In a typical stick-frame house, lumber occupies 27% of the wall area, leaving little room for insulation. Since the insulating value of softwood lumber (R-1.25 per in.) is less than that of fiberglass or cellulose insulation (R-3.6 per in.), each stick of wood lowers the wall’s overall R-value. Each piece of framing lumber acts as a thermal bridge, a conduit for heat to leak through the wall. Thermal bridging is more significant than many realize. Read more to see the interactive graphic that illustrates how heat flows around insulation and through the framing members in a wall, roof, or floor. It also shows you how to interrupt this heat flow with rigid insulation.
Read the article: http://www.finehomebuilding.com/pages/thermal-bridging