If you get only one thing under control in your remodeling, it should be moisture. Most houses don’t have major moisture problems, but when those problems do occur, the consequences can be serious. High moisture levels cause building materials to rot. (The term “dry rot” is a misnomer; all fungal growth requires moisture.) If the moisture content in the air becomes too high, mold, mildew, dust mites, and other microorganisms thrive. Mold and mildew make your house look and smell bad, and they may trigger allergic reactions in susceptible people.

In summer, moisture can enter a house via humid outdoor air; dehumidification may be an appropriate solution. In winter, with the house closed up, moisture can come from human activities such as cooking and bathing; good spot ventilation is the answer here.

You also want to avoid more serious sources of moisture, such as

  • leaks in the roof, plumbing, or around windows;
  • runaway humidifiers; and
  • wet crawl spaces.     

Even the best ventilation system won’t make up for these sources of moisture; you need to control them at their source.

Cold temperatures cause another kind of moisture problem. When warm air strikes a cold surface, the water vapor in the air will condense. An obvious example is condensation on windows. This condensation can lead to mold growth and the decay of the sash and framing. Another common condensation spot is air-conditioning supply grilles and nearby ceilings.

What’s the best way to ventilate? Do you need mechanical equipment to control humidity? That depends on the climate zone you live in. The right thing to do in Minneapolis is the wrong thing to do in Houston. Climate zones range from dry to very humid. In the North, Northeast, and West, the air is generally dry most of the year. In the South, it’s typically very humid. However, in any given region, humidity can vary greatly depending on the season.

In drier climate zones, exhaust ventilation in the kitchen and bathrooms can keep moisture under control most of the year. If you live in an area where summers are hot and humid, consult your air-conditioning contractor to discuss mechanical means to control moisture.

No matter where you live, you should build tight to help control where moisture flows, line your crawl space with a vapor barrier, keep up with maintenance, go easy with the humidifier, and install a ventilation system that’s appropriate for your climate.

Heat Recovery and Energy Recovery Ventilators

In winter, heat recovery equipment recovers “waste” heat from air on its way out of the house. It transfers this heat to incoming air by passing it through a simple heat exchanger. Up to 70% of the heat in the air being exhausted out of the house can be recovered.

In a hot climate, an HRV can also be used to precool outdoor air, reducing the air-conditioning load. The cooler outgoing exhaust air absorbs heat from the warmer incoming air, cooling it as it enters. But an HRV recovers only dry heat, not humidity.

An ERV can go one step farther. It will transfer not only the heat between the two airstreams, but about half of the humidity, as well. For this reason, an ERV is superior in humid climates. If you live in a hot and humid climate, however, an ERV alone will not dehumidify incoming air sufficiently. You will probably still need to consult with your HVAC contractor about adding dehumidification.

An HRV or ERV can reduce the cost of cooling or heating your ventilation air. The equipment has a high first cost, and it requires proper installation, but it is highly energy efficient. It is most likely to be cost-effective in cold or harshly cold climates, or in very hot, humid climates, especially where fuel costs are high. In milder climates, there is less of a temperature difference between incoming and exhausted air, so heat or energy recovery is less likely to be cost-effective.

If you install an HRV or ERV, you will still need a separate range hood in the kitchen and an exhaust fan in the bathroom.


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