Increased productivity and better employee well-being, two frequently cited benefits of daylighting, can be accompanied by brutal glare and unwanted heat gain if your skylight isn’t performing as well as it should.
Address your skylight’s issues by picking a solution that allows your building to gain the most light without making occupants uncomfortable.
Manage Light and Heat Gain
In the summer, beneficial natural light is frequently accompanied by unwanted heat, which demands more cooling to keep the space comfortable. In winter, however, the heat is desirable, says Grant Grable, vice president and value stream leader of Sunoptics. Systems that scale back artificial lighting in response to daylight actually increase the need for close thermal comfort monitoring because the lighting system is no longer contributing heat.
“The value of putting a hole for a skylight in a perfectly well-insulated roof is only about light, because it will never equal the thermal insulation value of the roof itself,” Grable says. “Many building codes have tried to mandate that you increase thermal insulation by increasing U-value, which will decrease light and reduce the solar heat gain, but they never realized in the old days that you were going to shut off the artificial lighting.”
Buildings with large atriums or glass domes also can suffer from too much light, says Steve Debusk, global energy solutions manager for window film manufacturer Solutia. In extreme cases, building owners are forced to install canopies, umbrellas, or canvases over their supersized skylights to stop the excess light and heat, negating the benefits of daylighting they originally sought.
“With that much light coming in, you also have a risk of too much UV,” Debusk says. “This damages the furnishings and carpet, requiring them to be replaced.”
If your skylight fits this description, there are a few basic retrofit projects that can help limit the light and heat entering the building without dashing your daylighting plans.
Window films help block heat while still letting most of the light through, but some films can create a disadvantage in the winter if they reject all solar heat instead of just insulating the building. Look for a film with a low-e coating and a high percentage of visible light transmission – the transmission rating is around 15% for most films, Debusk says.
Consider replacing standard glass with a product such as acrylic, plastic, polycarbonate, or tempered or laminated glass. Some products, such as the synthetic Kalwall material used at the Peterson Family Athletic Center at Middlebury College in Middlebury, VT, diffuse light over a larger area to give you more bang for your buck.
“Maximize the U-value of the materials used in the skylight itself,” recommends Mike Crowder, national sales manager for Structures Unlimited, which created the Peterson center’s subframing system. “You should be looking for an absolute minimum of 4.5 to 5.”
Software Solutions and Building Controls
A suite of smart technologies can help automate the process, allowing easier, more customized adjustments. Lighting or HVAC controls paired with light sensors and smart thermostats can adjust artificial light and HVAC in response to whatever comes in through the skylight, though they obviously can’t cut down on heat gain or glare.
Another option, automated shading, can vary the amount of light based on personal preference or preset schedules. Software packages can drive motorized shades for the entire building or a handful of zones.
“You could put a sensor on the roof that monitors whether it’s sunny or not, and algorithms will determine when and how to draw the shade,” says Jan Berman, president and partner of automated shading developer MechoShade Systems. “You can have a switch that’s operated by the users as they see fit. There are all levels of control and automation.”
The DOE offers tools such as Skycalc, a software program that helps determine daylighting needs, to make the process easier. But no matter what, Grable says, the key is to balance all of the space’s needs.
“If you have high visible light, diffusion, and lighting control, and you properly lay it out, the solar heat gain coefficient and UV values will have an effect on thermal comfort, but are so much less than what you’ll see by shutting off lights,” Grable adds. “You’re giving up pennies to get massive amounts of dollars.”
Janelle Penny ([email protected]) is associate editor of BUILDINGS.