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All Low e’s Are Not Created Equal

If you had the choice between two windows that were exactly the same except one had a slightly higher R-value glass than the other, which one would you pick to lower your heating bill? And yes they are the same price.

If you picked the higher R-value window you could be wrong. Wrong to the tune of many hundreds of dollars over the life of the windows. 

The answer is not always the higher R-value window. Why?

Windows contribute solar gains that help heat your house. Currently the type of Low e glass that insulates the best also stops more than half of the free heat from the sun. If your heating bill is bigger than your cooling bill this is a bad thing, a very bad thing. To understand how a window affects your heating bill you need to know not just how well it insulates but how well it collects free heat from the sun.

The Canadian Energy Rating (ER) is the only one number system that permits comparison of energy efficiency because it accounts for all the R-value losses as well as the solar gains. I’ll use it to show you how more is less and less is more when it comes to windows and your heating bill. 

The two extremes in the R-value/solar gain trade-off are Cardinal’s E2 and Libby Owens Ford’s Energy Advantage II. The example below is for our double glazed fiberglass casement. 

Fiberglass Casement (600mm x 1200mm / 24" x 48")

GlassProperties

 Window Properties

Transmittance

U-value

 U-value ER
Glass Code visible solar W/m2K Btu/ft2 SHGC W/m2K Btu/ft2 W/m2
211 (Cardinal E2, #2) 0.72 0.36 1.41 0.25 0.28 1.54 0.27 -13.5
211 (LOF, #3) 0.75 0.58 1.66 0.29 0.49 1.71 0.30 -1.9

The Fine Print:

  1. Glass Code

    1st digit - number of panes of glass 
    2nd digit - number of SuperSpacers 
    3rd digit - number of Low-e coatings and argon gas fills 

    The number after the Low e manufacturers name shows the location of the Low e coatings - surface #1 is the exterior surface 
      
  2. Glass and window properties were determined by FRAME PLUS

Note that the location of the Low e surface is different for the two options, but reflects where most manufacturers put the coating. Feel free to e-mail me if you want a more detailed explanation of this.

Comparing glass properties you can see the two glass options both transmit about the same amount of visible light. In other words they look very similar. But they aren’t. The LOF glass transmits 60% more solar heat than the E2. Solar energy includes visible light energy as well as invisible UV energy and invisible infrared energy. All three ‘flavors’ of solar energy contribute to reducing your heating bill. 

When the comparison moves onto the U-value (R = 1/U), the ranking has reversed. Cardinal’s E2 insulates 14% better than the LOF. 

Comparing window properties for this example you can see similar results. SHGC stands for Solar Heat Gain Coefficient. It tells us how much of the solar energy that strikes the outside of the window (including the frame), makes it through the window. Looking at the SHGC you can see not suprisingly, that the LOF Low e delivers more Solar energy than Cardinal’s E2. Similarly the Cardinal E2 window has maintained its lead in the U-value, although the lead has been reduced by the effect of the spacers and frame.

As far as your heating bill goes, the ER tells the tale. In this case the difference is 11.5 W/m2. Over a 212 day heating season (typical for Canada and the Northern US) this amounts to 1170 kWh or 4 million BTU’s for a typical custom house with 200 ft2 of windows. Over the next 20 years that difference accumulates to an expense that, depending on local energy rates, is about $1600 for electric resistance heating or about $600 for natural gas heating. (for more info on ER, see section 6.3 of Consumer's Guide)

This may or may not be a lot of money to you, but when you consider that Cardinal’s EE is either the same price as the LOF Low E or more expensive than LOF’s Low e it’s a pretty easy decision to make. 

  1. Frame In? Frame Out?
     
  2. All Low e’s Are Not Created Equal
     
  3. Double Hung Inherently Flawed
     
  4. The Road Less Travelled
     
  5. The Affordable Energy Efficient House
     
  6. Green Priorities - Skin Deep Not Deep Enough
     
  7. Embodied Energy - As Important As Low Energy Design?
     
  8. Energy-Star's Solar Eclipse Ending?

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