What does a builder need to know about windows and energy efficiency?

We recently took part in a Builder’s Breakfast with Layton Consulting hosted by HAVAN, where we did a deep dive into everything to do with windows.

With buildings in the Lower Mainland, energy efficiency has been both a great focus of new building regulations and the cause of the greatest confusion. Unfortunately, despite all the target goals, the step code, and industry compliance, the performance ability of windows and doors continues to be misunderstood.

So here is Part 1 of your windows 101.

1. What is U-Factor and why do we use it to determine a window’s energy performance?

In a nutshell, it’ll tell you how efficient your windows are. U-Factor (imperial measurement) or USI Value (metric) measures how well a product can keep heat from escaping from the inside of a room. The lower the number, the better the window or door is at keeping the heat in.

Each product’s U-factor actually comprises 3 separate U-values: the centre of the glass, the edge and the frame. An R-value is the inverse of the U-value. We also take into account the SHGC (solar heat gain coefficient) which is a measurement of how much radiant heat energy comes through the windows. For example, a product with an SHGC of 0.4 = allows 40% of solar radiation through, with 60% reflected back.

Both H2K (the modelling software Energy Advisors use) and the B.C. Energy Step Code Design Guide suggests that energy modellers view solar heat gain as a positive part of good energy design.

“Professionals who orient their buildings to maximize the solar-gain potential from the south can reduce heating demands by as much as 30 to 40%.”

While this strategy does not minimize heat losses per se, it takes advantage of passive heat gains but, designers taking advantage of solar gain must also be careful to avoid overheating in the summer months, by specifying the use of external shading.

2. What are the four metrics builders should understand about performance labels?

There are four things we look at when it comes to the glass in windows and doors. 1) U value, (USI value – metric) 2) SHGC, 3) visible transmittance 4) air leakage (how much air is getting through the seals and the frame).

There is a difference in air leakage between windows that are inoperable (can’t open) for example, a picture window and operable windows. For a picture window, no air escapes. On a slider window, there is a bit of air leakage. Awning and casement windows don’t lock as tight and let the most air through.

3. What are the challenges of ordering windows that meet the requirements on your energy reports?

As part of the energy model, energy advisors assign a U value and an SHGC for every window and door in the home. The next part is where it gets tricky. Window manufacturers test and deliver windows with set U-values and SHGCs. But when a builder is ordering windows to be energy efficient, there are a lot of variables – such as the direction the house faces, the shading inside or outside and what kind of window is being put in (i.e.: picture window vs. casement)

When you’re ordering your window package, work with us and we’ll figure out together what the right numbers are. That’s the advantage of working with an experienced EA.

You can discuss window performance with your EA to determine if the energy model (Step Code report) specifications can be changed to improve overall performance. There are often tradeoffs that can be made, but sometimes builders don’t realize that there can be flexibility if managed before construction starts.

As a builder, if you are not brought on to a project before the building permit, one of the first things to look at would be the window specifications from your energy model. Knowing that the U value and solar heat gain could impact how your clients feel about the performance of their windows, you should review with your EA if you have any concerns. Up next, is Part 2 of our deep dive into windows.