Ventilation

1. Passive vs. Active Ventilation

There are two primary approaches to home ventilation:

• Passive Ventilation: This relies on natural forces to move air. It includes open windows, doors, and purposeful vents in the attic or crawl space. Passive ventilation is simple and requires no energy, but it is uncontrolled. The amount of airflow depends entirely on external factors like wind speed and temperature differences. It is not a reliable method for managing indoor air quality or humidity.

• Active (Mechanical) Ventilation: This uses fans and ductwork to actively move air. It is a controlled system that ensures a consistent and predictable rate of air exchange. Examples include bathroom and kitchen exhaust fans, as well as dedicated systems like HRVs and ERVs.

2. Bathroom and Kitchen Exhaust

These are examples of spot ventilation, which targets specific, high-moisture areas. They are essential for removing pollutants and moisture at the source.

• Moisture: A single shower can release a significant amount of water vapor into the air. If not exhausted, this moisture can lead to condensation on cold surfaces, feeding mold and mildew growth in bathrooms and throughout the home.

• Pollutants: Cooking, especially on a gas stove, releases pollutants like carbon monoxide and nitrogen dioxide. A kitchen exhaust hood that vents directly outside is crucial for removing these harmful gases and the moisture and grease that they carry.

These systems must be vented to the outside of the building envelope. Venting them into an attic, wall cavity, or crawl space is a major code violation and a building science failure. The moisture and pollutants would be trapped in these unconditioned spaces, leading to mold, rot, and a breakdown of the building materials.

3. Ventilation Systems That Run 100% of the Time

In modern, tightly sealed homes, a dedicated mechanical ventilation system that runs continuously is often necessary. The most common types are:

• Heat Recovery Ventilators (HRVs): An HRV brings fresh outdoor air into the home while simultaneously exhausting an equal amount of stale indoor air. It transfers the heat from the outgoing air to the incoming air, pre-heating it in the winter to save energy.

• Energy Recovery Ventilators (ERVs): An ERV does the same as an HRV but also transfers moisture, which is beneficial in both humid and dry climates.

These systems provide a controlled, continuous flow of fresh air, which is essential for maintaining healthy indoor air quality and preventing the buildup of moisture and pollutants.

4. Recommended Ventilation by Building Scientists

Building scientists use standards like those from the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) to determine how much ventilation is needed. A common starting point is the ASHRAE 62.2 standard, which recommends a continuous ventilation rate based on the home's square footage and the number of bedrooms. The goal is to provide enough air exchange to maintain IAQ without wasting excessive energy.

• The standard calls for a base ventilation rate of 3 CFM (cubic feet per minute) per 100 square feet of floor area, plus 7.5 CFM per bedroom. For a typical 2,000-square-foot, 3-bedroom home, the required ventilation would be around 82.5 CFM.

5. Impact on IAQ and Structural Health

Ventilation is a foundational element of a healthy building.

• Indoor Air Quality: Without proper ventilation, indoor air can become a soup of pollutants, including volatile organic compounds (VOCs) from building materials, pet dander, dust mites, and carbon dioxide from occupants. Ventilation dilutes and removes these pollutants, creating a healthier living environment.

• Structural Health: By removing excess moisture, ventilation prevents condensation, a major cause of mold growth, wood rot, and a breakdown of building materials. This is especially true in attics and crawl spaces, where poor ventilation can lead to a host of long-term structural problems.

Here's why:

1. Inconsistent and Uncontrolled Operation

The ASHRAE 62.2 standard requires continuous, controlled ventilation. A bathroom fan is not designed for this. It's a "run-on-demand" system, meaning it's only turned on by the occupants, typically for a short period after a shower or when using the toilet.

• Inconsistent Air Exchange: A family might forget to turn the fan on, or they might turn it off too soon. This leads to long periods of no air exchange, allowing indoor pollutants and moisture to build up to unhealthy levels.

• Negative Pressure: When the fan is running, it creates negative pressure by pulling air out of the building. This can lead to the issues we've already discussed, such as backdrafting and pulling in unfiltered air from attics or crawl spaces. A system that creates a pressure imbalance is not a true whole-house ventilation solution.

2. A Spot Solution for a Whole-House Problem

A bathroom fan is designed to remove a specific source of moisture and odors from a small, contained area. It is a spot ventilation tool, not a whole-house ventilation tool.

• Limited Coverage: The fan only pulls air from the bathroom itself. It does not provide fresh air to the bedrooms, living room, or other areas of the house. This means that pollutants generated in those other areas (such as off-gassing from furniture, pet dander, or CO2 from occupants) are not being removed.

• Pollutant Transport: When the bathroom fan is running, it creates a pressure gradient that can draw air from all over the house, including pollutants and moisture from sources in other rooms, and pull them into the bathroom before exhausting them. However, if there are pollutants in a bedroom and the fan isn't running, they remain trapped there.

3. Energy Inefficiency

A bathroom fan's motor is generally not designed for continuous, long-term operation. Running it 24/7 would be highly energy-inefficient and would likely lead to premature failure of the motor. It is also not a heat or energy recovery system; it simply exhausts conditioned air (and all its heat or cold) to the outside, which is wasteful.

The Correct Approach

The ASHRAE 62.2 standard is a guideline for ensuring a healthy, energy-efficient home. The correct way to meet this standard is by using a system designed for the job, such as:

• Dedicated Whole-House Ventilation: A continuous, low-CFM exhaust fan (sometimes in a central location or a bathroom) that runs all the time.

• Heat or Energy Recovery Ventilator (HRV/ERV): These systems provide balanced ventilation, ensuring that fresh air is supplied while stale air is exhausted. They are the gold standard for meeting modern ventilation requirements because they also help recover energy, saving on heating and cooling costs.

In summary, a bathroom fan serves a critical purpose, but it cannot be a substitute for a properly designed and installed whole-house ventilation system that meets the ASHRAE 62.2 standard.