Rendered External Insulation Facade System

Insulation is fixed to the outside of an existing or new wall using adhesive, mechanical fixings or both, and a thin reinforced render is applied directly onto its face. Nothing rigid sits between finish and insulation, so the render's flatness, its resistance to knocks and its behaviour under driving rain are all borrowed from a light, compressible board and from whatever holds that board against the wall.

The nearest sibling is the direct-applied render system, and the test is what the render is standing on. At a window reveal the difference shows: over external insulation the opening is lined and thickened, the render turns a soft edge protected by a bead, and the frame sits recessed within a deepened reveal. Over masonry the reveal is the wall's own depth and the coats have keyed into a hard background. The two look alike across a street and behave nothing alike at a doorway or a bin store.

Because the whole build-up hangs off the wall rather than bonding into it, where the system starts is a design problem in its own right. It has to begin above splash and standing water on a supported, shedding edge that protects the exposed lower face of the insulation. The same question repeats wherever the render stops: at reveals and sills, under an eaves, at a party boundary, and at every interruption made for a pipe, a bracket, a meter box or a light.

Wrapping a wall in insulation also changes the wall. It runs warmer and takes far less rain, and its drying is then shared between an inward route into the room and an outward route through the insulation and the render, with how much the outward route carries set by the vapour openness of the boards, the base coat and the finish. That is why the finish is a decision about the wall behind it, and why a dense or closed one is the case to look at. Whether a particular wall can accept that change is assessed by a qualified professional who has examined it, and is not inferred from the finish chosen for the outside.

Nothing here supports itself. The finish depends on the base coat, the base coat on the insulation, the insulation on its adhesive and fixings, and all of it on a wall whose condition was assessed before any of it arrived. A weakness anywhere in that chain surfaces at the outer face, which is why a crack pattern following board joints is a message about the layer beneath rather than about the render.

The mesh does its work only where it sits within the base coat rather than against the insulation or on the surface. Buried too deep it leaves the outer face unreinforced and cracks appear over every board edge; left too shallow it is exposed to weather and the coat around it loses its cover. The same applies at overlaps between mesh sheets and at corners, where local reinforcement is added precisely because impact and movement concentrate there.

The base of the system is where every weakness meets. It is the lowest exposed edge of a soft board, the place splash arrives, the level at which any damp-proof provision in the wall passes, and the point where a starter profile has to hold the build-up, shed water clear and resist knocks. Get it wrong and the failure is not cosmetic: water enters the back of the insulation and travels behind the render, where no external repair can reach it.

Once complete, the wall behind is warmer and enclosed, so its drying now depends on the vapour behaviour of the render and finish rather than on exposure. Choosing a finish is therefore a choice about the wall as well as the facade, and the two cannot be decided separately. Whether interstitial condensation is a risk in any specific build-up is assessed by a qualified professional who has looked at the whole construction and its internal conditions.