How to Get the Ventilation That You Need in Your House.

If your house is stuffy, odors linger, or humidity is high in fall and winter, it is likely that your house does not have adequate fresh air. If you or your children have respiratory conditions, such as asthma, bronchitis or chronic colds, getting the proper amount of fresh air is even more important. Opening windows can be part of the solution, but open windows can be a security risk (in some neighborhoods), can cause comfort problems and can increase heating and cooling costs. Furthermore, opening windows may not improve indoor conditions under all circumstances. Using a mechanical ventilation system, such as an exhaust fan or a heat recovery ventilator (HRV), can be more effective.

We get some fresh air from natural infiltration. This is the amount of fresh air that comes into your house through leaks, and is sometimes shown as house air changes per hour (ACPH). An air-change rate of 0.5 ACPH means that half the house air is changed every hour, or that the amount of fresh air that enters the house every two hours equals the volume of the house. Exfiltration, or the amount of air that exits the house, always equals infiltration — if it didn’t, the house would either implode or explode.

The fresh air needs to be moved around the house, particularly to rooms with closed doors (such as bedrooms). This distribution usually requires fans and ducting systems. Imagine a two-storey house with all the fresh air infiltrating or being delivered into the basement where the clothes dryer is running. Without distribution, the fresh air would be removed from the house by the clothes dryer before it reached the occupants on the floors above. Only the basement would receive fresh air.

Ventilation and good air quality are sometimes under-appreciated. If your furnace breaks down in the winter and the house starts getting cold, you will notice that problem within a couple of hours at the most. Insufficient ventilation will generally not be noticed as quickly as it takes time for symptoms, such as stuffy air, to develop.

A good time to check your indoor air quality is when you enter your house, before you get accustomed to the indoor air. Does it have a distinctive odor? Is it fresh and neutral? People moving to a house with good ventilation from a house with bad ventilation will recognize that the indoor air quality in their previous residence was not as good as in their new home.

People need fresh air all the time, but the need for additional ventilation will change. In the middle of winter, when it is very cold or windy outside, the natural air-change rate of the house will be highest and you may not require additional mechanical ventilation.

However, most times in fall, winter and spring, having some mechanical ventilation may make sense. Mechanical ventilation is beneficial for mid-summer if you are using an air conditioner and do not open windows for extended periods of time. In fact, summer is when houses have the lowest natural air change rate. Figure 4 shows the results of recent research that monitored air change rates in an Ottawa home during the summer. Most of the time, the house air change rate was below 0.3 air changes per hour — a recognized threshold for good indoor air quality.