The Lowdown on Low-E

***WARNING: This information is specific to Santa Cruz, California!***

Low-emissivity (Low-E) glass coatings are an awesome window feature. With multiple pane insulated glass windows, there are several potential glass coating combinations. The standard offerings from window manufacturers generally perform well thermally in climates that require heating and cooling, but may not apply in all situations. What about coastal climates like mine that have very low cooling demand? What about a home design like mine with a large expanse of South facing glass for passive solar heating?

For this post, I ran an energy analysis of my home using HEED software. I ran the 4 different glass coating scenarios listed below.

1. All double pane windows with LoE³-366^TM on surface #2 (inside surface of exterior pane) and Argon fill.
   
   
2. All double pane windows with LoE²-270^TM on surface #2 (inside surface of exterior pane) and Argon fill.
   
   
3. All double pane windows with no coatings (Clear).
   
   
4. All double pane windows with LoE-179^TM on surface #3 (inside surface of interior pane) and Argon fill.

All other characteristics in the model remained the same for each analysis scenario. I compared total annual heating energy for each of the 4 scenarios. My home design is all electric, so the energy units are kilowatt-hour (kWh). The graph is shown below.


A few comments about the results:

• Two of the coating offerings from one of my potential window manufacturers (LoE³-366^TM and LoE²-270^TM) perform poorer than using no coatings at all (Clear). The reason is that they are designed to reduce solar heat gain in the summer as well as insulate in the winter. In my moderate coastal climate, these characteristics are detrimental to thermal performance. My design wants maximum solar heat gain in the winter. Passive solar heat gain through the windows during the day more than offsets the heat lost at night. The summers in Santa Cruz are not hot enough to require these coatings.
  
  
• LoE-179^TM is an ideal coating for the windows in my passive solar home design. It allows about 90% of the solar heat gain of clear glass, yet limits heat loss to about 66% of clear glass. I will need to specify this coating as a window option. Any added cost from this option is offset by a reduction in solar panels needed to generate heating energy.
  
  
• The analysis was kept simple for posting. All windows within each scenario were kept the same. Further performance gains can be obtained by tweaking individual windows or window groups. For example, windows on the North wall (in the Northern hemisphere) could use high U-value coatings like LoE³-366^TM.
  
  
• Thermal performance can be further improved by using thermal shades at night.
  

The bottom line is that home designers should be careful when specifying window glass coatings. Various climates require various coatings. Fortunately, glass companies can provide suitably coated glass to the window manufacturers for incorporation into your windows. Simple analysis tools can help determine the best coatings for any application. All that's left is to tell the window manufacturer what you want.

Thanks for reading. As always, your comments are appreciated.