Ventilation Strategies: Natural and Mechanical

• Mechanical Ventilation
• The Relationship Between Ventilation, IAQ, and Building Efficiency

Ventilation is not a one-size-fits-all solution. Different buildings and different situations require different approaches. Broadly speaking, however, ventilation strategies fall into two categories: natural and mechanical. Natural Ventilation

Natural ventilation leverages environmental factors, namely wind and thermal buoyancy, to circulate air. The aim is to create a breathable and comfortable indoor environment without relying on energy-intensive mechanical systems.

Wind-based natural ventilation is driven by pressure differences between the interior and exterior of a building. If you've ever felt a breeze come through an open window, you've experienced this phenomenon. The magnitude and direction of this wind-driven ventilation depend on factors like the size and placement of windows, the wind speed, and the wind direction.

Thermal buoyancy, or stack ventilation, is another natural ventilation strategy, which leverages the fact that warm air rises. As warm, stale indoor air moves upwards and out of the building, it can create a low-pressure area that draws in cooler, fresh air from outside.

Natural ventilation has numerous benefits. It can reduce the need for (and therefore the cost and energy consumption of) mechanical ventilation and air conditioning. It can also increase our connection to the natural environment, making indoor spaces feel more pleasant and less 'stuffy'. However, natural ventilation is less controllable and predictable than mechanical ventilation and may not be sufficient in all situations. For example, during periods of extreme temperatures or in highly polluted urban areas, relying on natural ventilation may not be practical or advisable.

Mechanical Ventilation¶

When natural ventilation is not enough to maintain good IAQ, mechanical ventilation comes into play. Mechanical ventilation systems use fans and ductwork to provide fresh air and exhaust stale air, offering a more controlled and consistent approach to ventilation.

There are several types of mechanical ventilation systems, but we can categorize them into four main types: exhaust-only, supply-only, balanced, and energy recovery ventilation systems.

Exhaust-only systems work by depressurizing your home. The fans remove air from your home, allowing outdoor air to seep in through the cracks and openings. These systems are relatively inexpensive and simple to install.

Supply-only ventilation systems pressurize your home. The system introduces fresh air, and the indoor air leaks out of your home through openings and cracks. These systems allow better control of the air that enters the house.

Balanced ventilation systems, as the name suggests, balance the air pressure by introducing and exhausting air in equal amounts. These systems provide fresh air and exhaust stale air, helping to improve IAQ while also maintaining comfort.

Energy recovery ventilation systems provide a balanced ventilation solution but with an added advantage: they precondition the incoming outdoor air, reducing the energy cost of heating or cooling it.

The type of mechanical ventilation system suitable for a home depends on various factors, such as climate, home design, and specific IAQ concerns. For example, in very cold climates, energy recovery ventilation systems can be particularly beneficial because they can reduce the energy costs associated with heating incoming fresh air.

The Relationship Between Ventilation, IAQ, and Building Efficiency¶

The key takeaway here is that ventilation is a critical component of both IAQ and building efficiency. It's a tool we can use to control pollutant levels, manage humidity, and maintain a comfortable temperature range.

However, ventilation doesn't exist in a vacuum. It's part of a complex system, interlinked with other components such as insulation, air sealing, heating and cooling systems, and more. The effectiveness of ventilation depends not just on the design of the ventilation system itself, but also on how well the entire building system performs.

Indeed, the relationship between ventilation, IAQ, and building efficiency is a delicate balance. On one hand, we need to ventilate to dilute and remove pollutants, but on the other hand, over-ventilating can waste energy by removing too much heated or cooled air.

Effective ventilation strategies should therefore aim for 'just-right' ventilation: enough to maintain good IAQ, but not so much that energy is wasted. And to achieve this, we need a good understanding of the building science principles that underpin it all. That's why we're spending so much time talking about topics like heat flow, moisture movement, and air pressure: because they're all part of the story of how we can create homes that are both healthy and energy-efficient.