Airtight homes offer lower energy costs and are easier to heat and cool but without proper ventilation welcome pollutants, moisture, odors, and stale and toxic indoor air. Ventilating a home using natural means such as opening doors and windows, aside from being inefficient, may not always be feasible due to security reasons or outdoor noise levels, rain, snow or other weather conditions. Mechanical ventilation allows for exchange of indoor air with outdoor air to reduce indoor pollutants, moisture, odors, and stale and sometimes toxic indoor air in your home year round.

Mechanical Ventilation:

• Installs as part of any central heating and cooling system
• Provides a constant, controlled supply of fresh air to your home year round
• Reduces excess indoor humidity levels
• Reduces unhealthy indoor air pollutants such as formaldehyde, volatile organic compounds (VOCs), radon, carbon dioxide, smoke, odors, dust, bacteria and viruses and more
• Saves energy by effectively retaining and utilizing the energy value from your indoor air

Natural ventilation: This is when air moves into the house through open doors or windows. Though natural ventilation can improve indoor air quality, it has several limitations such as:

• It is an inefficient means to dilute contaminants in a home because the air is not uniformly distributed.
• The flow cannot be controlled.
• Increased ventilation from outside can increase pollen and mold spore levels, if outdoor counts are elevated, and moisture.
• If there is not much wind outside and not much temperature difference between indoors and outdoors, there will not be enough ventilation in the home when windows are opened.
• If there are high winds outside or big temperature differences between indoors and outdoors, there will be too much ventilation that can make humidity levels too low inside. This kind of ventilation in the winter also wastes energy by heating outdoor air.
• If the outside air is humid, it can make the indoor air humid, which can create a favorable environment for dust mites and mold.

Properly installed and operated exhaust fans can remove stale, moist air from bathrooms and kitchens. However, there needs to be a way that fresh air can be brought in to make up for the air being vented out. Many homes actively exhaust air but rely on natural ventilation to bring air into the home.

When there is little infiltration, natural ventilation, or mechanical ventilation, the air exchange rate (fresh air coming in and stale air going out) is low and pollutant levels can increase. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends a minimum ventilation rate of 0.35 ACH (air changes per hour) for new homes. This air exchange rate measures how many times all of the air inside the house is replaced with fresh outside air.

Some people may choose not to "tighten" up homes that are "leaky" or that rely upon infiltration to bring in the needed outside air. There are several problems with doing that:

• Ventilation through infiltration cannot be controlled so there may not be enough ventilation in areas of the home that need it most. As a result, pollutants can build up in even "leaky" homes.
• Infiltration and natural ventilation rely heavily on outdoor weather so these conditions can drastically reduce the amount of outdoor air that enters a home.

"Leaky" homes can cost more in the long run since they are not energy efficient and cost a lot to heat and cool. "Tight" homes are ones that have been weatherized by sealing cracks, having good insulation, and quality windows. If indoor air pollutants are controlled, the use of mechanical ventilation in these homes can be more comfortable since the temperature and humidity levels can be controlled. If homes are tight, pollutants are not controlled, there is an inadequate number of air exchanges, and levels of pollutants can build up.