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How to prepare the substrate for installation of the insulation layer

Our guidelines for inspection and preparation of aerated concrete and brick walls before installation of ETICS.

Crucial to know

relative humidity lower than 80%

relative temperature +5 to +30˚C

even all hollows deeper than 20 mm

Tools needed

a brush

a stainless steel trowel

a drill with mixer paddle

Step 1

ceresit, infopack, etics, surface, inspection, preparation, CT17, roller

Substrate unevenness inspection


Each substrate should be strong, carried and free from grease, bitumen, dust and other substances decreasing adhesion. The adhesion to the existing plasters and paint coatings should be checked before starting the application. Hollow plasters should be removed. Any surface contaminant and other adhesion impairing substances, steam-tight paint coatings and the coats with low adhesion to the substrate should be completely removed, e.g. by means of washing devices operating under pressure. 

In case of mycological contamination with fungi, moss and algae, the surface of the façade should be cleaned with steel brushes and, then saturated with a fungicide solution of Ceresit CT 99 in compliance with technical instruction. The old, not plastered walls, strong plasters and paint coats should be dusted, then washed with water jet and left until they go completely dry.


Any losses and uneven surfaces of the substrate over 20 mm should be filled in with the filler CT 29 or covered with cement or cement-lime plaster. The best way to evaluate the substrate unevenness is to pull the strings from the top of the wall to the bottom and also crossing way to see what are the unevenness of the wall.

ceresit, infopack, etics, surface, inspection, preparation, CT17, roller


Evaluation of substrate quality should be done by a designer of facade insulation. In case of any doubts it should be checked with a pull off method (min. requirements regarding to the substrate is 0.08 MPa) or if when right equipment for thet measurement is not available it is possible to use EPS (TR 80) blocks (10×10 cm) glued to the substrate with a system adhesive. 

TR VALUE (one of EPS parameters acc. Standard EN 13163 – tensile strength) for EPS shouldn’t be higher than TR 80 for checking the substrate, acc. ETAG 004 min. requirement to the substrate is ≥ 0,08 MPa, so if the cube of EPS will be destroyed it means that our substrate i carried enough. Substrate pull off strength should be higher than that of thermal insulation material extension.

Step 2

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All absorbent substrates like old plasters, aerated concrete, silicate blocks should be primed first (in case of fixing the insulation boards with cement adhesives). Priming the substrate is the core point to avoid to fast drying of adhesive mortar and also due to fast drying, decreasing/loosing technical properties like adhesion strength of mortar to the original substrate). Ceresit CT 17 is recommended for priming.

Tips & Tricks

For professionals

How to tackle cavities under insulation without cutting out large parts of plastered insulation?

If there are cavities under EPS boards and you do not want to cut out large parts of insulation, make local injections with PU foam adhesive. 

Drill some small holes through all insulation layers to reach the wall. The drillhole can be as wide as the diameter of PU foam gun barrel. Inject the PU adhesive Ceresit CT 84 into the drillholes.

products used

  • ${/content/heliux/adhesive/ceresit/com/en/products/interoir-finishing/central-pdp#/ceresit-ct-17/SAP_0201JAC014F1/searchResult.productFields.MainImageAltText}
    Primer for surface strengthening of all absorbent substrates for indoor and outdoor application, prior to fixing ceramic tiles, pouring floors or fixing thermal insulation boards. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/interoir-finishing/central-pdp#/ceresit-ct-29/SAP_0201JDC014G9/searchResult.productFields.MainImageAltText}
    Grey, mineral filler to repair traditional plasters and thin-layer “putties” inside and outside the buildings. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/interoir-finishing/central-pdp#/ceresit-ct-99/SAP_0201JAC014N6/searchResult.productFields.MainImageAltText}
    Renovation concentrate for removing fungi, lichens, moss and bacterial contamination, suitable for indoor and outdoor areas. 

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