This diagram shows a thin slab of an unspecified building material. The slab is shown vertically on edge and is seen looking slightly down on a top corner. Two labelled arrows indicate that the slab is 1 metre square. Another labelled arrow indicates that it is ‘t’ metres thick. A broad red arrow showing the heat flow penetrates the slab from right to left and is labelled ‘Q’ at the left. The arrow is also labelled with two temperatures. At the right it is labelled ‘T subscript in’. At the left it is labelled ‘T subscript out’

2.2.4 Conductivity and U-values – the basics