“My Home Needs to Breathe” – Many of us have heard this phrase at some point from people who are unconvinced about the benefits of an airtight home. Quite often, one of the main concerns is over the impact it has on indoor air quality.
Traditionally, homes have been naturally ventilated as a result of cumulative (and for the most part, non-intentional) air leakages through the building envelope, meaning that there was no pertinent need to mechanically exhaust stale air and bring fresh air into the home.
The downside to this approach is that the air breathed indoors was in fact filtered through the building assemblies and insulation. If you have ever removed drywall to find dirty fiberglass insulation behind (or seen streaks of dirt coming from air registers), that is because the insulation has acted as a filter, catching dirt and pollutants from the air as it passes through. Although this can be considered a passive form of ventilation, it certainly doesn’t correlate to good indoor air quality.
Not only this, but gaps or cracks in the building envelope are the biggest sources of both heat loss and moisture infiltration into the home. In the days when energy consumption was not so much of a concern, the solution was to heat the house more. This would compensate for the loss of conditioned air but also help to dry out assemblies and eliminate potential moisture concerns.
When it comes to indoor air quality, there are several components to consider:
Particulate Matter
Fine particulate matter with a diameter <2.5 microns in diameter (PM2.5) which comes from sources such as natural gas appliances, bacteria and viruses, and wildfire smoke. These particles penetrate deep into the lungs, causing irritation of the airways and potential cardiovascular issues.
Carbon Dioxide
CO2 comes primarily from combustion and respiration inside the home. Studies have shown that high CO2 levels can impair cognitive function.
Carbon Monoxide
CO is derived from the incomplete combustion of fossil fuels. Breathing carbon monoxide is poisonous and potentially life-threatening.
Biological Contaminants
Biological contaminants such as mold, bacteria, viruses and allergens are strongly related to factors such as relative humidity levels and water damage in the home. They are often responsible for allergic symptoms or respiratory infections.
Humidity & Temperature
Both high and low humidity can affect occupant health and comfort. Relative humidity is strongly correlated to temperature and can have a strong impact on the presence of biological contaminants.
TVOCs
Total volatile organic compounds, such as formaldehyde, are compounds that can easily become gases or vapors. They are typically emitted from products such as furnishings, sealants, and paints over a period of time following manufacture or installation. Volatile organic compounds are responsible for a variety of adverse health effects. Over the long term, they are carcinogenic and can cause damage to the central nervous system.
Advancements in building science have taught us that a more airtight home is a more energy-efficient home. That being said, a house is a system, and the building envelope cannot be tightened up without having a knock-on effect on other areas in the home. With less natural air exchange between the interior and exterior environment, it is crucial to have a well-thought-out ventilation strategy, typically using mechanical ventilation, to remove moisture and pollutants, and ensure adequate indoor air quality.
Building Code Ventilation Requirements
Ventilation requirements were first introduced into the BC Building Code in December 2014. Since then, residential dwelling units are required to have a principal ventilation system that exhausts air from bathrooms and kitchens and supplies fresh air to bedrooms and living areas. In most cases, this requires a mechanical fan and is designed to create a continuous flow of air throughout the dwelling unit. In addition to the principal exhaust fan, every kitchen and bathroom is required to have a fan that exhausts air to the outdoors.
The ventilation requirements for each dwelling unit can be found in BCBC 9.32 and are defined by floor area and number of bedrooms. Some small dwellings in mild climates (such as laneway homes) are an exception and may still be permitted to supply fresh air through passive vents, providing certain other requirements have been met.
It is important to mention that while defining ventilation requirements by floor area and the number of bedrooms might be a good rule of thumb, it fails to account for the number of occupants, activities in the home, materials used, and fuel systems all of which are factors impacting indoor air quality. Variation in these factors could result in homes that are under or over-ventilated.
How to improve indoor air quality?
Passive ventilation
Passive ventilation typically refers to an intentional opening in the building envelope (such as a screw vent that can be opened and closed seasonally), designed to allow for the passage of indoor and outdoor air. In more practical terms, passive ventilation can mean anything as simple as opening a window or two for cross ventilation. The downside of this approach is that outdoor air quality may be poor and a passive opening does not filter for pollutants. Although the openings are intentional, passive ventilation negates the importance of airtightness from an energy-efficiency perspective.
Exhaust-only Ventilation
Exhaust-only ventilation refers to kitchen range hoods, bath fans, humidistats, etc. which can be set to run on a timer, left constant running, or switched on and off as necessary. These are great for exhausting stale air and removing moisture and pollutants, but they don’t bring in any fresh air from the outside or distribute it around the home. They also rely on unintentional leaks through the building envelope to replenish the exhausted air. If the home is sufficiently airtight, exhaust ventilation can induce negative pressure in the home and run the risk of back drafting exhaust fumes from combustion appliances. This is also not the most energy-efficient approach as conditioned air is intentionally exhausted from the home without heat recovery.
HRV & ERV
HRVs (heat recovery ventilators) or ERVs (energy recovery ventilators) are without a doubt the most energy-efficient approach to home ventilation. They exhaust stale indoor air, bring in fresh air filtered from the outside and exchange the heat between the two. The difference is that while an HRV only exchanges temperature, an ERV also recovers relative humidity. ERVs typically work best in colder climates where indoor air is dry in the winter, whereas an HRV is more appropriate for a humid indoor environment.
The type of filter installed (eg. MERV, HEPA, or charcoal) can play an important role in how and to what degree pollutants are filtered from outside. Picking the right filter can provide protection when the outdoor air quality is poor, for example during wildfire smoke events, which are becoming an increasingly common occurrence.
Inline or ductless HRVs are a newer technology that is currently being introduced to the market. They allow for the installation of a ventilation system without the need for dedicated or forced air ductwork. Many of these are not yet HVI tested and certified but offer enormous potential for retrofits in existing buildings where there isn’t central ventilation ductwork, or for individual ventilation in apartment units or secondary suites.
What should occupants consider?
Digital monitoring
Digital monitoring is an important and often overlooked aspect of indoor air quality, particularly because it can bring more awareness to homeowners, helping them notice patterns and correlations between how they feel and the quality of the air in their homes.
Indoor Activities
Monitor and be mindful of activities that result in a spike in humidity or indoor pollutants eg. cooking, smoking indoors, plants & pets, and operating combustion appliances.
Furnishing Fixtures and Finishes
Pay close attention to the furnishing, fixtures, and finishes that you use for the home. Some of these may have a higher VOC content than others and can considerably impact indoor air quality. Carpets, for example, can emit VOCs up to 5 years after installation and are known to retain more dust and allergens than wood or tile flooring.
Air Purifiers & Dehumidifiers
While air purifiers are typically small and don’t exhaust stale air or bring in fresh air from outdoors, they do still filter out pollutants and can be worth considering in a home suffering from indoor air quality issues. Likewise, a dehumidifier can lower the relative humidity in the home, impacting moisture levels and helping to reduce the presence of biological contaminants.
