There is no need to insulate the exterior walls of unheated basements, because there is no need for reducing heat loss. The exterior walls of heated basements (or interior walls separating heated from unheated areas) are a different matter, because heat loss is a concern here. An insulating layer having a thermal resistance rated at R-7 or greater should be applied. Figure 3-17 illustrates two methods for insulating these walls.
Exterior opaque metal walls (Figure 3-16) should have a maximum U-value of 0.07. This will require insulation having a thermal resistance rated at R-11 or better, which can be provided by a layer of rigid insulation either preformed or foamed in place.
Sandwich or layered construction will vary in composition and thickness but will typically consist of three layers: two facings and a core. Exterior opaque walls of a sandwich construction should have a maximum U-value of 0.007. The thermal resistance (Rvalue) must be calculated on the basis of its composition and thickness.