Rigid Foam Insulation
There is a LOT of house that is NOT insulated wall cavity. Within the construction of a standard stud wall, there is a bottom plate, a double top plate, studs every 16″, window and door headers, jambs, bracing and other areas that cannot be insulated with cavity insulation.
Wood is a fair insulator, at about R-1 per inch, but most insulations are at least R-3 per inch (over 3 times better at insulating)
Insulating foam board sheeting goes over the exterior, adding additional insulation to all areas of the wall, including the framing.
The first picture is foil-faced foam board, probably a polyisocyanurate board. This is the highest R-value board at R-7 to R-9 per inch, not counting any reflective value from the foil. Polyisocyanurate boards are generally some shade of yellow or off white, with a foil facing on both sides to both seal-in the gases in the foam, protect the foam, and add additional strength and workability.
Extruded polystyrene board comes in a variety of colors to distinguish the brand name. The most common are blue and pink.
Extruded polystyrene has an R-5 value per inch. It is also the only kind of foam board that is acceptable for use below grade.
Although the wall is well insulated, note how the insulation sheathing stops at the floor line; the band joist (aka:rim joist – the perimeter of the floor) is open. This not only leaves a lot of area not well insulated, there are several continues cracks that are left with one less barrier to the wind.
Notice also that the concrete basement wall is not insulated.
This insulation board should have been run all the way down, over the band-joist, over the basement walls and into the ground to the basement wall footing. See Basement Insulation.
Source: Text Bob Fegan 12/2008; pictures Bob Fegan