Q:I’m hoping you can give a brief explanation of the tankless water heater. I can’t seem to make my husband understand. Our water heater is on the opposite side of our house from the bathrooms and laundry room. Consequently, it takes running the water a long time before we get any warm/hot water. Isn’t the tankless heater something that allows you to tap into hot water on demand?

A: That’s true. Tankless units cost more than storage heaters, but since you are not heating a big tank of water 24 hours a day, they cost less to operate.

Although more complete examples are available at the Web sites of manufacturers –Bosch, Rinnai, Noritz, Paloma, Rheem, Titan, Takagi, PowerStar, Eemax, and Seisco – here are a few basics to present to your husband, who shares the skepticism of a lot of plumbers.

Heating water accounts for 20 percent or more of a typical household’s annual energy expenditures, according to the U. S. Department of Energy. The yearly operating costs for conventional storage-tank water heaters average $200 for gas units, $450 for electric ones.

Storage-tank water heaters raise the temperature to the setting on the tank, usually between 120 and 140 degrees, and maintain it there.

Even if no hot water is drawn from the tank (and cold water enters the tank), the heater will operate periodically to maintain the temperature.

This is the result of what is called standby loss –the heat conducted and radiated from the walls of the tank and, in gas-fired water heaters, through the flue pipe. Standby losses represent 10 to 20 percent of a household’s annual water-heating costs.

By providing hot water immediately where it is used, tankless heaters waste less water: You don’t need to let the water run as you wait for warmer water to reach a remote faucet.

Equipment life may be longer than with tank-type heaters because tankless models are less subject to corrosion. The expected life of tankless water heaters is 20 years, compared with 10 to 15 years for tank-type heaters.

Tankless heaters range in price from $200 for a small under-sink unit to $1,000 for a gas-fired unit that delivers 5 gallons per minute. Those numbers do not include installation, which can add $150 to $300 to the price.

Typically, the more hot water a unit produces, the higher the cost. Electric tankless heaters typically cost more to operate than gas units.

Q: You talked in a recent column about air sealing before insulating. What did you mean by that?

A: The energy efficiency experts I know say that without sealing, insulation doesn’t do its job. Before you insulate, you need to use foams and sealants to close any penetration to the outside, such as at the ends of joists at the front and back of the house.

You can seal by tucking white or black plastic bags into the cracks. That prevents heat from escaping and moisture from entering the attic, without interfering with the required ventilation.

The basement is another place where air sealing will reduce heat loss and moisture intrusion by reducing incoming air flow. Basement windows tend to be the worst cared-for, so make sure too much air isn’t escaping through them.

One of the primary sources of cold air in the basement is the point where the porch joists meet the exterior wall. In our house, it is a cold spot, so I tried to insulate without sealing first, and my efforts did not work.

To do it properly, I first removed the insulation –both cellulose-treated ground-up newspapers and kraft-faced fiberglass batts –and cleaned out the spaces, making sure there was no evidence of intrusion by rodents or other pests.

Then I filled much of the back of the cavities with insulating foam. Be sure to wear latex gloves when using the foam.

I then took some of the cellulose or unfaced batts and put it in white kitchen trash bags, stuffed it in the gaps, and filled any opening that remained with more foam insulation.