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.
“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."
4) Do You Have Zone Heating and Cooling Paired with Controls?
Depending on how deep your building is, keep in mind that operable windows will only benefit spaces close to the perimeter as fresh air can’t easily reach the core without mechanical help, notes Brinton. Open plans are also helpful as there are fewer barriers that could block air flow, adds Frank, though you can always add ceiling fans to help carry a breeze.
If you don’t want occupants guessing when it’s a good day to ventilate naturally, there are several ways to add automation.
- Full Automation: In addition to a remote panel or switch, “windows can be integrated with the HVAC system using sensors. This configuration balances the use of air conditioning with natural ventilation whenever it is feasible or desirable,” Jackson says. “An integrated HVAC system is best when approached during the design phase.”
- Notification System: You can install outdoor weather stations that measure air conditions. These sensors connect to your BAS and will trigger your perimeter HVAC to shut off when presets are met, Frank explains. This ensures your air conditioning isn’t running full blast when the windows are open.
- This ventilation sequence also includes a reset for the interior supply air or room setpoint temperature to ensure those units aren’t maxed out, he adds. If your HVAC isn’t connected to your BAS, note that this option will necessitate a controls upgrade.
- Interlocking Controls: Another possibility is to add interlocking controls to the windows. These use a binary contact that is broken when a window is opened, which in turn sends a message to the HVAC system to shut off.
In addition to these controls, you may elect to add a ventilator, notes Jackson: “These are typically small horizontal devices constructed within the framing system that allows air to flow into occupied spaces. When windows are in the open position, the ventilators provide a steady amount of outdoor air. They are usually equipped with screens to keep out insects and weather.”
5) Will Your Occupants Be On Board?
It goes without saying that for operable windows to be effective, they need to be easily accessed by occupants. That means they’re within reach, don’t require much force to open, and aren’t blocked by any objects, says Jackson.
If you don’t have full automation, you have to rely on occupants to open windows, and more importantly, close them, she adds. Without good habits, employees may neglect to shut windows at the end of the day or over the weekend. This can present energy, moisture, and security concerns if left unaddressed.
A little occupant training can go a long way, so take the time to educate users about the benefits of open windows and what they can do to be good environmental stewards. If that falls short, you can always task your cleaning crew to shut open windows just as they might already be doing with lights that are left on, advises Frank.
When hitting the sweet spot for the trifecta of temperate weather, acceptable outdoor air quality, and smart HVAC controls, operable windows can be a sustainable and energy-efficient way to supply natural ventilation to occupants.
Jennie Morton email@example.com is senior editor of BUILDINGS.