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What is Low E Glass and why do I need it?

An article in our “make an informed decision” series!

What is Low E Glass and why do I need it?

Low E glass or, more correctly, low emissivity glass is so essential to your home that the building code in Ontario virtually demands its use.

Start with a clear piece of glass and add a coating which reflects only specific parts of the light spectrum. The light you need to see through your windows  is short wave (ultra-violet light). Light that is already heat is long wave (infra-red light). The middle wave-length light is where you as a homeowner get to make an important decision that takes into account your level of comfort in your home in different seasons.

Think of low E glass as microscopic mirrors impregnated either on (soft-coat low E) or in (hard-coat low E) the glass. How these glasses are produced is not the issue. What is important is that with soft-coated glasses, those microscopic mirrors are slightly closer together than on their hard-coat counterparts.

All Low E glasses are designed to reflect existing heat back to its source. This keeps more heat from your furnace in your home in winter and keeps much of the heat of the sun outside in summer. Both types of low E let enough short wave light through that you can still easily see through your window.

The important difference is, because in some low E glasses (soft-coats) those mirrors are a little closer together, this type of glass reflects more intermediate wave-length light back to its source. The result is that the soft-coats tend to have slightly better U or R values because by reflecting slightly more light, they are measured as better insulators.

Is this a good thing? Maybe yes, maybe no. It is certainly a good thing in summer when you want to keep heat from the sun outside, however, in winter, from a purely heat cost point of view, losing a bit more free solar heat (called Solar heat gain”) is proven to cost you a bit more on your heating bill over a full year, in spite of the soft-coat’s slightly better U/R values. By letting in more intermediate light waves, hard coats do give more free heat in winter, but also allow that heat into your home in summer.

Now you get to make your decision. Do you opt to save a few more pennies over a full year on your combined heat and air conditioning bill, with hard-coat low E, (when both glass types will give you significant energy cost benefits), or are you willing to forgo the extra pennies saved to keep your home slightly cooler and more comfortable (with less use of air conditioning) in the summer with soft-coat low E?

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