Exterior walls are responsible for keeping the comfortable air in and the elements out. All building codes require that you insulate the exterior walls of your home. This is done in-between the wood studs of the frame. Here's a look at some energy-efficiency factors to consider for walls:

• Fiberglass Batts are pre-cut for a tight fit in between wall studs. They're easy to install and are used to reduce heat loss of air leakage.

• Sheathing is an added protective layer on the outside of the exterior wall. It's essentially plywood that is nailed or screwed into the studs of the building. Sheathing provides structural integrity, and its other function is to close off the studs.

  Note: Sometimes sheathing can be foam insulation — or rigid board. There's no structural loss using rigid board, and it's basically a better insulator for your home.

• An air infiltration barrier

  Other air infiltration barriers are polyurethane sheets that protect the house from moisture. The siding of the house is then applied on top of the air infiltration barrier. This can be a good wood paneling, a wire or a stucco surface.