North, South, East and West – Which Window Will Fit the Best?

One of the most complex issues the design team dealt with was the selection of glass in the windows. This post is fairly technical (the original draft came from the energy modeling experts) but gives you an insight into the fact that some of the building decisions were more than "common sense," and in fact involved highly technical work.

Buildings respond to the climate and to their environment, so green building design is site-specific. Berks County’s climate, for example, has more overcast days than clear ones.

Daylight is free, so we designed our building to take advantage of all Berks County could offer. Designing to take advantage of sunlight reduces the number of lights that need to be on at any given time, thereby reducing the building’s electrical load. Since the lights won’t be on, we’re also eliminating any heat the lights would generate. This in turn lets us use a smaller air conditioning unit.

One of the ways architects ensure that daylight is put to good use is to use different types of glass depending on the direction a window faces and the climate. That’s because the sun’s intensity shifts as it moves across the building during the day and throughout the year.

Bear with me, I’m going to get a little technical here.

The basic properties of glass are visual light transmittance (VLT) and solar heat gain coefficient (SHGC).

VLT is the amount of light that can pass through the glass. Single-pane clear glass windows have about an 89% VLT. The foundation’s building has three different VLTs: 56, 60, and 70.

The solar heat gain coefficient (or SHGC,) indicates how much heat passes in and out. The SHGC of the foundation’s building ranges from 0.32 to 0.39. Any value below 0.35 is good.

VLT and SHGC can often be at odds with each other in the glass selection process. But, by selecting a window that balances both values with its position on the building, you end up with the proper amount of daylight filtering through without making the tenants feel like they’ve taken up residence in a greenhouse.

As the sun comes up in the east, it’s low in the sky which can cause glare. Solar radiation in these low rays can quickly heat up a building that is cool from being unoccupied during the night. To compensate for the sun’s position as it rises, the windows on the east side of the building have a VLT of 56 and a SHGC of .38.

As the sun moves higher as it approaches noon, its direct light and heat can be blocked with overhangs and shading devices.

The south side of the building is affected by both morning and evening sun so the VLT on the south side is 60% with an SHGC of .38. I

In the winter in Berks County, the heat from the sun helps warm the building. As the sun begins to set, it has the same low angles as the morning, but the building is already warm from being occupied all day. For this reason, the VLT of glass on the west side is 56 with a SHGC of 0.32.

The north side of the building is not impacted by the sun’s movement, so the windows there are larger with a higher VLT of 70 and SHGC of 0.39. These windows need to let as much light in and keep as much heat in as possible in the winter.

Yes, it all sounds very confusing. But the bottom line is this: by working with our environment and climate, we’ve created a window scheme on all four sides of the building that will provide more than 80% of the building’s light without ever flipping on a switch. We’re saving energy and money on our heating and cooling units by being smart about our windows. It just makes sense.