Operable Windows Deliver a Breath of Fresh Air

04/01/2015 | By Jennie Morton

Use controls to balance mechanical and natural ventilation

Operable windows for ventilation

What would your occupants give to open their windows and let in a nice breeze?

More than the desire to enjoy pleasant weather, natural ventilation supports productivity, learning, and health for employees and students alike. It will also flush the space of potential VOCs and CO2 concentrations, which helps to reduce the likelihood of sick building syndrome. If you are pursuing a green certification, LEED, Green Globes, Living Building Challenge, and the WELL Building Standard all reward designs that improve ventilation and IAQ.

Passive ventilation is out of reach for most existing buildings without a costly remodel, so your best retrofit option is operable windows. The trick with using this type of mixed-mode ventilation, however, is to ensure that the fresh air doesn’t bring in outdoor pollutants or overwork your HVAC system.

Ask yourself these five questions to ensure that this solution won’t undermine air quality or energy costs.

1) Are You in the Right Climate?
Operable windows are best suited to regions that have temperate climates; otherwise cold, humidity, precipitation, and extreme heat limit the number of days you might be able to let in untreated air. If your building is in a region with unpredictable weather, you may want to keep the status quo with mechanical ventilation.

Outdoor Pollution vs. Sick Building Syndrome

Could fresh air alleviate indoor pollution levels or only create a bigger problem?

A 2013 NREL study called Natural Ventilation in California Offices: Estimated Health Effects and Economic Consequences set out to determine if opening windows was an answer or a problem to poor IAQ.

For buildings with natural ventilation, the researchers found that while fresh air could reduce sick building symptoms, it also increased the number of respiratory issues workers experienced. The cost to treat exposure to particulate matter was higher than the cost savings to reduce sick building syndrome. Despite these findings, you don’t need to abandon fresh air altogether.

“The health effects and costs from increased exposure to ozone and particulate matter in naturally ventilated buildings could be substantially reduced by keeping windows closed on the days with the 10% highest levels of ozone and particulate matter,” note the authors. “Mechanical cooling would likely be needed to maintain comfort, and mechanical ventilation would be needed to prevent an increase in sick building syndrome symptoms.”

“Everywhere in the country experiences those 68- to 72-degree days, but for many areas, they’re rare or only occasional,” says Marty Brinton, senior applications engineer with LG Electronics. “You need to look at the first costs of an operable window compared to a fixed one and calculate how many days of the year you can actually use the operable one.”

Wind resource – an assessment that measures how gusty your location is – also plays a role.

“While operable windows are a simple device, you are dependent on wind or temperature differences to create buoyancy for air movement to flow through your building,” notes Michael Frank, director of engineering with the firm McKinstry.

You can consult anemometers from your local airport or online maps of wind forecasting. Look for data from the National Wind Technology Center, the National Center for Atmospheric Research, or the National Climatic Data Center.

2) Is Your Outdoor Air Safe?
In addition to a mild climate, outside air quality is a major concern. It’s an unfortunate reality that many locations suffer from high levels of pollutants. You cannot open a window at the cost of exposing occupants to harmful toxins.

“Keep in mind that in a mechanical ventilation system, all of the air goes through a common filtration point. With operable windows, however, you don’t have a way to remove contaminants,” Frank cautions.

Not sure about your site’s air safety? Consult the National Ambient Air Quality Standards, a function of the Clean Air Act. The measurements establish harmful levels of lead, carbon monoxide, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. These standards are then used to determine nonattainment areas where air pollution is routinely high.

“If you’re in one of these EPA nonattainment areas and you’re bringing in untreated outdoor air, you are actually polluting your building,” Brinton stresses. “It would only increase your operating costs to filter properly.”

Even if your location has safe levels of particulate matter, there are additional sources of airborne contamination. Flying dust and plant debris may be of concern, as could vehicle exhaust if your windows are near street level on a busy thoroughfare, explains Frank. Even outdoor noise could drift in and become a distraction, adds Brinton.

To ensure you aren’t below the requirements for ANSI/ASHRAE Standard 62.1-2013 – Ventilation for Acceptable Indoor Air Quality, you can add IAQ sensors throughout your building to keep an eye on pollutant and thermal comfort factors.

3) Are You Ready to Upgrade Your Existing Windows?
Beyond air quality considerations, it makes the most financial sense to add operable windows during a facade renovation. Particularly if your current windows need better sealing or solar film, it’s a great opportunity to gain dual functionality.

“A window renovation may be necessary if the existing building was designed with only fixed windows,” says Lisa Jackson, manager for window products and sustainability with Kawneer Company. “Switching to high-performance operable windows can be a cost-effective way to introduce natural ventilation into occupied spaces and improve overall energy efficiency.”

Keep in mind that it may not be necessary to have all windows unlock and you can save on costs by limiting the number of openings to upgrade. Many properties will add operable windows only on one side of a building, a single floor, or even just a row, says Frank.

Before you commit to replacements, evaluate your site for cross wind, buoyancy, and the stack effect to determine which spots on your facade receive the most wind, Frank recommends: "Using advanced software to model multiple natural ventilation scenarios can help you determine the most effective location and quantity of operable windows."

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