Separating layer

What is a separating layer in a floor build-up?

A separating layer (separačná vrstva) is a thin sheet, usually polyethylene foil, that stops a screed from bonding firmly to the layer below it. Baumit's Slovak technology guide defines it as the layer that prevents a rigid connection between the screed and its base. In a floating screed (plávajúci poter) the layer below is the insulation, so the foil lies on the insulation boards. The screed goes on top of the foil.

The foil carries no load and gives no thermal insulation. Its job is separation. The screed rests on the foil and not on the base. The slab can then move freely when it shrinks or warms.

Where does the separating layer sit in a floating screed?

The separating layer sits between the insulation and the screed. Baumit's screed sheet describes the build-up this way. Floor insulation boards, or a layer of lightweight concrete, form the insulation layer. A separating foil divides that layer from the screed. The guide names the foil Baumit Separačná fólia FE and says it separates the screed from the thermal insulation.

The same guide sets two conditions on the layers around the foil. The insulation must be a material with small deformation under load. The screed must also be separated from walls and other vertical parts by a flexible edge strip, which is a different component. For that strip, see edge insulation strip.

Baumit names two cases where the foil is not used. If a special sound-insulating underlay is chosen, the screed is laid straight onto that resilient underlay without the foil. If a system board or mat holds the heating pipes, the foil is no longer needed. Both are Baumit's instructions for its own system, so ask the screed supplier before you leave the foil out.

How wide should the overlaps be and how should the foil be laid?

Lay the foil flat, without folds, on a prepared base, and pour the screed straight after it. Baumit's data sheet for its 0.05 mm PE separating foil recommends overlaps of at least 100 mm. Baumit Slovakia’s paper separating foil FE is a different product: its product page says to lay it with overlaps of 5 to 10 cm. It also allows butt joints taped with a suitable adhesive tape. The sheet gives the reason: protection against screed mix leaking through. It also says the foil must not span cavities in the base. The figures below belong to this one product. Another maker's sheet may give other values. The sheet of the product you install governs.

Detail	Baumit PE separating foil (Baumit data sheet, 2026)
Thickness	0.05 mm, LDPE
Joints	Overlap of at least 100 mm, or butt joints taped
Voids in the base	The foil must not span them
Screed	Apply the screed immediately after the foil
Temperature	+5 °C recommended for air, material and base during installation
UV light	Do not expose for long, UV resistance is not declared

This sheet gives no wall upturn height. No other manufacturer document opened for this page gave one for wet screed. Show any upturn on the detail drawing and agree it with the screed supplier. A UK manufacturer guide gives other overlap and upturn figures. It covers dry floors of chipboard and fibre boards, so this page does not use them.

Does the separating layer stop screed cracking?

Not by itself. The foil removes the rigid bond to the base, so the screed can move as a free slab. Baumit's guide says the separating layer must be continuous. A break in it would form a rigid connection that blocks the movement of the slab.

Other parts take up the movement. The guide uses an edge strip around the perimeter. It also uses movement joints across the screed, made with an edge strip and an L profile. For cement screed without heating, it limits bays to 40 m2. Sides are at most 8 m long, in a ratio of at most 1:2. These are one manufacturer's figures, so agree the bay layout with your screed supplier. See expansion joint for the joints in the screed field.

How does the separating layer affect impact sound?

The foil is not what insulates impact sound. In Baumit's build-up the insulation layer consists of floor insulation boards or impact-noise materials. The foil only separates that layer from the screed. A thin foil does not replace the resilient layer and should not be counted as part of it.

Sound can still bypass the floating layer where the screed touches a wall. Baumit says its edge strip also stops noise passing from the floor into vertical structures. This path has a name: flanking transmission. For the layer that does the acoustic work, see impact sound insulation.

How is a separating layer different from an insulation layer?

Both layers sit under the screed, so they are often confused. They have different jobs, as the table shows for a floating floor.

Feature	Separating layer	Insulation layer
Main job	Prevents a rigid bond between screed and base	Reduces heat loss and, in suitable products, impact sound
Typical material	Thin polyethylene foil, for example 0.05 mm in Baumit's sheet	Floor insulation boards, such as EPS or XPS or mineral wool, with small deformation under load
Thickness	Set by the product	Set by a thermal calculation, according to Baumit's guide

Confusing the two leads to specification errors. A floor with insulation and no separating layer lets screed mix run into the joints between boards. A floor with foil and no suitable insulation has no thermal performance.

What should a Slovak designer check on a floating floor?

This page draws on manufacturer documents from Baumit and Ceresit. It does not rest on a Slovak statutory rule for separating layers. No Slovak act was part of this check. Baumit and Ceresit both cite STN 74 4505 (2013), Podlahy. Spoločné ustanovenia, as the design basis for floors. Its text is behind a paywall, so this page did not read it. For a Slovak project, check these points:

• the foil's data sheet, for overlap, taping, installation temperature and UV exposure,
• the screed supplier's guidance, for thickness (Baumit recommends 45 to 70 mm for its floating screed) and bay layout,
• the insulation maker's data, for deformation under load and impact sound performance,
• the edge strip and any upturn, drawn on the detail and checked on site before the pour.

Use the Slovak names separačná vrstva and tepelná izolácia on the drawings, so the contractor can match them to the product sheets.