Retrofit Cavity Wall Insulation System

Retrofit cavity fill introduces an insulating material into the void of an existing masonry cavity wall, normally through holes formed in the outer leaf and made good afterwards. Nothing else about the wall changes. The same outer leaf, the same ties, the same trays and the same drainage exits remain in place, but the space they were all detailed around is no longer empty.

What governs the result is therefore the wall rather than the fill. How much water the outer leaf passes under driving rain, whether the ties are sound and sitting the right way, whether the void is clear of mortar droppings and builder's rubbish, and whether trays, closers and vents were built as drawn all decide what happens once the space is occupied.

The marker that separates this from masonry-cavity-wall-system is on the elevation itself. A cavity insulated as the wall was built leaves the outer leaf unbroken, while a wall filled afterwards normally carries a regular pattern of made-good injection holes across each treated face. What follows from that difference matters more: in new work the void, the ties and the trays were open while material was placed, and anything wrong could be corrected before the wall closed; here those same features can only be inferred from a survey through small openings. The survey, not the material, is the system.

That is why neighbouring houses of apparently identical construction can respond differently to the same work. Exposure varies with orientation, shelter and height. The condition of the outer leaf varies with its history of pointing and repair. What the void contains varies with who built it. None of that is a property of the fill, and none of it can be read from a product description.

The porosity of the outer leaf and the water resistance of the fill are a single pair rather than two properties. A leaf that passes little water can be backed by almost anything. A leaf that wets through under driving rain depends entirely on the fill refusing to pass that water on, and on the fill not settling later to leave a gap down which water can run. The same material therefore behaves differently on a sheltered elevation and on the one facing the weather.

Ties and debris become far more consequential once the space is occupied. A tie sloping the wrong way, or a bed of droppings resting on ties or collected at the base, was a local bridge in an open void that water could still drain around. Surrounded by fill, that same bridge is continuous with the material either side of it and nothing interrupts the path. Cleanliness that was almost cosmetic in an empty void becomes central to how the wall behaves.

Every existing interruption has to be found before it is buried. Vents, flue ways, ducts, boxes and tray ends each become either a place fill escapes into or a designed air path that is blocked. Because none of these is drawn on an ordinary existing house, they are located by inspection of both faces and of the roof space rather than assumed from the age or the style of the building.

The finished work cannot be seen, so the record takes the place of inspection. What was found in the void, where fill was and was not placed, how the openings were treated and how the drilling was made good is the only evidence anyone will have when a damp patch appears on an internal face long afterwards and the question becomes whether the fill is implicated in it.