Thinner PIR can give a similar thermal resistance to thicker polystyrene, but there is no single conversion factor for all boards. You need to know the lambda of both materials and check their application. With a small window reveal, saving a few centimetres can be valuable, but it should not mean replacing the whole system without assessing its other properties. Thickness tells you how much space the layer will take up. Its ability to limit heat flow is given by the resistance, denoted R. In simplified terms, you divide the thickness in metres by the lambda. A lower lambda allows the same resistance to be achieved with a smaller thickness.

A good-looking façade does not always mean the house is evenly warm. In a 1990s building, one room may cool down faster than the others even though the heating is working correctly. PIR from the inside is sometimes considered when no façade renovation is planned. First, however, it is necessary to establish what is already in the wall and where the discomfort comes from. Houses with very different walls were built around the same period. The masonry may have had a cavity, thin insulation or an additional facing layer. Later renovations may have changed only part of the building. It is not worth assuming the insulation thickness on the basis of the appearance of the external render.

Internal insulation can improve comfort where a wall is cold, but joints need careful planning. In a small room every centimetre counts, and after renovation it is difficult to return to a covered wall. The aluminium facing is part of the protection against vapour penetration. Its performance depends on the airtightness of the entire surface, including at the ceiling and window. Before choosing the board, check that the wall is dry and that water is not getting into it. Stains at the windowsill may result from an external leak. Dampness at the floor may have a different cause. Covering both places with insulation will hide the symptoms but will not stop moisture ingress.

For a wall standing on a plot boundary, adding insulation on the outside may be difficult. This is where the idea of internal insulation comes in. A thin PIR board can be helpful in a small interior, but the space saving must be weighed against the condition of the wall, the routing of services and the possibility of sealing the new layer tightly to the ceiling. The building's position must be confirmed on the basis of documents and the actual condition. The solution cannot rely on the assumption that a few centimetres beyond the boundary do not matter. Any agreements, access from the neighbour and requirements for the wall must be clarified before the detailed design.