Over the years, our industry has relied heavily on reducing the center of glass (COG) U-factor as a way of driving down overall fenestration U-factors (thermal transmittance). Strategies have included increasing the performance of low-emissivity (low-e) coatings, using multiple coatings, addition of inert gases, and more recently, moving to triple-pane units. However, the COG U-factor makes up only part of a window’s performance. To achieve the lowest fenestration thermal transmittance and highest condensation resistance, it is necessary to look more broadly at the window as a system, including the edges: The frame and edge of glass (EOG).
Think of a window as the thermal equivalent of damming a river at its center, yet forgetting to block water flow all the way to its banks.
Focusing on preventing heat flow through the COG only is the thermal equivalent of damming a river at its center, but not spanning the entire river. In the case of the river, the water just flows around the edges. Likewise, in the case of the window, the energy still flows through it even if the COG conductance is low. It does so by taking the path of least resistance around the edges of the window – through the frame and EOG.
Whole fenestration U-factor as a function of center of glass (COG), edge of (EOG) and frame thermal performance.
The graph illustrates how the window U-factor varies with the COG, frame and EOG (spacer) performance, and demonstrates how the specifications of the edges of a window control the overall U-factor performance. The data confirms that when specifying a fenestration system, the first focus should be on improving the performance of the frame (biggest influence) and EOG before specifying the COG package. Spec the edge.
Having a high-performance edge is an enabler for achieving a high-performance window system, and it can provide greater flexibility in glass choice because the highest COG U-factor may not be needed. A top-performing frame can achieve the same performance with a dual-pane glazing as a poorer-performing frame with triple-pane glazing.
Because condensation resistance is also driven by thermal bridging (or lack thereof) at the fenestration edge, improving frame and EOG thermal performance first has a dual benefit of increasing performance on this dimension too. For the example above, the condensation resistance of the non-thermally broken window wall is only 40 compared to 53 for the thermally broken system.
The moral of the story is that when designing windows for thermal and condensation-resistance performance, start with the edge first. Doing so delivers higher performance and more flexibility, reduced complexity and potentially reduced cost for the glazing in-fill. In other words, before getting caught up in the middle, spec the edge. To learn more, visit https://spectheedge.com/
Related Stories
Windows and Doors | Apr 5, 2018
Product roundup: 12 windows + doors products
Schlage's M Collection, Hufcor's Invista, and Sierra Pacific Windows' FeelSafe Bi-fold door are just three of the 12 products highlighted in BD+C's April Product Roundup.
75 Top Building Products | Mar 26, 2018
101 Top Products: Windows + Doors
Among the best window + door products included in BD+C's Top 101 Products report are Jeld-Wen's EpicVue windows, Yale/Assa Abloy's Yale Pivot Collection, and Zola Window's Thermo ALU75 Windows.
Sponsored | Windows and Doors | Nov 21, 2017
Daylighting promotes healing and wellness at the Florida Hospital for Women at Orlando Campus
Growing research demonstrates that patients recover faster and better from illness or surgery in settings that offer abundant daylight and views to the outdoors.
Sponsored | Windows and Doors | Aug 16, 2017
Pella Impervia® fiberglass windows provide luxury apartment residents maximum daylight, energy efficiency
Envisioning a luxury, suburban-meets-urban experience, the development team looked to Pella’s large combination Impervia® fiberglass windows.
Sports and Recreational Facilities | Jul 17, 2017
A new Rec Centre in Toronto links three neighborhoods
Community engagement impacts its design and programming.
Sponsored | Windows and Doors | Jun 5, 2017
Pella provides perfect blend of past and present using historically accurate windows solutions
The design team made the most of the structure’s strengths — like the alignment and shape of the window openings and skylights, which allow abundant daylight to penetrate deep into interior spaces.
Windows and Doors | May 5, 2017
Product Roundup: 16 hot window + door systems
Multiglide doors from Andersen Windows, SL73 from NanaWall Systems, and EpicVue from Jeld-Wen are three of the 16 products included in BD+C's May Product Roundup.
Sponsored | Windows and Doors | Apr 5, 2017
Tall windows and a unique vision - Welcome to the Beranger Condos
Combining product flexibility with outstanding performance, tall fiberglass windows and weather-friendly aluminum-clad wood doors were a natural choice for this mixed use building.
Sponsored | Windows and Doors | Jul 11, 2016
From past to present – bringing Mitchell Hall into the 21st century
With 60-year-old windows and high energy usage, the University of New Mexico building was in need of a historically accurate renovation that would reintroduce it as an environmentally sustainable education facility
Windows and Doors | Nov 3, 2015
Pop-out window concept offers 'more sky' for multifamily dwellers
Windows in the More Sky system jut out the side of the building and fan out like an accordion.