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The Engineer: Low-E lowdown

A triple-glazed IGU with two low-E coatings has a U-value 15 per cent better.

November 29, 2018  By David Heska P.Eng

The NHL season is in full swing, temperatures are falling and Canadians are getting ready for winter. At the same time, building owners are considering what projects may need to be completed in 2019 and preparing budgets accordingly.

Projects for next year may include the replacement of a small number of insulating glass units or renewal of an entire curtainwall facade. Regardless of the project size, selection of the proper glass is critical.

There are many things to consider when replacing glass units. What is the thickness of glass required?  What type of gas fill will be used? Will the glass be laminated, tempered, annealed or heat strengthened? What type of low-emissivity coating will be installed? And also which surface(s) will receive the low-E coating? This final question is the one that I’ll consider briefly here.

I remember a few years ago arriving at a construction site only to have a very young foreman ask me, “Does it matter if I install the new glass with the sticker facing inward or outward?” To many of us this is a silly question; obviously, it matters. But why? Does it really matter if the low-E coating is on surface #2 or #3 for a double-glazed IGU?

First, let’s state that a low-E coating is a metallic coating that minimizes the amount of ultraviolet and infrared light that can pass through glass without compromising the visible light that is transmitted. Surfaces are numbered from exterior to interior, with surface #1 always being on the exterior. For double-glazed IGUs, surface #4 is on the interior, but for triple-glazed IGUs it’s surface #6.

For double-glazed IGUs there is debate as to whether the low-E coating should be installed on surface #2 or #3. Regardless of which surface the low-E coating is on, the light transmittance and U-value of the unit will be the same. The benefit of installing the low-E coating on surface #2 is that the solar gain is absorbed by the outer pane and mostly rejected to the outdoors. As a result, the solar heat gain is reduced and the cost of air conditioning the building in the summer is reduced. The benefit of installing the low-E coating on surface #3 is that the coating absorbs incident solar energy and warms the inner pane of coated glass.

As a general rule for commercial buildings in most urban Canadian cities, the low-E coating should be on surface #2. This is because reducing solar heat gain and reducing cooling costs is a priority. Having the low-E coating on surface #2 also helps avoid overheating the south-facing parts of the building in the winter. If the HVAC mechanical systems are sophisticated enough that they can be tuned for different conditions, consideration may be given to installing the low-E coating on surface #3 on the north elevation. This is not popular on building restoration projects, but it is becoming more popular in new construction.

As the number of triple-glazed windows being manufactured increases, we are seeing more IGUs installed with multiple low-E coatings. Studies have shown that a triple-glazed IGU with two low-E coatings has a U-value 15 per cent better than the same IGU with only one coating. For triple-glazed IGUs the coatings are placed on surfaces #2 and #4 or #5.

So as you can see, multiple factors intersect to determine how your IG units should be coated. One design most definitely does not fit all, even within one project. In all cases, it’s important for the engineer, manufacturer and owner to understand the orientation and daylighting of the building, how the glass is intended to function and the HVAC limitations. And, yes, if you install the glass backwards, it is installed wrong.

David Heska, P.Eng. is a director with WSP’s building sciences team in southwestern Ontario. He oversees the operation of the Hamilton, Kitchener and Windsor offices. David has been involved on window simulation projects as well as the design and replacement of windows in commercial and residential buildings. He can be reached at

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