03/11/2026
This is a great article about window specifications. In my work as an Energy Advisor and as an HVAC designer, I frequently see windows that qualify as Energy Star but are really not appropriate for climate of northern Ontario. That is because Energy Star allows the use of Energy Rating (ER) to qualify.
Windows may qualify as Energy Star or Energy Star Most Efficient based on either the Energy Rating (ER) or the U-value. U is the reciprocal of R; so, windows with lower U-value have a better insulating value. ER is an all encompassing metric that takes into account air leakage and solar gain, as well as U-value. While it sounds good that ER takes into account all characteristics of the window, what it means is that a window may qualify because its glass allows a lot of solar gain while its thermal efficiency is poor. That may work well in more southern areas where winter days are longer than ours and it doesn’t get as cold. But in northern Ontario, a window with high ER but poor thermal efficiency (high U-value) will be cold to sit near on cold nights and will not deliver the energy savings it promises. Therefore, I recommend choosing windows with as low a U-value as possible (less than 1.2 W/m2K or, better yet less than1.05 W/m2K if that fits your budget)
Choosing windows for your home can be complex. Learn how to balance performance, cost, and aesthetics for different climates and designs.
