Just as you have to spend money to make money, you sometimes have to use energy to save energy -- at least if you want stay healthy.

That's because today's houses, designed to conserve energy, and older houses that have had energy upgrades, are tightly sealed around doors, windows and ducts that penetrate exterior walls and roofs. And while the well-sealed home keeps warm air inside during the winter, it also traps airborne germs, dust, dust mites, formaldehyde, noxious gases from plastic products and moisture that can support mold.

Constant exposure to these irritants or toxins can cause problems ranging from wheezing to heart trouble, serious nervous disorders and even cancer. Thus one of the dilemmas of green design is that the more energy-efficient we make houses, the less healthy they can become.

While it might seem that the solution is to seal houses less diligently so they ";breathe" the way leaky older houses do, that introduces too much unreliability into the design. Who decides how much a house should leak, and where the leaks are placed? What are the health effects on calm, temperate days when the interior and exterior air temperature and pressure are the same and no air is exchanged?

It's far more reliable to seal the house tightly and then install a whole-house ventilation (WHV) system that uses an electric fan to evacuate the interior air and draws in outside air through carefully designed passive vents or ducts equipped with an intake fan. A WHV system replaces air at a constant rate -- ideally about one-third of house's total volume per hour -- so pollutants don't have a chance to build up and there is too little moisture to support mold.

A typical WHV system features a quiet exhaust fan mounted in a central location -- above a stairwell, for example, where it can draw air from the house's major open spaces. Ducts might be needed to exhaust air from remote rooms or basements.

The exhaust fan creates negative air pressure throughout the house, so outside replacement air can enter through simple, passive vents. Intake vents should be carefully placed so they don't take in radon or molds from a crawl space, or fumes from a garage.

Systems that use fans (active systems) must be balanced so the inside air pressure remains even or slightly positive so you can breathe easily. Balancing air flows and optimizing the performance of a WHV system requires a good deal of knowledge and experience, so the services of a heating-ventilating-and-air-conditioning engineer are recommended.

Bringing in cold outside air will increase your energy bill. One solution is to install a heat recovery ventilator (HRV), which routes the exhaust air through a series of pipes called a ";heat exchanger." The heat exchanger passes the heat from the outgoing warm air into the incoming cold air stream, recycling about 70 to 80 percent of the heat in the exhaust air.

A prototypical super-insulated house in Berkeley, California, has no heating system whatsoever and relies on a HRV to capture and return heat generated by its refrigerator and other appliances, along with the body heat of its occupants to maintain a comfortable temperature. However, Berkeley has a mild climate, and its likely that there would be a lag in warming such a house to a comfortable temperature if its occupants returned from a trip on a cold day.

According to the US Department of Energy, the current HRV systems are most cost-effective in very cold climates (or very hot climates, where air conditioning is an issue). Also, since HRVs are not yet common, there are not many experienced technicians to install them. The HRV concept is solid, however, so they're likely to become a standard energy-efficiency solution.

The downside is that their fans have to run most of the time when windows and doors are closed, so we're forced to use energy to live in air-tight houses. However, the energy used by a well-designed WHV system is far less than that wasted by a leaky house, and it can be generated using green technology. A WHV system could be the best energy solution for your ecological house.