On average, Americans spend 90% of their time indoors, according to the U.S. EPA. In cold weather, that percentage increases. Unfortunately, in most homes there’s a good chance the Indoor Air Quality (IAQ) is contributing to allergies, asthma and even the spread of influenza and the common cold. In colder weather, fewer air changes between indoors and outdoors further compromises Indoor Air Quality. This is especially the case with energy-efficient homes.

There are only two basic ways air can enter or leave a house:

1) Infiltration– the natural flow of air into and out of the house through openings and cracks in walls and joints and around doors and windows. In energy-efficient homes with airtight windows and doors and high R-value insulation, infiltration is minimal. In these homes, ventilation (see below) is required to maintain Indoor Air Quality, according to the U.S. Department of Energy.

2) Ventilation– the deliberate, assisted exchange of air between the inside and outside of a house. Ventilation can be achieved naturally, by opening windows or doors, or mechanically. Mechanical systems range from very simple exhaust systems to advanced energy-recovery and heat-recovery ventilators.

Natural vs. mechanical ventilation

The quickest and easiest way to ventilate a home is with natural ventilation. This includes simply opening windows. But as much as a third of a home’s heat loss is through windows even when they are closed, according to the Natural Resources Defense Council. Opening a single window dramatically increases heat loss.

Amechanicalventilation system is the solution when energy efficiency is a priority. There are four basic types of mechanical ventilation systems.

Types of mechanical ventilation

The four basic types of mechanical ventilation include:

1. Exhaust Ventilation:A relatively simple and inexpensive solution. An exhaust ventilation system typically uses a single large fan to expel air from the house. Adjustable vents allow fresh air to come in from outside. At the same time, however, they also draw in unfiltered air, which may contain pollutants, from outside.

2. Supply Ventilation:This system is also relatively inexpensive and easy to install. A fan and ducts bring outside air into the house. One major advantage of this type of system is it allows the incoming air to be filtered or dehumidified. However, the incoming air is not tempered (heated or cooled) as it comes in, so additional energy costs may be incurred.

3. Balanced Ventilation:These systems combine supply and exhaust ventilation. Because they include supply ventilation, the air coming in the house can be filtered. However, like supply systems, they don’t heat or cool incoming air so additional energy costs may be incurred.

4. Energy Recovery Ventilation:These systems temper (heat or cool) the outside air as it comes in. It does so by heat transfer. There are two types: heat-recovery ventilators (HRV), which only transfer heat, and energy-recovery ventilators (ERV), which transfer both heat and water vapor to help humidify the house. HRV and ERV systems are more expensive than other mechanical ventilation solutions, but are more controllable and minimize energy loss. With these systems, air coming into the house can be filtered.

Mechanical ventilation will usually improve the Indoor Air Quality in a home, regardless of which type is used. But not all systems are equally effective if the outdoor air contains pollutants such as particulate air pollution from nearby traffic or industry, smoke from fireplaces, or other air contaminants. When outdoor pollutants are an issue, the Supply, Balanced and Energy Recovery systems, in combination with high-performance air filtration, are clearly the best mechanical ventilation choices to improve Indoor Air Quality year round.