Fair comparison of glazing whole life carbon

No pane, no gain

We’re still hearing repeats of old analysis showing that the embodied carbon of an extra pane of glass in triple glazing means it isn’t worth the reduced operational carbon, so not worth the pane. It’s a good pun, but the calculations don’t stack up, it is worth the pane, and it’s very little pain! We’ve shared some calculations at the end of this post, so you can make up your own mind.

Triple glazing gives lower whole life carbon than double glazing. The key difference in our analysis is that we’ve looked at the building and system as a whole, rather than just the glass. There are knock on system impacts of using more heating, simplified analysis of just the glass in isolation does not give the whole story.

Low operational AND embodied carbon

It is not useful to compare operational and embodied carbon emissions of individual products. To meet carbon reduction targets both need to be reduced as far as possible, and this includes a radical reduction in heating energy down to near Passivhaus levels.

Picking on triple glazing in isolation is another example of well intended analysis providing a convenient result, similar to our previous post on embodied carbon of PV. The embodied carbon of materials is changing quickly, and it’s important you consider the whole system. If you are searching for options to reduce embodied carbon, target structure, materials and finishes that have no impact on operational carbon first.

Counting the impact of the whole system

The simple direct comparison between the operational (heating) carbon saved by triple glazing, and the embodied carbon of manufacture for the extra glass misses some of the impact by focussing only on the window itself. What about the generation and distribution of heat? These diagrams show how the balance can change depending on what we include in the analysis:

Simple comparison of just the glazing
Simple comparison of just the glazing but using a different glass manufacturer
Sophisticated comparison using assumptions for the system changes.

The glazing affects the requirements for the building services and energy network, so to make a fair comparison we should also include some assumptions for this. Some knock on effects of reducing the specification to double glazing could be:

  • Extra length of heating distribution pipe to get the heat emitters under the window. This is needed to offset the winter radiant surface temperature for double glazing, but not for triple.
  • Larger radiators or heat emitters in each room to offset the extra heat loss.
  • Larger building heating system to supply the extra radiator capacity.
  • Larger local electricity distribution capacity and cable sizing to supply the additional power.
  • Additional renewable generation to supply the power.

OK, so lots of these will be small, and in practice absorbed by safety margins, but at scale the impact will be there. All stacked up, even with very small estimates for the impact of some of these things, the balance quickly starts to sway in favour of better energy efficiency. In colder climates, glass with low embodied carbon, or for glass that is horizontal for rooflights, even quadruple glazing would start to make sense.

Reducing the embodied carbon of windows

Window sizing

We can’t mention glass without thinking about the facade design of the building as a whole. Good daylighting is crucial to a successful building interior, but floor to ceiling glass is not needed to deliver this and there is often an opportunity to reduce the window area in overly lit spaces, or to reduce the glare. 

Reducing the window area reduces the operational energy and overheating risk or cooling load, which reduces whole life carbon. Depending on your wall construction you may see an embodied carbon reduction from reduced window areas as well. Lots of reasons to get that daylight analysis right early in design. 

Manufacturer choice

Early design decisions can be taken based on material databases such as ICE or averages from software like Bionova OneClick, however the most reliable source of embodied carbon data is direct from manufacturers through third party assessed Environmental Product Declarations (EPD). 

The product level calculations are finely balanced, so even excluding all the other impacts we’ve highlighted, a high carbon glass product for double glazing can have a higher whole life carbon than triple glazing with a lower carbon glass product.

The frame choice matters more than the glass

The glazing industry in the UK are working hard to reduce carbon emissions from glass manufacture. But frame materials can have fundamental differences depending on the material used. An aluminium frame double glazed window or door has much higher carbon than its equivalent timber frame triple glazed counterpart. Therefore when you consider materials and embodied carbon in your design, softwood timber framed or hybrid window systems should be considered.

Modern stabilised timber and coatings have 25year + warranties, and can have 5-10 year maintenance cycles. Timber frames are easy to repair and in houses and older schools examples of timber windows over 150 years old are common. Hybrid windows with an external aluminium capping could reduce requirement for painting, if you must, but ‘maintenance free’ is often unrealistic.

Calculations

Please email for a copy of our calculations.

Notes on calculations

  1. Follows RICS guidance Whole life carbon assessment for the built environment, and the LETI Whole Life Carbon defining and aligning guidance. Uses 60year design life.
  2. We’ve assumed one window replacement and four maintenance cycles over the 60 year period. 
  3. Operational energy comparisons assume the same air temperature. For equal comfort the same operative temperature should be used, so a balance between radiant and air temperature. Triple glazing increases the internal surface temperatures of the glass, meaning potential for reducing the air temperature inside for the same comfort and so further energy savings.
  4. Our assessment includes a good low carbon heating system, a heat pump with a delivered sCOP of 3. For higher carbon heating sources triple glazing makes even more sense.
  5. It is currently not possible to know the embodied carbon impact of the building services and energy network beyond the pipe connections to the radiators. A simple assumption of 1kgCO2/sqm glass has been made, this is wrong, but probably less wrong than assuming no impact.