Ways To Sustain

When someone talks about trying to be energy conscious, there is often a love hate relationship with regards to windows.  It is best to first understand the interaction between your window and the surrounding environment, and then you can decide what works best for you when it comes to improving your window energy efficiency.

Feeling the Heat

Most often a home owner will only notice a window impacting the room’s temperature when they either feel hot during a sunny day or a cold draft during the winter months.  The variation in temperature is caused by heat transfers that happen when there is a temperature difference.  Here are four processes by which heat transfer takes place.

Conduction

You have probably experienced heat transfer by grabbing a handled on a metal frying pan or a spoon that has sat in a hot liquid for too long.  Noticed I said that when a spoon sat in hot liquid too long.  How fast something heat up is all dependent on the type of material they are made out of since different materials conduct heat at different rate.  You’d want a frying pan for cooking to heat up fast so that you can start cooking as quickly as possible and not waste energy.  However you don’t your coffee cup or spoon to heat up fast or you won’t be able to use them with out burning yourself.  In the case of your windows, you want it constructed out of material that has low conduction rates so that the outside temperature doesn’t change the indoor temperature you are trying to control.

When choosing a window, talk to your window manufacture about the material that make up the sash, and frame.  They should be able to give you options or identify the overall thermal resistance (metric RSI value or imperial R value).  Since the window pane or glazing itself does little to prevent heat loss, you will have to rely on the air trapped between panes.  Air is a poor conductor of heat and in some windows inert gases such as Argon are used instead because of Argon’s lower heat conductivity.  

Convection

You have probably seen a hot air balloon floating across the sky and never realized that what keeps the balloon in the air is exactly the same thing that you’re trying to reduce with your windows.  This happens by:
•    The cold winter winds carries heat away from your windows
•    Air currents generated next to windows as warm air next to the glass cools and sinks to the floor
•    Air between the glazings as the cool outer pane absorbs the heat from the warm inner space.

Radiation

One of the most pleasant experiences of radiant heat is felt when standing in front of campfire or fireplace.  It is not so nice when you walk past a window in the middle of a heat wave and if feels like someone put you under a magnifying glass.   What you feel is commonly referred to as solar gain or radiant energy.  The amount of radiant energy transmitted into your house can depend on the number of layers of glass (glazing) you have on your windows.  Now there are commercially available films such as Low-E film that can reduce the amount of radiation transmitted.  

Window styles

So now the question is how many panes of glass do you want or need.  That really depends on your situation.  A single pane of glass has no air trapped so it provides very little thermal resistance, dual pane has some space for air or gas to be trapped so it does a much better job than single pane, and triple pane provides yet additional thermal resistance