GUEST POST: Clever Glass – The Science Behind The Savings

Once these metal frame, single-pane windows were considered adequate – but not today.
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Man landed on the moon 43 years ago, an amazing feat of technology, so it’s hardly surprising that building materials have really improved significantly, impacting the lives of virtually everyone in the USA and other countries on the bleeding edge of energy efficient architecture.

Up to 30 percent of home energy losses are through the windows of a property. Why is this loss so huge? Many homes use poorly made windows, which can lose a very large amount of heat when it’s cold outside (we’ll start by focussing on this process rather than concentrating on how windows can cause a room to warm up when it’s hot outside and cool inside). There are several basic processes that cause windows to lose heat, which are as follows:

1 – Conduction

Warm air particles collide with the window, transferring their energy into heating up the window itself. The heat is transferred through the window to the outside world, which then heats up: goodbye energy!

2 – Convection

This is a process that speeds up heat loss, but doesn’t directly cause it. When air cools down, it becomes denser and therefore sinks to the bottom of a room, whereas the warm air flows to the top of the room. If your room is being heated, but you have several cold windows, when the warm air comes into contact with the window, it will quickly lose its heat through conduction. Convection then means that the cooled-air will sink and hotter air will then – again – come into contact with the window. Convection essentially means that there is always a greater temperature difference between your windows and the air in contact with them inside the room, hence there is a greater rate of heat transfer.

3 – Leakage

Simple to understand – warm air may literally flow out through gaps i the window frame.

4 – Radiation

Heat energy isn’t just rapidly vibrating air particles – it can also be infrared heat, which is actually a type of light and is technically a wave. This is why if you shine a magnifying glass on a piece of dark paper on a hot day (dark paper absorbs infrared heat better than light paper), it will start a fire. Please don’t try this at home! Infrared radiation can travel straight through glass, quickly leaving your nice warm room.

How can clever glass combat these issues?

Leakage is the simplest problem to tackle – well-made windows will have tightly fitting frames, which will be well sealed and won’t let through any air. They also have clever locking mechanisms, which will mean when the window is shut, it will shut properly and won’t rattle around, which would otherwise indicate that it is too loose.

Conduction can be minimized through double or even triple glazing. Every material has a Standard Heat Capacity (SHC) – a figure that indicates the amount of energy (measured in joules) necessary to heat up 1kg of that material by 1 degree Celsius. If a lot of energy is required to heat up a material, then it has a high SHC and does not conduct heat well – a high SHC means it’s difficult to heat up a material. Glass has an SHC of approximately 840J/kg deg. C, compared to 1020 J/kg deg. C for air. This means that glass heats up more easily and will therefore quickly heat up when warm air comes into contact with it inside your home. This heat will then be lost on the other side of the glass as wasted energy. The air gap found in double-glazed windows is a lot more difficult to heat up, and therefore heat is lost more slowly. With triple-glazing, there is yet another air gap that the heat must travel through before it is lost, and therefore it is more efficient again. Argon gas is used in better windows because it has a higher SHC than air and therefore is liable to lose even less heat. The rate at which a window loses heat through conduction is its U-factor, so in every climate, a lower U-factor is better than a high U-factor.

Convection is where modern windows really can impress homeowners. The Solar Heat-Gain Coefficient (SHGC), measured as a value between 0 and 1, tells you how much heat enters a building through the window when the sun is shining. In cold climates, it’s clearly more beneficial to have a higher SHGC because it means that more infra-red radiation will enter your home, thus heating it up, when the sun is out during the day, even if it’s cold outside. If you live in a hot climate, you probably don’t want your property heating up during the day, since this will mean you spend more energy on air conditioning to keep the place cool. Therefore, make sure the SHGC of the windows you pick is suited to your region.

How do manufacturers control the level of convection to achieve a range of possible SHGC values? The coating of the surface of the glass makes the difference – the highest SHGC windows, best in hot climates, when coupled with strong U-factors, can actually gain energy rather than lose it because they let in more heat than escapes!

So there you have it – plenty of information on what to look for next time you’re shopping for new double or triple glazing. Make sure that your windows suit your property!

Written by James Hawkins from a home improvements company – my knowledge of double-glazing is thanks to a chat with our team providing a free cheap conservatory prices comparison across the UK. 

Photos: GR Meyers,  U.S. Army Corps of Engineers Los Angeles District