Getting the most out of windows, Part II
By Paul Duffy
Window performance in code varies based on region
By Paul Duffy
Part I of this article can be found here.
Window performance required in codes varies based on type of building and climate in which it is situated. How do you compare the other measures of performance? Here is a quick primer on how to use the available ratings:
- U-value is the inverse of R-value. A higher U-value represents a higher rate of heat loss. Better windows will have a lower U-value.
- SHGC or Solar Heat Gain Coefficient is the fraction of solar energy passing through the window. A window with a lower SHGC has a greater tendency to block solar heat gain.
- VT is a measure of the visible transmittance of light through the window. The higher the VT, the greater the amount of daylight passing through.
- Air Tightness is measured in classes from A1 through A3. A3 is the most airtight.
- Water Tightness is measured in classes from B1 through B7. B7 is the most waterproof.
- Wind Load Resistance is categorized with values from C1 to C5. C5 has the highest resistance to wind.
- Resistance to Forced Entry is also broken into groupings; F1 is the minimum level and F2 is the highest level of resistance.
So you now have information to compare your window options. Can things still get screwed up? Yes, absolutely, because any product can get messed up with a poor installation. Usually, the issues fall into three broad categories:
1. Product selection: I assume that the previous discussion gives at least some guidance on getting your product choices right. It can get complicated, affecting HVAC sizing and other issues. You might want to engage an expert—more about that later.
2. Window sizing: Simply understanding that windows are usually the least thermally efficient elements of your building is an important step. In most new houses, the opaque walls have five to 10 times the thermal resistance (R-value) of the windows and in the summer can be responsible for huge temperature swings affecting comfort and air conditioning loads. Most experts agree that when window areas exceed 15 per cent of wall area, comfort problems and other performance problems become more pronounced.
3. Installation: Beyond the windows themselves, the rough openings can be responsible for considerable air leakage and water ingress into the concealed elements of the building. With water comes rot and corrosion. The results can be disastrous. The need for proper training of installers is critical to getting windows that perform.
Recognizing that proper installation goes hand in hand with better windows, most manufacturers participate in a program for training and certifying installers called “Window Wise” provided through SAWDAC – the Siding and Window Dealers Association of Canada. The program provides:
- training of installers;
- random inspections; and
- certifications of installations for warranty purposes.
Most Canadian manufacturers participate in the Window Wise program and support its use. Proper installation of windows could be the focus of a whole other article so, for now, I am going to leave it at installers have to be properly trained.
Window performance also can have a huge impact on heating and cooling loads as well as distribution so I also recommend that major renovations including window replacements be accompanied by an energy/heat loss/gain analysis. If you have ever worked with programs such as the R-2000 Program, EnergyStar, Eco-Energy, etc., you will have some idea of the expertise I am suggesting is needed.
Finally, you may also be aware that window retrofits qualify for some grants and incentives provided by various governments and utilities across the country. You will notice I left this point to the end of this discussion. I don’t think the grants are significant enough to drive customers to change all their windows because grants and incentives are available, but there is probably enough money available to help you pay for the experts you need—qualified designers and contractors—to do it right. Most grants and incentives come with a requirement that a qualified expert review and sign off on the installation. I would recommend searching for a local energy evaluator or rater in your area.
In a new house or in a renovation, good quality windows can be the “icing on the cake” that produces stunning results. A high quality window installation can also boost your reputation and referrals. Take the time to do a proper job so your customers won’t be disappointed—deliver high performance that matches the beauty of your work.
An independent building science, engineering and management consultant, with more than 30 years experience, Paul has contributed to Canadian Contractor numerous technical articles about energy-efficient construction, codes and innovations in recent years. His strength is translating technical information into plain language concepts that are easy to understand. He is based in Toronto.