Energy Efficient Windows

Before I write a blog about window efficiency, I suppose that I should first ask the question,"Efficient for who?".  Making a window energy efficient for a northern climate means Triple Pane Glass, large I.G. units and heavy inert gas between the panes.  Southern climates depend more upon high quality LowE.

The Low E (Low Thermal Emissivity) Situation:
Objects that are highly thermally emissive simply get hot when heat is applied.  Imagine walking bare-foot across a black pavement parking lot in the summer.  Black pavement has a HIGH low-e rating because it doesn't reflect the suns heat.  Black pavement absorbs the suns heat and radiates it.  White pavement, on the other hand, reflects more of the suns heat, meaning that it has a lower rating.

Glass actually has a fairly high "Low E" rating (like Black Pavement).  However, quality glass manufacturers can add a metallic mixture consisting primarily of tin or silver to the glass that significantly reduces the "Low E" rating.  The idea is that eventually this Low-E metallic mixture will only effect the UV spectrum and block 100% of the invisible "Heat Light" while allowing 100% of the visible light.  Unfortunately, we aren't there yet.  Very Energy Efficient windows are now capable of blocking up to 80% of the heat-light (UV) while only blocking 5% of the visible light (barely tinted).  Severely tinting (reducing the amount of light that comes through) your windows is no longer the smart way of reducing the amount of heat.

Pane Thickness (Double and Triple):There is the issue of thermal conductivity.  Think of it this way... if you had a ruler that was made of iron and you heated only one end, that heat would eventually conduct all the way to the other end.  Thermal conductivity also applies to glass.  If the sun heats up one side of the glass, how long will it take before the other side gets hot?  The obvious next question is: if the sun heats up one side of the glass, how long until the inside of your home gets hot?   We can significantly reduce this conductivity by adding another pane of glass with a large airspace between them.  This means that heat (or cold) will have to conduct through the glass, heat (or cool) the airspace to such extremes that it then conducts through the second pane of glass.

Gas (Argon / Krypton):Hot air rises and cold air falls.  This also happens in the air space between the two (or three) panes of your window glass.  This "rolling" effect causes heat (or cold) to conduct faster and a window to break down quicker.  We can reduce this effect by replacing the air with gas.  Argon and Krypton gas are much heavier than air and both resist the rolling effect.