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Why Streamlined Energy Performance Testing is Good News for Manufacturers

Some important news was announced by the National Fenestration Ratings Council (NFRC) in February that has the potential to change the way residential window and door makers simulate thermal performance for new products. And it could have some big, positive implications for those manufacturers to innovate more efficiently.

Per DWM Magazine:
According to the council, its new Linear Energy Analysis for Fenestration (LEAFF) methodology leverages the simulation process that it has used for three decades producing fair, accurate, and credible ratings which measure the energy efficiency of fenestration products.

The LEAFF trendline methodology uses product-line characteristics to determine trendlines that replace the need to run numerous simulations separately. This establishes the windows’, doors’, or skylights’ energy performance with different options such as glazing and frame types.

I had the privilege of helping to develop LEAFF with some of the brightest minds in the industry, and I believe it could be a big deal for window and door makers.

The way the industry has traditionally calculated thermal performance ratings has been complex, time- consuming, costly—and therefore inhibitive of innovation. Two separate elements go into the total rating. The first, known as center-of-glass U-factor or Ug, includes the glass itself, the airspace width(s), any gas fills, and the type(s) and location(s) of any low-E coatings. This value can be obtained from a relatively quick, simple calculation in the WINDOW software app.

But a full window model also requires the frame U-factor (Uf) and the edge-of-glass U-factor (Ue), which includes the IG spacer. The calculation of these values, using the THERM 2D modeling app, is much more involved and time-consuming than the quick calculation of Ug. And with numerous types of energy-saving framing and spacer technologies available, this became the source of the old system’s complexity. Changing elements of the glass package influence the total product U-factor; for example, a more thermally efficient glass package may divert heat energy toward the frame, impacting the frame’s performance and therefore that of the entire system.

This meant that any change, no matter how small, required a completely new simulation and calculation from the ground up, requiring both WINDOW and THERM, to determine a new thermal performance rating for the complete window system. Each new calculation might take 30 minutes to an hour to complete, including new framing calculations—and when a manufacturer might want to test dozens of potential tweaks to their glass package, the process became prohibitively time-consuming. LEAFF streamlines the process by using a standard calculation discovered by some industry professionals a few years ago: that when the choice of spacer and framing is left the same, the total window U-factor is directly proportional to the center-of-glass U-factor.

LEAFF uses this formula and enables window manufacturers to simulate design tweaks to glass choice, low-E coatings and other factors without the need for complete ground-up re-simulation every time. As such, the tool can result in a huge reduction in the cost and time historically required to obtain simulated thermal performance figures. With these time savings, manufacturers can of course test new spacer and framing packages as well, granting manufacturers greater visibility into exactly how big of an impact can be made by choosing certain components over others.

I’m excited about what LEAFF has the potential to do for the industry. The LEAFF trendline methodology will soon become the standard NFRC 100 modelling method for residential glass, and I think all stakeholders will realize the benefits. Innovation needs to be at the heart of the fenestration industry as we work toward new thermal performance targets—this new tool makes innovating easier.

Questions or comments? Contact me at

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