Letting the fresh air in
A discussion on ventilation requirements for Canadian homes
One of my favourite feelings in the world is the first time I throw open a window in my home after a long, cold Canadian winter and take a deep breath of fresh air as I look outside and watch nature wake from her slumber. There's nothing quite like taking that first, deep breath of fresh air or sleeping with the windows open for the first time; it feels like I'm letting mother nature air out my home and push out the stale, lingering feeling of old man winter.
My home was constructed in the late 1970s as part of a planned subdivision in my community. Tract homes are notorious for being built quickly and to minimum code requirements, and my home was no exception. Up until the early 1980s, homes were ventilated through the envelope by negatively exhausting the interior of the home, typically by turning on a mechanical exhaust device in the home such as a bathroom exhaust fan, kitchen range hood, clothes dryer or central vacuum. These devices exhaust a considerable amount of air in a relatively short amount of time, creating a pressure imbalance and forcing outdoor air through the envelope to flush out any contaminants produced in the home. This is referred to as an 'exhaust only' ventilation system and was pretty par for the course here in Ontario.
After the multiple energy crises of the 1970s, designers, builders and building code standards turned their attention to improving the insulation levels and airtightness of building envelopes. The thought was that by reducing the location and quantity of infiltration points for fresh air to aspirate through the envelope, energy costs could be reduced to manageable levels. The byproduct of this is that indoor air quality suffered, and people started contracting respiratory illnesses associated with Sick Building Syndrome.
In older, drafty homes, the use of natural ventilation through infiltration may have been sufficient in ventilating a home. This wasn't the case for newly constructed homes, however, and many studies conducted through the mid-late 1980s and early 1990s have confirmed that natural ventilation through infiltration does not provide enough ventilation to flush out the home or protect against depressurization. As we get better at the building science side of things, we're focusing more on the construction of the envelope, including the location of the air barrier, increased insulation levels, minimizing penetrations through the envelope AND addressing fresh air requirements for buildings and their occupants.
Re-thinking the Thought on Fresh Air
In 1989, the Canadian Standards Association (CSA) released CAN/CSA F-326-M89 'Residential Mechanical Ventilation Systems'. This standard laid the framework for defining ventilation requirements for single-family residential homes under the National Building Code (NBC), including self-contained ventilation systems and mechanical exhaust equipment such as bath fans and kitchen range hoods. While this took significant strides towards addressing ventilation requirements in homes, it wasn't mandated by either the NBC or provincial level building codes.
In late 1990, the Canadian Mortgage and Housing Corporation (CMHC) held a Residential Ventilation Symposium to address the challenges and shortfalls of CSA F-326, and discuss strategies for increasing builder and homeowner awareness of the need for proper ventilation systems. It was noted that the standard was too complex for a layperson and more education was needed on the importance of indoor air quality. The standard was superseded the following year by CAN/CSA-F326-M91 and was adopted by the next release of the NBC and OBC.
Okay, now that you know the history of ventilating homes in Ontario, let's focus on the actual ventilation requirements.
Since the mid-1990s, single-family residential homes have been required to have mechanical ventilation systems in place per Section 9.32 of the Ontario Building Code (OBC); the code states that a mechanical ventilation system is required in every dwelling unit that is supplied with electrical power. Natural ventilation alone, while admirable from an energy efficiency standpoint, is not acceptable by the building code which dictates minimum ventilation requirements for each space in the home.
Here's where things tend to get muddied up a bit - there are two approaches a designer can take when designing a ventilation system for a home; OBC Section 9.32 lays out prescriptive requirements, including ventilation rates and distribution requirements, and is meant for homes that are:
3 storeys or less
less than 600 m2 (6,460 ft2) total building area
contain 5 or fewer bedrooms
a self-contained ventilation system serving only one dwelling unit
If your home meets all of these conditions and doesn't have an induced draft or natural draft appliance, then the ventilation system can be designed to Section 9.32.
If at least one of the above requirements are not met then the ventilation system must be designed using Part 6 of the OBC which is where CSA-F326 comes in. Part 6 is considered to be the performance-based approach where the end-result is listed; the materials, methods, equipment and other details are left to the discretion of the designer. This allows more freedom in interpreting various sections of Part 6 and CSA-F326 as long as the end-goal is met.
When talking about ventilation and forced air systems, there are two units of measurement we're looking at, volume and velocity. Airflow volume is measured in cubic feet per minute (cfm) or, for those that prefer the metric equivalent, litres per second (L/s). This is the total amount of air that will be delivered in a specific timeframe. The quick and dirty rule of thumb is 1 L/s = 2 cfm, though some jurisdictions use the hard conversion factor of 1 L/s = 2.118 cfm. The ventilation rates between OBC and CSA-F326 are generally the same, with Master Bedrooms and Basements requiring 20 cfm and other habitable spaces requiring 10 cfm.
Air velocity is the measurement of speed and is used to calculate how fast an airstream travels. An airstream velocity of 200 feet per minute (fpm) is, in practice, equivalent to 1 metre per second (m/s) and these units are used interchangeably between the imperial and metric systems.
We'll talk about acceptable velocities in a later blog entry, but for now, what you need to know is that airflow volume and velocity are directly related. Let's take an 8" x 18" duct for example.
For a duct with a constant cross-sectional area, the volume increases or decreases proportionally to the velocity. In the example above, the trunk duct supplying 500 cfm of volume has a cross-sectional area of 144 in2 or 1 ft2; using the formula FPM = CFM / Area, we get a velocity of 500 fpm. The branch duct takes 100 cfm of supply air to distribute to an end-location. Since the trunk duct doesn't decrease in size with the reduction in air volume, the velocity reduces to 400 fpm.
So why is this important? As mechanical designers, we need to ensure that not only are we designing our systems to provide sufficient capacity (volume) to each space, we need to ensure we're delivering it at an acceptable velocity to keep our duct sizes at the right size while ensuring the supplied air isn't delivered at such a high velocity to create noise and turbulence in the system. Every design decision we make has an impact on the overall mechanical system, occupant comfort and the indoor environmental quality of the spaces we're conditioning. It's all connected.
In the next entry, we'll talk about the difference between exhaust only ventilation, supply only ventilation and balanced ventilation systems.