Science and material U values

[CharlesYork]

A wall panel is to have the following: Outer metal sheeting, foamed polyurethane board, air gap and 15mm of lining board. The relevant value of thermal conductivity in w/mk are:

1) polyurethane board 0.025

2) lining board 0.16

3) (metal sheet is ignored)

The standard thermal resistances, in m�k/w are: outside surface 0.06, inside surface 0.12, air gap 0.18. calculate the minimum thickness of polyurethane needed to give the wall panel a U value of 0.35w/m�k.