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Ventilation and Airflow Control



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What Is Ventilation?

A proper ventilation system exhausts stale air, supplies and distributes fresh air throughout the house, and can be controlled. The National Building Code of Canada requires that new homes replace one third of the total volume of air with outside air every hour. How does your home compare?

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The Difference Between Ventilation and Air Leaks

While some homeowners believe that air leaks allow fresh air to enter the house, they don't realize that air leaks often have negative consequences. Air leaks are a sign of a poorly sealed or degrading house. In cold, windy weather, too much air could be drawn into a leaky house, causing high energy bills, cold spots, drafts and moisture damage. In spring and fall, with changes in wind and air pressure, there might not be enough fresh air.

Air leaks are uncontrolled air exchange; ventilation means controlled air exchange.

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How Much Ventilation?

Recognizing that homes are now built with tighter air barriers, the National Building Code of Canada now requires that new homes include a mechanical ventilation system with a minimum exhaust capacity of one third of an air change per hour. This means that one third of the total volume of air in the home is replaced by outside air every hour. Not every retrofitted home will need this much mechanical ventilation capacity. Even after extensive air-sealing work, considerable amounts of air will continue to flow through the envelope of most older houses. Humidity levels and the appearance of condensation can provide a rule of thumb for judging ventilation needs. Generally, if there is only occasional or a small mist of condensation on double-glazed windows on the coldest days, then sufficient ventilation is present. Of course, in houses where there is little moisture generated but where activities such as hobbies and smoking generate pollutants, more ventilation will be required.

You can assess the tightness of an air-barrier system with a blower door test.

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Why Control Airflow?

Controlling airflow in your home gives you:

  • greater energy and dollar savings
  • a more comfortable home without cold spots and drafts
  • protection of the building materials from moisture damage
  • improved comfort, health and safety
    • stale odours and stuffiness are eliminated
    • a safe supply of air for combustion appliances is assured
  • a cleaner and quieter home

Controlling airflow involves three relatively simple activities:

  • preventing uncontrolled air leakage through the building envelope
  • providing for fresh-air supply and the exhaust of stale air
  • providing draft and combustion air for fuel-burning appliances

These activities must always be done together. Halfway measures will not do.

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Air-Leakage Control: Wind Barriers, Air Barriers and Air Sealing

To be effective, insulation must trap still air. It must be protected from wind blowing through from the outside and from air escaping from the inside of the home.

The wind barrier is located on the outside of the envelope to protect the insulation from the circulation of outside air. Standard building materials such as exterior sheathing and building paper or new sheet materials such as housewraps act as the exterior wind barrier.

The air barrier blocks airflow from the inside to the outside. By doing this, it serves two important functions:

  • It reduces heat loss by preventing air from passing in and out through the envelope.
  • It protects the insulation and structure from moisture damage caused when water vapour condenses in the envelope assembly.

The air barrier can be installed at any location in the envelope; it may even be combined with the wind barrier. It's usually installed on the inside of the envelope where it is kept warm. This protects the material from temperature extremes that can affect its durability. When installed on the warm side, the air barrier is often combined with the vapour barrier.

If located on the inside, the air barrier will also prevent convective heat loss when air circulates from the house into the wall space.

To make sure that you've accounted for a continuous air barrier, make a sketch of the wall, attic or foundation that you plan to retrofit. Then take a coloured pencil and trace a line through all the air-barrier components on the sketch without lifting the pencil from the paper.

Because of the many components that make up the house envelope, including walls, foundations, doors and windows, it's impossible for any one material to surround the house completely and form the air barrier. The air barrier is actually a system made up of many different components that are sealed to each other.

Here are some typical components of the air-barrier system:

  • Polyethylene, drywall or plaster are used for large surfaces such as walls and ceilings.
  • Windows, doors, hatches and vent dampers seal openings in the envelope.
  • Even structural parts of the building, such as sill plates and rim joists, can form part of the air-barrier system.
  • Caulking, gaskets and weatherstripping seal joints between the components to ensure that the air-barrier system is continuous.

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How Tight an Air Barrier?

For an air barrier to work, it must be continuous and well sealed. But if the air barrier is tight, how will fresh air get into the house?

  • First, most older houses are so loosely built that air will still be able to come in to provide ventilation even after extensive work to control air leakage.
  • Second, the air barrier is just the first step in the control of airflow. It is also necessary to provide ventilation air and air for combustion. These steps may be necessary in houses where extensive retrofit and renovation work has been done, where the house is heated with a reduced flue-action heating system (such as electricity or a high-efficiency gas system) or where there are special ventilation needs.

It is a good idea to take a systematic look at the moisture balance and ventilation needs of your house.

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Healthy Ventilation

It's important to remember that your house works as a system. Keeping your home airtight is one part of the system. Airtightness combined with proper ventilation leads to greater energy and dollar savings, improved comfort and protection from moisture damage.

Control your airflow by:

  • preventing uncontrolled air leaks
  • providing ventilation (supplying fresh air and removing stale or polluted air)
  • providing draft and combustion air for fuel-burning appliances

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Providing Controlled Ventilation

Years ago, ventilation for homes was provided by open windows and doors and by uncontrolled air movement, but this method was not always comfortable or effective. In cold, windy weather, too much air could be drawn into the house, causing high fuel bills and uncomfortable drafts. Often, in spring and fall, not enough fresh air would be supplied.

With uncontrolled airflow stopped by an air-barrier system, it is now possible to provide comfortable, effective ventilation year-round.

A system for controlled ventilation consists of four basic parts:

  • a means of exhausting stale air and excess water vapour
  • a means of supplying fresh air
  • a way of distributing the fresh air throughout the house
  • controls for operating the ventilation system

Many homes already have parts of a complete ventilation system; it's just a matter of putting them all together and adding the missing components.

The exhaust function can be provided by kitchen and bathroom fans; these are already located conveniently in areas of high humidity. Clothes dryers should also be vented outdoors.

Fresh air supply can be arranged for homes with a forced-air heating system by connecting an outside duct to the forced-air system. Air is distributed by running the furnace fan on low speed, even when the furnace is not heating.

In homes with individual room heaters, fresh air can be provided by installing a central supply with a duct to each room and a fan to move the air. This system is easiest to install in bungalows. In colder regions, the incoming air may need to be tempered or preheated.

Controls are usually connected to the exhaust side of the system; the supply side responds passively to replace the amount of air exhausted. One control method uses humidity as an indicator of how much air needs to be exhausted. There is usually an automatic setting on the exhaust fan for routine operation, with a manual override for cooking, showers or times when there are more people in the house.

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Providing Air for Combustion Appliances

A combustion appliance is any device that burns fuel; furnaces, fireplaces, wood stoves, gas stoves, gas water heaters and gas dryers are all combustion appliances. Combustion appliances require air to burn the fuel and to provide draft for the appliance's chimney.

Older homes without a tight air barrier typically provide plenty of air combustion and draft control through cracks and holes in the building envelope. Tighter houses and houses in which fans, exhaust appliances and fireplaces may be competing for air can experience insufficient draft and even backdrafts. This can be a serious safety consideration where the draft in the chimney can be reversed, and dangerous combustion gases can spill back into the house.

To control air leakage in your house, you must make sure that each combustion appliance has enough air to work properly.

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