Open Windows and Ventilation

Written by Mike Woolsey.

Several construction concepts exist that intend to dramatically reduce building energy consumption. Among the concepts is Passive House, which describes ways to design and construct buildings so that their heating and cooling energy use is 75% less than most new buildings. A certified Passive House will have met both energy-savings and comfort goals.

The Passive House concept is used to design and build virtually every imaginable type of building, including high-rise commercial offices, hospitals, hotels and single-family residential. Five features of Passive House buildings are measurable airtightness, climate-specific insulation, minimal thermal bridging, and certified high-performance windows and energy-recovery air handling units. Of the five, only the air handler consumes energy, which is odd, considering the goal of Passive House is to save energy.

Why not just throw open the windows for comfort, like our predecessors?

The airtightness of the Passive House technique prevents airborne contaminants from escaping on their own, and the robust insulation along with minimal thermal bridging increase the risk for unacceptably high indoor air temperatures.

Operable windows exist that meet Passive House thermal and solar criteria. And these operable windows can be strategically selected and located to meet ventilation requirements in some climates and in some buildings, like single family residences. At the other end of the spectrum, imagine a meeting room in the middle of a code-compliant commercial office space, with no exterior windows. How does enough fresh air get into that conference room from an open high performance window on the perimeter of the space, especially when the room is at capacity on a hot day with the door closed for privacy?

A proven method to continuously replace contaminated indoor air with fresh conditioned air throughout a building is to use air handling units.  Air handling units condition outside air to adjust the temperature and remove excess moisture, and they also pressurize the air so that it flows through ducts to every occupied space in the building including rooms without operable windows. Passive House certified air handling units (often referred to as Heat or Energy Recovery Ventilators) perform these critical functions using energy recovery to reduce heating and cooling costs.

Also consider thermal comfort. Passive House requires occupant comfort most of the time. Similarly, ASHRAE Standard 55 allows for most occupants to be comfortable all of the time. Passive House certifies air handling units partly on the basis of their ability to capture heat leaving the building when it is cold outside. The same process of energy recovery also rejects heat and moisture from the outside air entering the air handler. In this way, energy recovery air handlers temper outside air so that on most days, little or no additional energy is required to warm or cool the air for occupant comfort. Only on the most extreme of hot and cold days, small amounts of energy are consumed to heat or cool the Supply Air leaving the air handler to keep occupants comfortable.

Another advantage of air handlers over open windows is that they filter air. Not all airborne contaminants are created indoors. Allergens and fine particles like dust originate outdoors. Well-designed filtration in air handlers removes most of those fine particles, contributing to occupant well-being and overall building cleanliness. Filters contribute to life-cycle costs because they require routine replacement and because they add a pressure drop to the air handler that causes the air handler to work harder and consume more electricity. Filters reduce life-cycle costs, however, by keeping air handler energy recovery devices clean, ensuring peak energy recovery effectiveness at all times, and virtually eliminating the need to clean the energy recovery element.

Opening the window may be an effective method of providing occupant comfort in single family homes, or even along the perimeter of large non-residential spaces. But this natural ventilation alone cannot be expected to provide code-required ventilation in all types of buildings all the time. Natural ventilation may play a valuable role in the year-round energy use profile, but chances are, your Passive House office building will benefit from energy-efficient mechanical ventilation over a large part of the year.

*Image courtesy of Zola Windows