Internal Wall Insulation Retrofit System

Internal wall insulation is a lining built on the room side of an existing external wall: an insulating layer, a means of holding it there, usually a layer controlling air and vapour movement, and a finish. It is applied to walls that were never designed to receive it, and its outcome depends far more on the wall it is applied to than on the lining chosen.

The physics is unavoidable. Heat that used to warm the wall no longer does, so the construction behind the lining runs colder and, in most climates, stays damp for longer after rain. Moisture already reaching that construction - from driving rain, from the ground, from an overflowing gutter - now has less energy available to drive it back out again. The existing wall's exposure and condition therefore become the deciding factors, not the lining specification.

The distinction from internal-wall-lining-system is what the lining contains and therefore what it changes. A lining may be built simply to give a flat, serviceable face on an irregular wall, and it alters the construction behind it very little. This entry covers the case where the lining carries insulation and so cools that construction. The bare phrase dry lining belongs with the former; insulated dry lining belongs here.

The lining also has to stop somewhere. Where it meets a floor, a ceiling, an internal partition built into the external wall, a party wall or a window reveal, the insulated line is interrupted by construction that runs on through to the outside. Those interruptions become the coldest interior surfaces in the room. How far the insulation is returned along them, and whether it can be returned at all, is usually the hardest part of the design.

The insulation and the layer behind the finish are one decision, not two. The more the insulation cools the existing wall, the more it matters that internal moisture cannot reach it; but the tighter that layer is made, the more completely the wall is asked to dry outward alone. Where the outer face is heavily exposed, coated with something dense or permanently shaded, the outward route may already be weak, and the two requirements then work directly against each other.

Whatever holds the lining up also crosses the layer meant to be continuous. Adhesive placed in dabs leaves voids behind the board where air can move around the insulation; mechanical fixings and a battened frame puncture the control layer at every point and leave timber or metal in contact with the cooled wall. Choosing between them trades one kind of discontinuity for another, which is why the fixing method is a hygrothermal decision as much as a practical one.

Returns at junctions are where the assembly's performance is decided rather than improved. An intersecting wall, a floor built into the masonry or a window reveal carries the outside condition inward past the insulation, so the room-side surface there stays close to the temperature of an untreated wall. Carrying insulation along those surfaces lengthens the path but never removes it, and each return consumes reveal depth or room dimension that may not be available.

Where the strategy is vapour-open rather than sealed, the finish stops being decoration and becomes part of the moisture path. The insulation, the plaster over it and whatever is painted or papered onto that plaster then work as a single resistance, and a later occupant choosing a coating for its appearance is altering the assembly. That dependency does not exist in a sealed build-up, where the control layer carries the duty and the finish in front of it is free.

Services placed in front of the control layer keep it whole; services chased into the wall behind it undo the purpose of building the lining at all. The difficulty is that the finished lining looks like an ordinary wall, so later occupants drill, fix and chase into it without knowing what lies behind. A record of where the layer runs is therefore part of the assembly rather than paperwork about it.