Energy Efficiency out the Window

This past week I attended the Cleantech Forum in San Francisco and much of the discussions in energy efficiency focused on sensors, networks, data analysis, and other technologies that will require more robust electrical systems.   However, I listened to one panelist that claimed that his product could reduce energy use worldwide by 5-6% with a technology that is already commercially available.  What is this technological breakthrough?……… A window.  According to Serious Materials CEO Kevin Surace, “39% of all emissions are tied to building operations, with 38% of that for heating and cooling. Up to 40% of that energy – and cost – literally goes out the window. The wasted energy results in over 250 million tons of emissions per year.”  (  Now obviously not everyone in the world in going to replace their windows, but this seemed like low hanging fruit in the grand scheme of energy efficiency. 

Over the past few decades home energy efficiency has focused primarily on better insulation of walls and attics with only marginal attention paid to insulation technology in windows.  Insulation materials are generally rated in R-Values, which is the resistance of heat flow of a material, calculated by (hr x degrees F x sqft) / Btu.  Window insulation is actually calculated as a U-value (just the inverse of the R-value), but I’ll show the R-value of the window for comparison.  Today a typical wall has an insulation rating of around R-13, meaning that if it is 70o F inside and 0o F outside then it would require 5.38 BTU to maintain the 700 F temperature for 1 sqft.  Now compare that typical R-13 wall to a typical dual paned Energy Star window with an R-2.8 rating, which would require 25 BTU to maintain the 700 F for 1 sqft.( With these drastically different insulation levels heat simply rushes out the windows as can be seen in the thermal image below:


Some window manufacturers have been able to achieve R-4 to R-9 ratings using triple pane low-e glass, which reflects heat and reduces heat transfer, and are filled with inert gases between panes that slow heat transfer further.  However, this is not a material breakthrough; it is simply stacking more pieces of glass together.  The result is a more expensive window with less applications due to the increased weight.  Serious Materials on the other hand is using two panes of low-e glass with a suspended film sandwiched in between and fiberglass frame to achieve R-6 to R-14 ratings.  By not dramatically increasing the weight of the window, larger windows can be used for more applications thus increasing natural lighting and reducing the need for energy used for lighting.  The figure below shows the increased energy savings that can achieved through better insulated windows compared to a typical dual pane window. (


The data for this chart is based on the U.S. Department of Energy’s (RESFEN) model and is an average taken from 5 typical cities across the U.S.

My point in explaining this is not to shamelessly plug a company, but to point out that technologies exist that are not being fully utilized.  The Department of Energy is promoting the use of more energy efficient technologies in windows through energy efficient tax credits worth $1,500. (  However, the requirements to receive these tax credits are that the window have a U-rating of less than .30 (R-rating of 3.33), which were increased in March 2009 from a U-rating of .35 (R-rating of 2.8)  If products exist that are two or even three times as effective at a similar price, then why does the DOE not set higher requirements?  Why should we settle for incremental improvements when significant ones exist?


1 Comment

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One response to “Energy Efficiency out the Window

  1. tom0mason

    With all that heat leaving through the windows, make you wonder how those glazed greenhouses stay so warm.

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