From my latest post on the Arborus Efficiency Journal:
Adding insulation to building walls and roofs is a simple and straightforward energy saving strategy. By resisting the flow of heat through the building envelope, insulation keeps buildings comfortable while reducing the energy required for heating and cooling. Unfortunately, many wall assemblies, and details at, for example, corners, parapets, and windows, are designed in such a way that large amounts of heat can bypass the insulation by flowing through structural components and framing elements, leading to decreased comfort and increased energy consumption. Quantifying the impact of these thermal bridging details can difficult, requiring precise three dimensional calculations and measurements of heat flow rates through assemblies under carefully calibrated conditions. Fortunately, a great deal of this work has already been done.