Most modern homes need mechanical ventilation systems to provide satisfactory indoor air quality. Many poorly ventilated homes have mold and mildew growing in wall cavities, high levels of airborne chemicals, combustion byproducts, or high utility bills.
Fortunately, there are proven construction methods for every climate that allow builders to construct comfortable, efficient, and healthy homes. These best practices don’t require hard-to-find materials or a change in construction sequence. Best practices do require a “whole house” approach to building that recognizes how each part of the home works together to provide a comfortable, safe, and durable shelter. Mechanical ventilation systems are an important part of these advanced new homes, known as Zero Net Energy (ZNE) Homes.
Ventilation Systems Protect People and Preserve Buildings
Properly designed ventilation systems perform three important tasks.
- They protect human health by diluting airborne pollutants and by supplying fresh air.
- Ventilation improves comfort by eliminating odors and reducing drafts.
- Ventilation systems preserve structures by controlling airborne moisture, thus preventing moisture condensation.
Mechanical ventilation systems move air through the home using fans and ducts. Electric controls regulate airflow to provide ventilation where and when the home needs it.
Contractors built many homes over the years that ventilate only by bathroom, kitchen fans, openable windows, and unintentional air infiltration through the building envelope. This un-controlled ventilation sometimes provides good indoor air quality and sometimes not.
Air Leakage is Wasteful and Polluting
Controlling air leakage through the building envelope is the first step to designing healthy and efficient buildings.
Air leakage happens when wind, temperature differences, or mechanical equipment force air into and out of a building through unintentional leaks in walls, floors, ceilings, windows, and doors.
Interestingly, building cavities may provide some filtering of particulates for infiltrating outdoor air. Unfortunately, infiltrating air may also contain air pollutants derived from a contaminated path it travels into the building. Infiltrating crawl-space air, for example, may contain moisture, biological pollutants, or radon.
The timing of infiltrating airflow seldom fits the needs of the occupants, over-ventilating the home during severe weather and under-ventilating the home during mild weather.
Three Requirements for Ventilation Systems
A ventilation strategy should dilute and/or remove both the background emissions and the occupant-related emissions, in order to prevent unhealthy chronic and acute exposure. As a result, current standards and regulations, such as ASHRAE 62.2-2016 and others in Europe, often prescribe ventilation strategies requiring three constraints on airflow rates:
- A constant airflow based on a rough estimation of the emissions of the buildings: for instance, one that considers size of the home and the number and type of occupants, or combinations of these
- Minimum airflows (for instance during unoccupied periods);
- Provisions for short-term airflows to dilute a source pollutant generated by activities as cooking, showering, house cleaning, and painting.
Assessment of Indoor Air Quality Benefits and Energy Costs of Mechanical Ventilation.” In Proceedings of the 2011 32nd AIVC Conference and 1st Tightvent Conference, Toward Optimal Airtightness Performance. Brussels, Belgium, 2011. “