In order for air to flow, a difference in pressure is required as well as a continuous path between the two areas.

High Pressure

Low Pressure

Air flows from a high pressure to a low pressure, never from a low pressure to a high pressure.

The objective to keep in mind when building a home is to reduce air flow from the interior to the exterior as much as possible. By reducing the amount of air flow through a building, you are reducing the amount of heat loss

Air Leakage Locations

Some leakage areas within a home are unavoidable and will always occur. Examples include: vents, drains, chimney, plumbing stacks, etc.

Insulating around these areas will ensure that the leakage area is reduced as much as possible. As an insulator it is important to make sure that all areas that require insulation are insulated or air sealed to reduce air flow. (This is because insulation should not be used around chimneys for various reasons.)

Air Flow Effects

Air flow in buildings is complex, time dependent and multi-directional. A perfectly airtight air barrier system is unlikely to be achieved in practice, so it is also desirable to control the air pressure differences driving the air flow. A tight building envelope minimizes the amount of air required to provide pressurization. In winter, cold air coming in from the exterior through leakage is heated when entering the conditioned space.The more air that is brought in, the greater the heating load, the greater the operating cost and the greater the drying effect (referred to as over-ventilation). In parts of Canada during the summer, outdoor air brought in from the exterior must be dehumidified and cooled. Air pressure differentials contribute to the transfer of pollutants, radon gas ingress, and spillage and back-drafting of combustion appliances

There are three main factors that contribute to air flow in a home:

• Each effect creates pressure differences across the envelope
• Flow is always from high to low pressure
• Pressure differences + cracks and holes = air flow and heat loss

The amount of air flow and the resulting heat loss depends upon the size of the openings and the pressure difference across the envelope.

During the heating season, air infiltration generally occurs in the lower part of the building while air exfiltration occurs in the upper part of a building.

For air flow to occur, there must be both:

• A pressure difference between two points
• A continuous flow path or opening connecting the points

Primary mechanisms that generate the pressure differences required for air flow within and through buildings include:

1. Wind forces act on all buildings, usually positive pressure on the windward face and negative (suction) pressure on the other walls.
2. Stack effect or buoyancy: pressures are generated by differences in air density with temperature (i.e. hot air rises and cold air sinks).
3. Mechanical air handling equipment and appliances: fans and blowers cause the movement of air within buildings and through enclosures. If more air is supplied to a room than is exhausted, the excess air leaks out of the space and the room is said to be under a positive air pressure.
4. The magnitude of the pressure difference can also vary considerably depending on the shape of the building, the exposure, the height and local conditions of outdoor temperature and humidity.

Air Flow Control

Involves 3 interactive strategies:

• 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

CAUTION:
ALL 3 STRATEGIES INTERACT AND MUST BE IMPLEMENTED TOGETHER FOR HEALTH AND SAFETY REASONS.

The important point to stress is that these 3 strategies MUST ALWAYS be carried out as if they were a single strategy. Otherwise, conditions can be created within the house that could impact upon the homeowner's health and safety as well as the structural integrity of the building.

Why Control Air Flow?

COMFORT	DURABILITY	ENERGY EFFICIENCY
Comfort benefits: Reduced drafts and cold spots Stale odours and stuffiness are reduced Improved indoor humidity control A cleaner and quieter home	Durability benefits: Protection of building materials from moisture damage	Energy efficiency benefits: Results in greater energy and dollar savings

Illustration Source: Keeping the Heat In, Fig.2-5,Pg.14

Not Managing Air Well Means…

• Houses that are stuffy and damp (high humidity)
• Houses that smell, odours linger
• Houses that are dry (low humidity)
• Drafts and chills
• Back drafting appliances (woodstoves, fireplaces)

Uncontrolled air leakage through the enclosure is often a major cause of performance problems (e.g. comfort, health, energy, durability, etc.). Low humidity can result in respiratory problems, dry skin conditions, increased allergy problems and bacteria and virus propagation. Backdrafting Combustion Appliances can KILL your client. This is of the utmost importance.