Why windows can be a pane

Why windows can be a pane
Tags: Heating & Cooling, Home Plans, Insulation, Remodeling, Windows & Doors
July 28, 2010 - Jason Warnock

When setting out to design our homes, we face the interesting challenge of maximizing natural daylight as much as possible while controlling the heat and glare that is generated through that light. There is no arguing that natural daylight creates a more pleasant, comfortable and satisfying environment to live and work in. However, adding more and more windows is essentially a double-edged sword -- we need the connection with the outside world, but with that, much of the sun’s energy is able to pass directly into our spaces and create heat, glare and possibly damage or fade our possessions.

The light from the sun is made up of several bands of light and energy, only some of which we are able to perceive with our eyes. The majority of the heat energy from the sun is transmitted as short wave radiation. When this radiation hits an object like the walls or carpet in our homes, it is converted to long wave radiation which generates heat. Some of this shortwave radiation is converted when it hits the window, but the clearer the glass, the more is transmitted through and turned into heat once inside. Once the heat has been generated inside the space we are often left needing to burn a large amount of energy through our cooling systems (like air conditioning) to remove or reduce it.

Shading systems allow us to create a barrier at the window to reduce the amount of short wave radiation hitting our internal surfaces. Internal systems like blinds, drapes and roller shades are designed to allow some of the natural daylight to pass through, but keep much of the heat contained at the window and even radiate some of it back through the glass. External shading systems like awnings and louvers are designed to block the shortwave radiation before it is transmitted through the window at all.

Reducing the amount of shortwave energy passing through the glass is only part of the benefit of proper shading. Natural daylight is made up of three main components: ultraviolet, infrared and visible light. Ultraviolet light or “UV” can also be damaging to our internal finishes like carpets, sofas and especially art objects like paintings. Shades can actually allow much of the visible light through, but block the UV light, protecting and extending the life of our products.

Shades can also play an important role in controlling energy costs in the colder winter months. Even our modern double or triple paned glass windows are relatively weak at reducing the amount of ambient heat or cold that passes through them. Our walls and insulation are designed to contain the heat inside our homes in the winter, but windows allow between 7 to 10 times more of that heat to escape. As the natural daylight is so important to us, our windows often become 35-45 percent of our external walls, meaning our insulation is only 55-65 percent as effective. Closing internal blinds and shades in the winter creates a blanket-like effect at the window and can reduce heat loss by as much as 25 percent depending on the type of shade system.

Being mindful of our windows through reduced energy transfer can reduce both our heating and cooling costs all year long, lowering our carbon footprint and saving us money.