Highly Insulating Glazing Systems
Using Non-Structural Center Glazing Layers

Windows in the United States use aproximately 2 quads a year in heating energy, approximately one third of all building space heating energy used and the largest single end use attributed to windows. Even if all existing windows were replaced with today’s ENERGY STAR low-e products (U values < 0.35 Btu/hr-ft2-F), windows related heating would still be over 1 Quad.

Because heating loads are strongly tied to conductive losses, technologies which lead to lower window U-factors are the key to reducing heating energy. A 0.1 Btu/hr-ft2-F window is targeted as a product, which will meet the requirements of zero-energy homes. Dynamic control of solar gains will further reduce heating needs by allowing winter solar heat gains to be effectively utilized while limiting cooling season gains. Significant cooling load savings can also be expected from lower U-factor windows in certain climates and from dynamic windows in all climates.

Current strategies used to reduce heat loss through windows are triple or quadruple glazing, which adds significant weight, or suspended films, which are costly. Because of these weight and cost disadvantages, very highly efficient multi-layer, low-emissivity (low-E) gas-fill window products account for less than one percent of today’s window sales.

In existing multi-layer units, the center layer is positioned between two spacers. This adds to production costs and doubles the path length for gas loss leakage. In this project, we examine the thermal potentials of three layers designs utilizing various means of positioning a non-structural center layer between the inner and outer layers of a standard IG (i.e. a drop-in insert in a typical IG). Some designs had thermal performance similar to that of a standard three layer low-e/gas filled unit, indicating the possibility of developing a lower cost, more durable, lighter, highly insulating glazing system.

This project focuses on high performance glazing systems; other R&D efforts are needed to focus on reductions in frame/edge heat transfer.

Optimal Gap Width study
We performed a brief study on the optimal gap width of double and triple glazing systems. Comparing US/NFRC standards with European EN 673 standards.


Future Advanced Windows for Zero-Energy Homes, J. Apte, D. Arasteh, J. Huang, 2003 ASHRAE Annual Meeting, 2002

Performance Criteria for Residential Zero Energy Windows, D. Arasteh, H. Goudey, J. Huang, C. Kohler, R. Mitchell, 2006, submitted to ASHRAE


For more information contact Robert Hart

Traditional triple glazed window with 4 paths for gas leakage.

Window with non-structural center layer, utilizing only 1 spacer, with 2 paths for potential gas leakage

DOE Fact Sheet

Detailed Test Data