In homes or buildings with frame construction over a basement or crawlspace, the floors are usually supported by framing members called joists, which rest atop the foundation sill. The outside ends of these joists, and the space between them, are covered with a band joist or "header". This band joist is the same size as the floor joists, and runs around the entire perimeter of the building.

An uninsulated band joist can account for a significant portion of a home's heat loss, as the only thing separating inside from outside is two inches of wood and the siding material covering it. If the basement is heated, or contains heating or water heating equipment, the loss through an uninsulated band joist is even greater.

Insulating a band joist is an easy way to improve a home's energy efficiency. Unlike finished walls, the band joist is usually readily accessible from the basement (unless the basement has a finished ceiling). The amount of materials needed is minimal due to the relatively small area involved; there are no expensive or specialized tools required; and the skills required are very basic. As a result, the cost of insulating a band joist is relatively low compared to the potential energy savings.

Band joists are usually insulated with either fiberglass batts or rigid foam insulating board. If using fiberglass, use R-13 (minimum) on structures with two-by-four sill plates, and R-19 with six inch wide sill plates. If the fiberglass has a facing or vapor retarder, be sure that the insulation is installed with the vapor retarder towards the heated space. If using rigid foam board insulation, three to five inches of thickness should be sufficient, depending on the R-value per inch of the material. Foam board insulation materials will generally range from R-3 to R-7 per inch, with the R-value indicated on either the packaging or the insulation board itself. Use enough to achieve a total R-value of 13 to 20.

Other than the insulation itself, the only materials needed are some type of fasteners to hold the insulation in place. If using fiberglass, the insulation can be stapled by the paper or foil facing, or fastened with wire fasteners called "tiger claws". If using foam board, the insulation can be "friction fit" in place if it is measured and cut carefully.

On the two sides where the floor joists are perpendicular to the band joist, cut the insulation material to a snug fit, and gently push it into place between the floor joists. Be sure that it fits snugly against the band joist, without being compressed (compressing insulation reduces its effectiveness).

On the sides where the floor joists are parallel to the band joist, cut longer pieces of insulation (sections of four feet or less are easiest to work with). If using fiberglass, the insulation can be held in place with staples (if faced), "tiger claws", thin wire, or fishing line criss-crossed around tacks or nails. With rigid foam board, the insulation can be "friction fit" or glued directly to the band joist for additional holding power.

Be sure to fit the insulation snugly around any pipes, wires, or other penetrations through the band joist. Any penetrations should be caulked or otherwise sealed prior to installing the insulation. If any water or heating pipes run along the band joist area, be sure that the insulation is fitted between the band joist and pipes, with the pipes on the warm side of the insulation.

Before installing the insulation, be sure to check for any air leakage at the foundation sill joint, and caulk or seal this joint as needed.

In most cases, the cost of insulating a band joist is minimal. The primary factors affecting the potential savings are the average temperature in the basement and typical winter outdoor temperatures. The greater the difference between warm basement temperatures and cold outdoor temperatures, the greater the heat loss through the band joist, and the greater the potential savings. In most cases, insulating a band joist will pay for itself within a few years or less.