9/17/2014

Natural Ventilation vs Mechanical Ventilation

Proper ventilation is a critical consideration for homes and buildings. Poor ventilation can cause a buildup in indoor air pollutants like dust, pollen, mold, and household chemicals. An efficient and reliable ventilation system works to remove polluted air while continuously introducing fresh, clean air. While natural ventilation may be a viable option, there are a number of variables and reliability issues that should be considered. Instead, many energy-efficient builders and homeowners opt for the efficacy and reliability of a mechanical ventilation system such as an energy recovery ventilator (ERV) or heat recovery ventilator (HRV).

What Is Natural Ventilation?

As the name suggests, natural ventilation lacks a controlled, mechanical process to ensure that a home or building is consistently ventilated. Natural ventilation can occur through air infiltration or open doors and windows. Air infiltration occurs most often in less airtight homes as air “leaks” through small openings and cracks in the building’s envelope. The practice of opening doors and windows typically provides more adequate ventilation than infiltration leakage, but still falls short when it comes to optimal indoor air quality.

Today’s modern buildings and lifestyles hinder natural ventilation due to:

  • Security, privacy, energy, dust, and noise concerns associated with opening doors and windows
  • The profusion of indoor living pollutants such as household chemicals
  • Tighter home and building envelopes, which reduces the flow of incoming and outgoing air
  • Nuances in the complexity and size of home design and construction

Shortfalls of Natural Ventilation

Natural ventilation through cracks, openings, doors, and windows often proves unreliable in the long term. This is because the process depends on several varying factors that work together to dictate efficiency in removing old air and introducing new air. The main shortfalls of natural ventilation involve the driving forces of the process, the difficulty in controlling airflow, and the potentially harmful impact improper ventilation can have on the building’s envelope.

Driving Forces

There are three main driving forces of natural ventilation: the wind effect, mechanical effect, and stack effect.  While homes may experience ideal conditions for these driving forces to properly ventilate a home, the conditions are highly variable and dependent on external factors. These factors include the amount of wind, the area’s climate and temperature, and how they interact with the size, construction, and layout of the home. As a result, ideal airflow very rarely occurs on a consistent basis.

Difficulty in Controlling Airflow

Due to the constant shifts in the driving forces of natural ventilation, it’s difficult to control the building’s airflow. Depending on the state of the weather, holes, and openings in the building’s envelope can unpredictably act as both entry and exit points for air.

Building Envelope Impact

In cold winter climates, the rising of warm, moist indoor air can result in the upper wall and attic moisture issues. Likewise, hot and humid summer air entering an air-conditioned building can create condensation in the building’s wall cavities. Over time, these issues may compromise the building’s durability and longevity.

The efficiency of Mechanical Ventilation

Mechanical ventilation can help overcome the shortfalls that occur from natural ventilation.  A Zehnder HRV or ERV works to exhaust air from moist rooms like bathrooms and kitchens while introducing a balanced amount of fresh air into habitable spaces like bedrooms and living rooms. With these mechanical ventilation systems, home and building owners have greater control of proper building ventilation, regardless of external factors like the weather or the time of year.