Professional installation of flow screeds onto insulation boards

Flow screeds based on calcium sulphate (anhydrite) or cement are almost self-compacting during installation and therefore have consistently high flexural strengths over their whole thickness. For screeds on insulation, these are decisive for the stability of the screed slab under load. Respect the traffic load-dependent layer thicknesses which are specified in national standards (for ex. DIN 18560 part 2). Calcium sulphate flow screeds may therefore be installed in lower layer thickness compared to traditional sand/cement screeds. Underfloor heating systems are optimally wrapped, which means that the heat is transferred very well.

Simple & safe to the goal

  1. Cleaning

    The untreated floor must be cleaned. Residues of concrete or mortar must be removed. Deeper irregularities must be levelled out prior to the installation of insulation boards. In case of pipes on the floor, level out either with cut-to-size boards or better with the quick-setting lightweight levelling compound weber.floor 4520.

  2. Application of acoustic insulation foam strips

    All walls and upstands (for ex. door frames etc.) within the floor should be separated from the floor construction with an acoustic insulation foam strip (thickness ≥ 8 mm); it must reach downwards from the load-bearing substrate up to the upper edge of the final covering. The strip may only be cut off after completion of the top floor covering (finish), so as to ensure that there will be no acoustic bridge due to residues of levelling compound or adhesive.

  3. Installation of insulation layers

    1. Insulation layers must only consist of approved insulation materials in accordance with national standards (for ex. DIN 18164 parts 1 and 2 or DIN 18165 parts 1 and 2). In case of a combined application of impact acoustic insulation boards and thermal insulation boards, the impact sound insulation boards must be installed as first layer. Exception: when levelling over pipes, the thermal insulation follows first, e.g. with the quick-setting lightweight screed underlay weber.floor 4514.

  4. 2. Insulation boards should lie on their entire surface, with tight joints and without offset. In the case of multi-layer insulation, lay the joints offset against each other. A maximum of 2 layers may consist of impact sound insulation materials. The top insulation layer must be placed under the foil of the foam strip.

  5. Cover of insulation layer

    1. It is recommended to cover the insulation layer with a weldable and waxed paper sheet (water vapour permissive polyethylene-coated natron kraft paper). Lay the paper sheets crosswise to the direction of the screed flow and the direction of laying is opposite to the direction of the screed flow. Thereby the individual sheets are pressed on top of each other like shingles.

  6. 2. It is recommended to weld or tape the overlaps. The overlap should be 10 cm. In the wall connection area, the cover must be placed on the foil of the foam strip and must not stick up at the edge.

  7. Adjusting of installation heights

    Place meter levels every 2 - 3 meters for checking the height differences. A construction laser or a hose spirit level can be used for this purpose. It is advisable to set up some levelling tripods in order to achieve the desired screed thickness or finished height. In general, the screeds should be installed in the most uniform layer as possible.

  8. Application

    1. Before pouring the first mixture of the flow screed, a slurry of anhydrite binder is mixed in a bucket and pumped through the hoses. This lubricates the hoses and the screed mortar flows better. The slurry is disposed of in a container as waste. Do not use it for the screed mortar.

  9. 2. The flow performance is determined by the water amount. The consistency must be adjusted in accordance with the right slump without water separation (i.e. the mortar must not show any bleeding). Over-watering will result in a too soft surface. The consistency is measured in a slump test (via 1.3 liter tin, flow ring and flow table).

  10. 3. The screed is distributed evenly on the substrate by swinging the casting hose back and forth in order to obtain a homogeneous layer (uniform grain structure). It is therefore wrong to place the hose in the middle of the room and to wait for a level screed with a uniformly good surface to be achieved.

  11. 4. Once the right height is reached, the screed mortar is immediately beaten through, using either a screed broom or the wobbling bar weber Schwabbelstange, first lengthwise and strongly, then crosswise and somewhat smoother. Such wavelike movements bring a good levelling and de-aerating effect.

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