Solid wood headers above windows and doors are often oversized for the load they need to carry. And the solid wood creates a cold spot because there is no insulation. On a non-load-bearing wall (a gable-end wall), the header space can often be left open so it can be insulated like the rest of the wall. On load-bearing walls, the builder can install headers with rigid insulation sandwiched between structural framing. By sizing and locating windows to align with wall framing, the builder can avoid having to put extra studs in the walls to support the windows, which also allows more room for insulation.