The topic of energy efficiency in existing homes, and in new construction is dominating much of the industry media. With third party energy standards such as LEED, EnerGuide and R2000 for new construction, and incentive programs from BC Hydro for existing homes, there is a considerable push to install better windows and doors, increase and optimize insulation, upgrade air-sealing, as well as upgrades to mechanical equipment and appliances with the goal to minimize energy costs and save money.

Ironically, we are not made to focus a great deal on measures to improve occupant comfort in our homes; specifically relating to indoor air quality. Perhaps this is because it does not benefit our wallets, nor relieve pressures on utility infrastructure.

The lesser appreciated aspect of better insulation, better windows, and general building envelope efficiency upgrades is the affect they have on ventilation and indoor air quality. In the era of ‘Green Building’ and energy efficiency, we will more and more be relying on mechanical ventilation systems opposed to natural ventilation in typical house construction. To better understand the implications, we need to understand the basics of indoor air quality, house ventilation, and the relationship between ventilation and whole building air tightness.

What is Good Indoor Air Quality?

Good indoor air quality is simply defined as fresh, clean air. When we think of air pollution we don’t often think of it as an indoor problem, although many types of indoor air pollution can be more very detrimental to our health. Tobacco smoke, chemicals from household products, and airborne toxins released from synthetic fibres and most manufactured furnishings all contribute to poor indoor air quality. When we consider that many of us spend up to 90% of our time indoors, the air we breathe has serious implications to our long-term health.

Ventilation for Indoor Air Quality

Adequate ventilation is essential to achieving good indoor air quality. We measure the ventilation of buildings in Air-Changes per Hour (ACH), or the number of times the entire volume of air in the space is replaced. For housing, an acceptable air change rate is between 0.2 and 0.35 ACH, with 0.30 ACH being the recommended level, which equates to changing or replacing 30% of the volume of air in a space each hour. The fall season always presents a great opportunity to discuss the need for ventilation in our homes, as this time of the year presents the peak ventilation demand. We will expand on this later on.

How Does Air Move?

Generally speaking, air pressure differentials are the primary means by which air is moved in and out of a space. This is caused naturally by wind forces, stack effect, or mechanically by the use of fans. When wind flows around a building it places a positive pressure on the front or windward walls, while creating a negative suction pressure on the leeward walls. Stack effect relates to natural convective air currents. Warm air in a heated building is lighter (less dense) than cold outside air, and therefore rises and escapes through the cracks and holes in the building envelope at the top of a building. As the warm air escapes, cold air is drawn in through cracks and holes in the building envelope at the base of the building. This flow is reversed in the summer; however, the temperature and pressure differentials are significantly less than in the winter season. Mechanical fans exhaust air from a space creating a negative pressure which draws in fresh outdoor air through intake louvers, but also the same cracks and holes in the building envelope.

Whole Building Air Tightness

Whole building air-tightness is the overall ability of the building envelope to resist forces from moving air through it under peak pressure differentials. Buildings are required by code to be constructed with a ‘continuous air barrier system’ for specifically this purpose. The air barrier system is comprised of various materials (membranes, coatings etc.) and other components (windows, doors, etc.) integrated together to provide a continuous air-tight plane. In effect, the intent of a continuous air barrier system is to limit the number of cracks and holes in the building envelope where air can un-intentionally pass through. More simply put, the intent is to limit unintentional natural ventilation.

Why Are We Concerned with Air Leakage?

Bulk air leakage on average accounts for 35% of the heat loss in a house. While it also provides natural ventilation in the process, we cannot ignore the inherent energy inefficiency.

The average pressure differential across the building envelope for a typical house is in the 5-10 Pascal (Pa) range, but at peak times, most codes and reference standards identify a 50 Pa pressure differential, which is roughly equivalent to a sustained 35 km/hr wind pressure. Whole building air tightness is determined based upon the Air Changes per Hour the building will experience at the 50 Pa peak pressure differential (ACH 50). To provide some context, an older house in Victoria might have an ACH 50 rating of 8-10, while a new home constructed to ‘energy efficient’ standards will be anywhere from 3-5 ACH 50. To recap, this means that for an older house, 8-10 times the volume of air in the space is replaced every hour. When we consider that even new houses with a ‘continuous air barrier’ still experience an unintentional natural air change rate of 3-5 ACH 50 we need to appreciate both the power of pressure differential forces, but also the general ineffectiveness of air barrier systems constructed today.

Many people will still argue that new buildings today are ‘too tight’ and ‘don’t breathe’ like the old ones, leading to poor indoor air quality, moisture and mould issues. Unfortunately for these individuals, paying to heat spaces where 8-10 times the volume of air in their home leaks outside every hour is beyond excessive. The reality is that buildings need to become more and more airtight to meet the energy efficiency targets we will see in the near future. Standards already in the marketplace such as the ‘Passive House’ target an ACH50 of less than 1. Natural ventilation will become increasingly limited the more airtight a building becomes, with the need for mechanical ventilation systems correspondingly increased to compensate.

Mechanical Ventilation Systems and Peak Ventilation Demand

The majority of houses, new and old have some sort of mechanical ventilation system installed already. In fact, the building code requires at least one mechanical exhaust fan be installed in every house. There are exceptions, but these are not important as they do not apply to the majority.

We mentioned earlier that the fall season represents the peak demand for ventilation. In this season, the temperatures on either side of the building envelope (interior and exterior temperatures) equalize, which neutralizes stack forces. Wind then becomes the sole natural mover of air. On calm fall days, natural air movement can become negligible. We know that we require 0.3 ACH even at static pressure differentials, thus mechanical means are required to create enough of a pressure differential to instigate the required movement of air.

Types of Systems

We are going to examine the two most common types of ventilation systems historically used in houses, the exhaust only and supply only systems, as well as the most common system used in new houses today, the balanced system. For the older systems, it is important to understand how the systems are intended to work, and their inherent limitations. These are especially important if you are considering renovations that may affect the natural system dynamics. The new systems present many advantages, and also may be worth considering in retrofit applications for older homes.

Exhaust Only Systems

Exhaust only systems use a series of fans to exhaust air from the house. Typically they come in the form of bathroom fans, and kitchen exhaust fans in houses where there is no forced air system. The basic principle of the system is that exhausting air creates a negative pressure in the space, drawing in fresh air through intentional air intakes, but also the cracks and holes in the building envelope, providing fresh ‘make-up air’. To satisfy the continuous ventilation requirements, typically a bathroom exhaust fans is designated as the principal exhaust fan, and must be connected to an interval timer to run at set times.

Many people don’t appreciate (or simply don’t understand) the function of this fan, and they remove the interval timer in favor of manual switch to avoid what they consider as unnecessary fan operation. These are the people who will really notice poor air quality in the fall season when natural air movement is limited.

The more airtight a building is, the more an exhaust-only system can struggle to provide the minimum ventilation requirements. Furthermore, these systems can be problematic when naturally aspirated combustion appliances such as gas fireplaces or ranges are used in the house, as the negative pressurization can draw combustion fumes into the house rather than being exhausted outside.

Supply Only Systems

Many traditional forced air furnace systems operate as supply only ventilation systems. With these systems, air is brought into the house mechanically, creating a positive pressure inside the house, driving air out through intentional openings or leakage through the building envelope. One problem with supply only systems is that forcing warm, moisture-laden interior air through holes in the building envelope can result in condensation and mould problems.

Some supply-only systems are connected to a principal exhaust fan (such as a bathroom fan) to provide mechanical exhaust each time the furnace is switched on in an effort to create a more balanced system.

Balanced Systems – the Way of the Future

Balanced ventilation systems provide an even combination of supply and exhaust air, such that the system itself does create undesired pressure differentials. The use of an HRV (Heat Recovery Ventilator) in combination with a forced air furnace or heat pump / air handler combination is the most commonly used system in modern energy efficient homes. The HRV performs two functions: to provide controlled ventilation, and also to pre-heat fresh outside air by passing it through a heat exchanger with warm exhaust air. By controlled ventilation, we mean the HRV’s ability to filter all fresh incoming air, limiting toxins from being brought into the house from outside. We often don’t consider that unintentional air infiltration through the building envelope brings outdoor pollutants and toxins, and also can pick up dust and other toxins such as mould as it passes through wall, roof and floor assemblies.

Closing Thoughts

From a building science perspective, the perfect house ventilation system is achieved with a completely air-tight building envelope, and a well-balanced mechanical ventilation system. While many people who think of green building and energy efficient construction would object to adding more mechanical systems to a house, we have demonstrated the necessity moving forward. This type of system does not aim to replace the ability to provide natural ventilation with doors and windows in the summer months, but rather gives the occupants the ability and flexibility to provide the required 0.3 ACH throughout the entire year, and specifically during peak demand shoulder seasons, while maintaining good indoor air quality.

There is no denying that it is possible to provide adequate ventilation to a building via completely natural means, however this generally comes at the expense of floor area and function that will never be representative of mainstream building practices.