News Brief

Twenty-Year Payback for Embodied Carbon of Triple-Glazed Windows

It takes almost 20 years for triple-glazed windows to save enough energy to overcome their additional embodied carbon, according to a new study.

Comparing Carbon Footprint of Frame Materials

Over the lifetime of a window, frame material makes more of a difference for carbon footprint than the number of panes, according to researchers.

Credit: Craig Jones, Circular Ecology
Triple-glazed windows may save more energy than double-paned windows, but a recent study conducted by U.K.-based consulting firm Inspired Efficiency and footprinting expert Circular Ecology finds that in terms of their life-cycle carbon footprint, they don’t necessarily come out ahead.

Researchers determined that the embodied carbon of an average triple-glazed window is 51 kgCO2e greater than a double-glazed window with the same frame type because of the carbon dioxide emissions that are released from extraction, refinement, transport, and processing of the additional layer of glass and pocket of gas between the panes. It would take almost 20 years for a triple-glazed window to pay back this additional embodied carbon—longer than the lifetime of many windows.

That doesn’t mean you shouldn’t specify triple-pane glazings; there are other benefits, including operational cost savings, construction cost savings from smaller mechanical systems, improved thermal comfort, and acoustics to name a few. Furthermore, if customers are seeking to reduce their carbon footprint, frame choice has a far larger impact, according to the researchers. Choosing wood over PVC saves 25kgCO2e—10 years’ worth of operational savings—so over 20 years, a triple-pane, wood-framed window will have a lower carbon footprint than a double-pane window with either a PVC or aluminum frame.

For more information:

Circular Ecology

circularecology.com/news/

Published August 3, 2014

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Comments

July 29, 2014 - 11:53 am

Sounds like the study just looked at triple pane windows with three panes of glass and perhaps did not examine triple pane using technologies like Heat Mirror.

July 29, 2014 - 11:54 am

I am curious what our Carbon payback on granite countertops is vs laminate or other options?  or perhaps wood floors vs carpet?  Seems like we focus on those things that actually have a financial payback.

I see you do show other reasons like comfort, acoustics, operational costs, HVAC cosntruction cost savings, etc.  

July 29, 2014 - 12:17 pm

I'm surprise at your statement "they don't necessarily come out ahead".  We certainly expect windows to have much much longer than 20 year life cycle, easily 50 years or longer.  So they still come out ahead even just on its own merit.

July 29, 2014 - 8:53 pm

Sorry Patricia, not so. Insulated glass unit (IGU) products collapse the durability of raw float glass from indefinite to a mere two or three decades. IGU durability is a function of fabrication quality, which varies widely, but even the highest quality producers are only beginning to aim at products with an intended 50 year service life. Warranties are in the 5 to 10 year timeframes. The IGU often becomes the weak link in the facade system: the component with the shortest lifespan. Failure is typically the result of seal failure.

August 4, 2014 - 3:36 pm

Yes, we see frequent seal failure leading to condesation, fogging, even an orangish speckled growth, on insulated glass units over time.  I have seen as much as approximately 30-40% failure in 15 years and as much as 70-75% in 20 years - especially where there is high contrast between shadow and full sunlight on the glass - throughout the day or from season to season. 

Can anyone offer advice on how best to specify to help assure seal durability over the long term?

July 29, 2014 - 12:34 pm

The carbon payback of any building envelope assembly is greatly dependent on climate - the greater the "delta_T", the greater the energy savings potential.  I'm surprised that this article didn't state the assumptions inherent in the study - is the 20 year payback for windows in the UK?  in Florida?  in Newfoundland?  I live in Alaska, where triple-pane has been the high-performance standard for many years, and I assume that the carbon payback is much less than 20 years when you are dealing with 14,000+ Heating Degree Days.  Making blanket statements relating to envelope energy savings undermines the importance of designing for your climate.

July 29, 2014 - 9:03 pm

There seems to be a negative bias to this article right up to the end, where the many good reasons to consider triple-glazed products are finally mentioned. I think it may be confusing to many. Triple-glazed products certainly provide more efficient thermal and acoustical performance than the typical double-skin facade, for example. There are many good reasons to use triple-glazed products. It is an excellent point, however, that materials must be considered (and too often are not), and this is a good example where the additional material can have a significant impact on overall carbon footprint. Its not all about operational energy, which is rightfully the dominant focus. But as buildings and systems become increasingly efficient, embodied energy will become an increasingly larger share of the energy debt, and must be accounted for.

July 31, 2014 - 2:14 pm

I appreciate everyone's thoughtful comments on this article and this topic.

While not explicitly referenced, this article, the time value of carbon is much on our mind here. See Embodied Carbon: Measuring How Building Materials Affect Climate. Reducing carbon emissions in the near term from reduced materials impact has an immedate benefit, while operational savings accrue over a long period of time. The next 10 years will be critical in determining the extent of global climate change.

While not explicitly stated I do not think the study considered films like Heat Mirror. I have always heard that the Europeans have not accepted the durability of that kind of glazing and it has not caught on. Clearly there are benefits to it, including a potentially lighter frame.

Also, Mic, it's a three-paragraph article! It takes about two paragraphs to briefly explain the study, and one to put it in context. Hardly like Fox News, which would entirely omit the context.

July 29, 2014 - 9:06 pm

By the way, the graphic does not read correctly in the article, at least in my browser (chrome). You may have to follow the Circular Ecology link to the original report to view the graph correctly.

July 31, 2014 - 3:50 pm

Mic, I also use Chrome and am not having any problem with the graph. Can you contact me at customersupport (at) buildinggreen.com and let me know what the issue is on your browser? Thanks.

August 4, 2014 - 3:29 pm

Our energy modeling on a 43,000 sf office building in the St. Louis climate showed a 5% improvement in the envelope performance of a wood framed window system as compared to a thermally broken aluminum framed system. A sensible approach can be to match or at least balance teh thermal properties of the frame and glazing system. For instance if one is using a high performance glazing system with a R-value of say 3.5 to 5 then one would wle ant a frame system that has a similar insulative value; and not a frame system something like a R-2 frame value - all other things (like infiltration rate) being equal.

I too question the durability of internally suspended film products such as Heat Mirror based on personal experience. That said however, I am an avid believer in the promise of such products as a very important tool in high performance window systems. Compared to triple glass glazing a double glazed unit with a suspended layer of Heat Mirror offers higher visible daylight transparency and essentially equal thermal performance at lighter weight. Their higher daylight transmittance allows us to accomplish the same daylight levels with smaller windows resulting in an overall higher wall insulation value.

April 20, 2016 - 11:45 am

In a recent meeting with an acoustical engineer, I was infromed that dual pane glass generally has higher STC values than triple pane glass.  This is because the tighter air space in triple pane glazing transmits vibrations more efficiently.  He said that he routinely challenges people to show him test data that supports the idea that triple pane windows have better acoustical properties, and no one ever delivers.