Emerging Energy Economy – High Performance Windows

Whether you are renovating or building, energy efficiency is the best defense to fight the soaring cost of heating our homes. One area to consider in overcoming winter cold or summer heat is in window design and construction.

Windows are our weakest link in energy loss. They have the lowest R-Value (an industry rating for the resistance to heat loss) in the building envelope. Older style homes with single-glazed windows typically have an R-Value of only 0.9, and is like having a hole in your wall one third the size of the window!

In new construction, regular vinyl windows have an R-Value of 2.04. When you compare that to the wall surrounding the windows – 4 inch thick wall construction is R12 and 6 inch is R20 – you can see why glass is the weakest link in our construction practices.

There are key points to consider when choosing windows, so let’s cover how high performance windows are made. To begin with, all windows have a rating for A) Air leakage B) Water leakage and C) Wind load resistance and have to meet a minimum of A1, B1 or C1 CSAA440 standard. Look for this minimum or higher when shopping for windows. "Casement” style windows have a higher rating than "sliders”, but generally cost about 25% more.

There are a variety of frame types such as wood, aluminum and fibreglass. However, vinyl frames are the most common because of their combination of low conductivity and low maintenance.

Glazing is a term you will come across and refers to the transparent part of the window. Single-glazed, double- and triple-glazed refer to the number of panes of glass in the window unit.

The spacer bar is what separates the double glass panes. Traditionally, spacers were made of hollow aluminum; but nowadays it is imperative to look for an insulated spacer bar. An insulated spacer bar significantly reduces energy loss. The two most common are thermal edge and swiggle. It is important to ensure your thermal panes are double sealed on the sides and bottom of the spacer bar. A dual seal will last two to three times longer than a single seal and usually comes with a 10 year warranty against fogging inside the glass.

There is also the application of low-emmisive (Low-E) coatings or films. These are thin invisible, metallic layers that are suspended between glazings and increase R-Value, reflect heat back into the house in winter and reflect heat out of the house in summer. There are three ratings for Lo-E: Good, Better and Best.

Good: Low-E hard coat is applied to the inside surface of the outer pane. This increases the R-value by 1, but does very little for heat gains in the summer.

Better: Low-E soft coat, known as Low-E2, is applied the same as the hard coat but is far superior in performance. It increases the R-Value by about 2.

Best: Heat Mirror is an invisible, metallic layer suspended in the middle of the thermopane. The two most common are "SC75” for southern facing windows and "TC88” for Northern facing windows. This application increases the R-Value to 4.4 for SC75 and 5.4 for TC88. This offers the best winter and summer performance available.

The final step in building a high performance window is the insertion of an inert gas replacing the air in the thermopane. Argon and Krypton are the two most common gases used. As they are much more dense than air, the heat transfers through them at a much slower rate.

So consider these tips and ratings when researching your new windows. Think high performance with lower energy bills and more comfort in your home.

John Lindsay works with SIP Building Systems and can be reached for questions via cell phone at 250-618-2440.