The headers are an area where there is a lot of thermal bridging where the floor joists meet the wall. A header must have the same effective R-value as the wall. In new homes, the wall cavity and header will often have the same insulation, but in existing homes this may not be the case. Because of the way headers are sealed, they often have a much deeper cavity than a wall. This cavity should be completely filled with insulation in order to avoid convection currents. Placing one layer of batt on the outside face or the inside face of the cavity only, leaving a larger air space inside the cavity, is not acceptable. Unfortunately this occurs often.

Headers are often a major area of air leakage and requires a lot of attention when air sealing. It is difficult to install polyethylene in the header area and great care should be taken to do so, or other methods can be used, such as using rigid foam board and expanding foam instead of polyethylene.