The temperature outside was a brisk 23 degrees last Saturday morning when about two dozen D.C. residents learned what they could do to keep warm this winter while conserving energy and minimizing heating costs.

Red-nosed and watery-eyed, they gathered in All Souls' Church at 16th and Harvard Sts. NW to participate in the morning session of a two-session Home Energy Fair being presented by the D.C. Cooperative Extension Service of the University of the District of Columbia (Van Ness campus).

In a one-hour lecture and slide show given by John Polgreen, an extension service agent, residents learned the basis of do-it-yourself weatherproofing.

"The most important energy-saving device," said Polgreen, "is people."

After the slide presentation, several workshops were conducted. Tables were set up where people, guided by extension service agents, could practice what they had just learned.

One table was devoted to leaky faucets. Several faucet fixtures, mounted on pegboard, were dismantled and reassembled numerous times by participants.

A leaking faucet can waste as much as 6,000 gallons of water a year, participants were told. If hot water is leaking, the waste will also show up in higher heating bills.

Other tables were devoted to solar heating, caulking, weatherstripping, insulating and weatherproofing windows by covering them with vinyl sheets.

Armed with hammers, screwdrivers, putty knives, staple guns and caulking guns, the participants insulated display walls, floors, windows and doors. In the process, they also learned how to weatherproof their own homes.

By the time the morning group had thinned out, the city was somewhat warmer 28 degrees.

The Cooperative Extension Service periodically offers a series of basic home repair workshops free to D.C. residents. For information, call 282-7410.