Preparing the surface & dealing with damp

Preparing the substrate before installing external wall insulation: the key steps

A solid wall for long-lasting insulation

Preparing a substrate before installing external wall insulation involves ensuring the wall is clean, dry, cohesive and sufficiently level so that the adhesive and wall plugs will hold for decades. It all starts with an assessment of the façade:

Testing cohesion. Look out for powdery render, blistering paint and hollow areas when probing the surface. According to Fassa’s technical data sheets, these layers with poor cohesion must be mechanically removed until a solid, compact substrate is reached.
Clean. Oils, grease, moss and stubborn dirt are removed using detergent and a brush. The wall is then left to dry.
Even out absorption. A bonding primer should be applied to any substrate that absorbs unevenly or remains slightly dusty. Centrale ITE offers two primers:
- FX 526, a pigmented primer with fine aggregate, at a rate of 0.12 to 0.16 l/m²;
- Primertech 101, a single-component acrylic primer.
Rectify any irregularities. Significant defects must be rectified before installation. On an existing insulation system, the Technical Assessment requires verification that it does not shift under pressure and that it shows no signs of delamination or significant cracking.

This assessment also determines the substrate category (concrete, solid brick, hollow brick, aerated concrete block, etc.). This determines the choice of wall plugs.

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External wall insulation and damp: treat the wall before insulating it

Insulate a dry wall, don’t trap the water

External insulation does not solve a damp problem. If the wall is affected by rising damp, water ingress or saltpetre, the cause must first be addressed; otherwise, water will remain trapped behind the insulation. Before starting work, three points should be checked:

the condition of the gutters, downpipes and window sills;
the drainage at the base of the wall;
any signs of damp inside the property.

From the moment installation begins, the Fassatherm Technical Approval requires that the insulation boards be protected from rain and damp, both before and after installation. Damp or warped boards must never be installed. The choice of insulation is also important for an old, ‘breathing’ wall. Gutex Thermowall-L wood fibre (µ 3) and rock wool (Euroclass A1) are vapour-permeable and allow the wall to release its moisture. In the lower part of the building, where it is exposed to splashes, it is preferable to use insulation that is not very sensitive to water, such as extruded polystyrene, which can also be bonded using A 96 adhesive. The base coat and top coat must then be applied at temperatures above +5 °C, and with no risk of frost within the following 24 hours. A façade that is sound from the outset is one where the external thermal insulation does not blister, stain or peel away.

How to choose your external insulation?

Steps involved in an EWI installation

Which fixing for which substrate?

Preparing the substrate before installing external wall insulation: the key steps

Before you start gluing, ask yourself the right questions

Le mur doit être propre, sec, cohésif et suffisamment plan. Grattez l'enduit existant : s'il s'effrite, farine ou se décolle, il doit être traité ou retiré avant la pose de l'isolant.

Identifiez d'abord la cause (remontées capillaires, fuite, gouttière défectueuse) et traitez-la avant d'isoler. Une ITE ne doit jamais enfermer un mur humide : il doit pouvoir sécher.

Sur un support poreux, farinant ou à l'absorption irrégulière, oui : le primaire régule l'absorption et améliore l'adhérence de la colle. Sur un support sain et peu absorbant, il n'est pas toujours nécessaire.