Express Course

How to fix EPS insulation boards with adhesive mortar

Learn how to install insulation boards with Ceresit mortars CT 81 and CT 83 and fix them with mechanical fasteners.

Know upfront

Tools needed

  • a trowel
  • a long float or a level
  • a drill and a power screwdriver
  • a fastening system for EPS boards

CRUCIAL to know

  • mortar’s pot life 90 min
  • anchoring after min. 2 days
  • coverage with adhesive 40% of the board surface


Substrate check and preparation

The adhesion to the existing plasters and paint coatings should be checked before starting the application. Any losses and uneven surfaces should be filled or covered with cement plaster. Any contamination, adhesion impairing substances, steam-tight paint coatings and the coats with low adhesion to the substrate should be removed, e.g. by 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 to dry for at least 4 hours. Before fixing the EPS-boards, the adhesion of the mortar should be checked by fixing polystyrene boards (10×10 cm) on the prepared substrate and tear them off after 2-3 days.

Mortar application on EPS boards

We recommend CT 81 or CT 83 Strong Fix for fixing EPS insulation boards. The prepared 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 (dots and dab application). The properly applied mortar, when pressed, should cover minimum 40% of its surface.

Attaching EPS boards to the wall

Press the boards 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. In case of even, smooth substrates, the mortar should be applied by using a toothed long float (teeth 10–12 mm).

The boards should be fixed tightly one to the other with the preservation of ”brick like manner” of vertical connection. The joints between the slabs should be free of adhesive, in order to avoid gaps between adjoining slabs. Boards should be cut precisely with a hot wire knife so that they fit each other, without any gaps. Ensure slabs contact with substrate. Tooth-like arrangement of slabs should be done around the corners. To avoid slab joints around windows and doors, make the cut-out from a single slab.

When the mortar is set (after approx. 48 h), any unevenness of the boards should be polished with abrasive paper. Now the boards can be reinforced with mechanical anchors.


How to use dowel fastenings properly

All the fixings should be designed for each type of the wall material (concrete, blocks etc.). The flange diameter of fixings for EPS boards is minimum 6 cm. The anchoring depth should be minimum 6 cm, in case of solid building material, and 8 cm, in case of porous building materials. Anchoring should be done not earlier than 48 h after fixing the boards with CT 83 mortar. For installation dowel fixings use a hammer drill or a standard drill. Remove all the dust from the holes. At least four anchors should be fixed per one square meter. All fixings should be acc. to ETAG 014 approvals – guideline for mechanical fixings.

The polishing of the insulation material should be done on the areas where the slabs are not faced e.g. corners, joints. The general idea is to reach as much even surface as possible, before application of reinforcing layer. After polishing, remove the dust from the façade.

Installation of anchors with plastic flange

You need to cut a round cap off the EPS board. There will be lots of polystyrene particles around, which need to be removed with a vacuum cleaner so they don’t disturb further work. Then you drill a hole through the insulation board deep into the wall and put the anchor inside. Now you can screw it in with a power screwdriver. Some anchors can be hammered as well. Cover the anchor with the previously cut EPS cap.

Installing anchors with a plastic head and prefabricated EPS caps

This system is simpler as the polystyrene caps are already prefabricated. Just drill a hole for the fastener, place it there and screw it in deep enough so that the polystyrene cap does not overhang the face of the EPS board.


Installing fasteners with EPS caps cut off a board.

Installing fasteners with the prefabricated EPS caps.


How many anchors per one meter of insulation?

Minimum number of fixings ≥ 4 units per 1m2. Installation of additional fixings is determined by the wind load on the facade.

Number of fixings should be increased in corner zones of buildings, which depends on width of the building.Minimum number of fixings ≥ 4 units per 1m2. Installation of additional fixings is determined by the wind load on the facade.