An exposed single skin masonry wall such as a concrete tilt slab internally insulated with a foil faced rigid thermoset PIR insulation. The insulation is mechanically fixed directly to the interior face of the masonry, with minimal thermal bridging, and must be taped and sealed to form a continuous air and vapour barrier preventing internal moisture vapour from condensing on masonry at below dew point temperature. This solution is often used as a compromise where external insulation is not an option. As the PIR insulation is deemed combustible, this solution can only ever be used in type A or B construction if approved as part of a fire engineered performance solution.