Features and Benefits

For fixing Expanded Polystyrene (EPS) boards for thermal insulation of buildings by means of light wet method of buildings


  • economical in use
  • good adhesion
  • resistant to weather conditions

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Ceresit CT 81 mortar is designed to apply EPS-boards within Ceresit ETICS (External Thermal Insulation Composite Systems) with a light-wet method. CT 81 is part of Ceresit Ceretherm Popular. CT 81 mortar is used for applying to the newly erected objects as well as the buildings to be thermo renovated. The applied boards require additional fixing by means of mechanical anchors, i.e. proper expansion pins made of plastic. Ceresit CT 82 mortar should be used to apply fibre glass armoured layer on the EPS-boards.

Step By Step

Preparation of the substrate


CT 81 has very good adhesion to sound and carrying substrates, such as wall, plaster and cement substrates 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 losses and uneven surfaces of the substrate should be filled or covered with cement plaster. 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.

Substrates with high water absorption, e.g. walls made of aerated concrete blocks or silicate blocks should be primed with CT 17 and left for drying for at least 4  hours. Before fixing EPS-boards the adhesion of the mortar should be checked by fix polystyrene boards (10x10 cm) to the prepared substrate and tear them of after 2-3 days.

Preparation of the adhesive mortar


CT 81 should be poured into the measured amount of cool clean water and stirred with the drill by means of a mixer until the homogenous mass is obtained without lumps. The ready mortar should be applied with a trowel along the board edges forming a strip of 3-4 cm wide and a few spots with the diameter of approx. 8 cm.

Installation of the insulation boards


The board should be pressed to the wall with a few slight blows of a long float. The properly applied mortar when pressed should cover minimum 40% of its surface. When CT 83 is set (after approx. 2 days), any unevenness of the boards should be ground with abrasive paper, then the boards should be additionally reinforced with mechanical anchors.