BUILDING SCIENCE

A Window of Opportunity

By Lisa Bergeron

For as long as I can remember in the construction industry, the perception was that windows were the weakest link among building envelope thermal components. And in fact, they are-—no argument there. When you dissect a wall assembly into its components, windows do have the lowest R-values and can’t compete against insulation. This story, however, is not about competing with insulation; it’s about complementing insulation and finding opportunities to improve the whole wall effective R-values in ways that benefit builders, homeowners and the environment.

Window to Wall Ratio: An Important Factor
In the last 15 years, the average amount of glass in residential housing has nearly doubled, going from 8% before 2000 to nearly 17% in 2016. Homeowners want more natural light and need to be comfortable in their highly glazed homes.
JELD-WEN began to look at the impact that fenestration had on the overall building envelope and how higher Window to Wall Ratios (WWR), coupled with high performance windows could influence more than just the building envelope. This was the beginning of our journey.

The Study
In order to bring credence to our theory, we had to demonstrate how windows significantly impacted the building envelope through simulations. We partnered with Building Knowledge Canada and set out to model four archetype homes in three Canadian energy zones with six different WWRs. We modeled minimum code compliant windows versus highly efficient triple-glazed units in both high solar gain and low solar gain with four different insulated wall types. The result was over 1400 HOT2000 simulations.

The Study Results: Expected and Unexpected
While we were confident that the numbers would show that the most efficient and simplest way to improve the effective R-value of an above-grade wall assembly was to put in high-performance triple-glazed windows, we were surprised to see how great an effect the choice of window has on thermal performance of the entire wall. Also surprising was data showing the extent to which choosing the right glazing packages influences the cooling loads. We indeed had a very good story to tell, both about the wall as a system, and the house as a system.
Above 13% WWR, a high-performance triple-glazed window had a greater positive impact on the effective R-value of a wall assembly with R22 batts than simply adding R5 insulated sheathing.
Above 20% WWR, a high-performance triple-glazed window had a greater positive impact on the effective R-value of a wall assembly with R22 batts than simply adding R10 insulated sheathing.
High-performance Low Solar Glass triple-glazed windows could reduce air conditioning loads by up to 50%+
High-performance triple-glazed windows improved the total effective R-value of above grade wall assemblies by: 16% at 15% WWR, 22% at 20% WWR, and 24% at 25% WWR.

Conclusions
The greater the WWR, the more important choosing the right window becomes for a builder. Choosing the right window package can reduce total costs for builders, save on energy consumption, reduce GHG emissions, virtually eliminate builder call-backs for condensation build-up on windows and improve overall homeowner comfort by increasing the interior surface temperature of windows.  Windows really are the biggest opportunity in the building envelope!
This story is about giving builders information to help guide them in their decision making process. The path to Net Zero Energy homes combines high-performance triple-glazed windows, better insulation and air-tightness as well as right-sized mechanical systems recognizing that a holistic approach to building science is often the most efficient one.  
Make sure you are looking at the entire data (whole window values, not centre of glass) regarding your window selection including U-Values, the Solar Heat Gain Coefficient (SHGC) and ask about condensation issues related to the glass option selected.  Whether you are on the Net Zero Energy path or simply trying to make the right choices for your clients, looking at your window options as a way to improve your building envelope, reduce energy consumption and increase homeowner comfort is a relatively new concept that now has strong scientific data behind it.

From the data in their study, JELD-WEN has developed a comparative tool that will help builders make the right window selection. For more information on this study or on JELD-WEN products, contact the author at lbergeron@jeldwen.com or your local JELD-WEN representative.

Lisa Bergeron is Government Relations Manager for JELD-WEN of Canada. She is an Executive Board Member of Fenestration Canada, chairs Fenestration Canada’s Fabricator’s Council and has been a member of NRCan’s Energy Efficient Fenestration Steering Committee, which dictates ENERGY STAR fenestration criteria, since its inception.

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