A vestibule saves even when the back door is not being opened. The dead air space provides additional insulation value to the back door and frame area. Also, most well-used exterior back doors no longer have a good airtight seal. A vestibule reduces the air leakage past your back door weatherstripping.

By reducing the drafts inside your house, you may be able to set your thermostat a degree or two lower and still be comfortable. Depending on your climate, you will save from 1 percent to 3 percent on your utility bills for each degree you lower your thermostat setting.

In addition to saving energy, a vestibule offers extra storage area for coats, wet boots, etc., and keeps muddy little feet out of your house. If you have allergies, the "air lock" it creates helps block the free flow of pollen and mold spores into your house each time the door is opened.

The simplest do-it-yourself exterior vestibule utilizes basic 2x4 wall framing and a simple built-up roof. You should install a large pre-hung door. Since the vestibule is not heated, an inexpensive non-insulated door should be adequate. Good weatherstripping, though, is an energy plus.

Install a window for natural lighting. An inexpensive single-pane window is adequate. If the window faces south, you should install a large window. The vestibule can then also function a passive solar collector to further reduce heat loss from your house. You should still wire up a single overhead light for nighttime use.

Although wall insulation is not required, you may want to use insulating foam sheathing instead of plywood. This is especially true if the window faces south for some solar heating. You can cover the walls with aluminum or vinyl siding for low maintenance.

You can write to me for UTILITY BILLS UPDATE No. 168 showing do-it-yourself instructions and diagrams for building an exterior back door vestibule.

Write to James Dulley, Deseret News, 6906 Royalgreen Drive, Cincinnati, OH 45244. Please include $1 and a self-addressed STAMPED BUSINESS-SIZE envelope.

Q - I am told that I should tighten up my home to save energy. Can you tell me how much heat is contained in that air that leaks out? W.K.

A - Each cubic foot of air holds very little heat, only .018 Btu per degree temperature. This means it takes .018 Btu of heat to raise one cubic foot of air one degree higher in temperature.

With an older house, it is not uncommon to have two complete air changes per hour. A 1,500-square-foot home with 8-foot ceilings has a volume of 12,000 cubic feet. It can lose 24,000 cubic feet of air each hour.

If it's 40 degrees outdoors and 70 degrees indoors, your furnace must heat that incoming cold air 30 degrees. By multiplying 24,000 cubic feet of air times 30 degrees and then that number by the .018 factor, you get the total amount of heat lost to air leaks. For the example above, it would average about 13,000 Btu per hour.