The table gives the approximate insulation value (or K value) of various materials in column E. The lower the K value the better the material is as an insulator. However the best insulating materials cannot always be used in particular situations such as where there is dampness, where they need to be load bearing to some extent or where they need to be self supporting. These factors are shown in columns B to D.
The K value of materials can vary considerably depending on several factors such as moisture the material may have absorbed, how compressed it becomes and the way it is made. Manufacturers tend to test their materials under ideal conditions and there is some doubt as to how reliable many of the values are in practice.
Timber is included in the list to show how timber members may contribute to thermal bridging. Structural timber’s thermal conductivity has traditionally been ignored in calculations, particularly for studwork.
Both aerogel and vacuum panel insulation are still in the early stages of development.