I grew up near Seattle, where floor-to-ceiling windows are the façade of choice for most downtown high-rises. The beautiful Emerald City skyline is practically made of glass. I've even heard that there are so many windows that a major earthquake could leave Seattle-ites wading through multiple feet of broken glass. As for my childhood house — a place I still call 'home' — practically every exterior wall surface that can accommodate a window does. This includes huge north-facing windows in the dining room, along with two large skylights. So I've definitely grown to appreciate the beauty and benefits of windows.

The wonder

Foremost amongst these benefits is the connection with nature. In western Washington, the color green is practically omnipresent outdoors (this is one of my favorite parts of coming home . . . after, of course, spending time with my parents). Without all of these windows keeping us connected with the outdoors, the feeling of being cooped up in a school, office, or home all day could drive us crazy; but we've already got our contrast-resistant winters for that. Apparently, I'm not the only one who thinks so highly of this characteristic: a variety of studies have shown a correlation between greater daylighting and improvements in learning and attentiveness of children and, similarly, in the productivity of adults.

Well-designed daylighting can also result in savings to three phases of a child care center's life: construction, operation, and maintenance. During construction, fewer lights will need to be installed in daylit areas. Daylighting will help with the operation by reducing the need to turn on lights and by offsetting some of the space heating load during cold winter months. Finally, fewer lights being used less often means fewer bulbs needing to be replaced each year. If the windows are operable, they can potentially provide additional savings in all three building phases as there will be less need for mechanical ventilation systems to maintain indoor air quality.

The waste

As is so often true in life, these benefits have some strings attached. For starters, notice that I started my previous paragraph with the phrase 'well-designed.' Many of these benefits can quickly be lost by poorly designed or installed ­window systems. Too much daylighting can cause thermal discomfort and ­distracting glare for occupants, not to mention UV damage to furniture. Acoustic disturbances and visual distractions could also be a problem if not properly taken into account.

Then there is the issue more near-and-dear to my energy efficiency-loving heart: windows often represent the weakest link in the armor of what is known as a building's 'thermal
envelope' — the group of surfaces that form a continuous separation between conditioned (indoor) and unconditioned (outdoor) space. Windows are generally rated for how much heat they let through by two separate mechanisms:

1. Thermal conduction — This is the heat transferred, by direct contact, through the building envelope. For example, this is like the heat felt when touching a cup of hot tea: heat from the tea is conducted to the cup and then to your hand.

2. Solar transmission — This is the amount of the sun's thermal radiation that makes it into the indoor space. You feel thermal radiation when putting your hand close to that same cup of tea.

Ratings for these mechanisms are represented by performance values called the U-factor and the Solar Heat Gain Coefficient (SHGC), respectively. For both, lower values mean less heat transfer. In general, the lower the U-factor the better, as this will help ensure the indoor conditioned air stays conditioned; on the other hand, lower SHGCs can be useful in some circumstances while higher are preferred in others.

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