The calculator can work out wall layers in a few moments. Most of the work, however, goes into establishing what is actually inside the building element. Before renovation, you need to separate the verified condition from assumptions. Only then is it worth comparing PIR thicknesses, assessing the risk of moisture and preparing data for the contractor. Start with the wall cross-section. Record the order of materials from the interior to the surroundings. Include renders, masonry, the cavity and any existing insulation. For each layer, give the thickness and how it was determined. A measurement in an opening gives a different level of certainty from the owner's memory of a renovation twenty years ago.

The thickness of a wall PIR panel should follow from the conditions in the room and the project requirements. In an extension, matching the existing cladding is added. A thinner panel may make the connection easier, but it does not always meet the thermal objective. A thicker one will in turn change the details of openings, the plinth and fixings. Note whether the room will be heated continuously, periodically or not at all. Humidity and the method of ventilation are also important. Extending a dry warehouse with a production area may change the requirements, even though from the outside both parts look similar.