Base coat

External thermal insulation base coat: definition, function and composition

The layer that gives your insulation its strength

The base coat of an external thermal insulation system (ETIS) is a mortar applied directly to the insulation, into which a fibreglass mesh is embedded. Together, they form the reinforced base layer which protects the insulation from impact, accommodates thermal expansion and serves as a substrate for the finishing render. At Centrale ITE, the product of choice is Fassa’s A 96 adhesive-plaster. This is a mortar made from white Portland cement and limestone sand, which is used both to bond the panels and to plaster them. It is designed for:

expanded or extruded polystyrene;
mineral wool;
concrete.

Its performance characteristics (EN 998-1, GP-CSIV-W2):

particle size < 1.4 mm;
compressive strength ≥ 7 N/mm²;
flexural strength ≥ 3 N/mm²;
vapour resistance factor µ 25.

For more mineral-based substrates and insulation materials, Centrale ITE offers two alternatives to lime:

the BIO KB13 417 undercoat;
the lightweight Eco-Light 950 adhesive and base coat, containing natural hydraulic lime (NHL 3.5).

All are suitable for use within the certified insulation system (Technical Approval or DTA).

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Applying the base coat: thickness, coverage and drying time

A precisely controlled thickness, a crack-free façade

The base coat is applied in two coats, with the mesh embedded between them, to achieve a thickness of at least 4.5 mm once dry. This is a requirement of the Fassatherm Technical Approval.

Mix each 25 kg bag of A 96 with approximately 6.5 litres of water using a mixer.
Apply the first coat using a notched trowel.
Press the mesh into the render using a stainless steel trowel, ensuring at least 10 cm of overlap between the strips.
Apply a second coat to completely encase the reinforcement.

The A 96 data sheet recommends 5 to 6 mm on polystyrene and mineral wool, compared with 2 to 3 mm on concrete. The coverage is approximately 1.5 kg/m² per mm of thickness. In practice, allow:

approximately 4 kg/m² on expanded polystyrene (EPS);
approximately 6.3 kg/m² on rock wool (4.2 + 2.1 kg/m² according to the Fassatherm Plus MW Technical Data Sheet).

Application conditions:

apply at temperatures between +5 °C and +30 °C, away from direct sunlight and strong winds;
do not expose to frost for the following 24 hours;
allow to dry completely – several days in cold and damp weather – before applying the primer and top coat.

How to choose your external insulation?

Steps involved in an EWI installation

Which fixing for which substrate?

External thermal insulation base coat: definition, function and composition

Getting your base coat right first time

En général 3 à 5 mm au total, en deux passes avec la trame noyée entre les deux. Respectez l'épaisseur préconisée par le fabricant du système.

Souvent, oui : de nombreux mortiers ITE servent à la fois au collage de l'isolant et à la couche de base armée. Vérifiez cette mention sur la fiche produit.

Laissez sécher selon les préconisations du fabricant, souvent quelques jours selon la température et l'humidité, et protégez l'enduit frais de la pluie et du plein soleil.