retaining the same inputs.

Choose the standard appropriate to the project: STN for Slovakia, ČSN for Czechia or ÖNORM/OIB for Austria. The English version initially defaults to the Austrian mode unless a manual choice has already been saved. Check the active mode — the calculator remembers your selection. The Austrian mode uses L’nT,w for the OIB comparison; it must not be treated as identical to L’n,w.

The calculation is indicative. It does not replace acoustic design, structural assessment or measurement of the completed building. If inputs are unclear or the result is close to the requirement, consult a specialist before installation.

Checklist before pouring

• I know the actual structural slab and complete floor build-up.
• I have the technical data sheet and installation instructions for the specific insulation.
• The screed and loads have been designed for the chosen system.
• Perimeter strips, penetrations and connections follow the design.
• The insulation is undamaged and rigid acoustic bridges are avoided.
• I distinguish ΔLw from L’n,w and do not treat L’nT,w as identical to L’n,w.
• If I use the calculator, my build-up is within its scope and the correct standard is selected.

Most importantly: Before installation, check not only the product but its use within the complete build-up and the connection details.