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Triple Glazing Windows

Is triple glazing as good for the environment as it is claimed, once the impact of its materials and processes is considered? Paul Hicks, ACIAT of VELUX looks at its performance and its environmental and financial impact.

There is an important debate developing as to whether triple glazing is as good for the environment as it is claimed.

Recent desktop research from Inspired Efficiency [1] (IE) and Circular Ecology [2] (CE) shows that triple glazed windows are not always the most effective low carbon option due to the environmental impact of materials and production processes. Consequently, the resulting embodied carbon [3] can make the choice of a triple glazed option less favourable and this should be taken into account when calculating the performance and impact of a building.

There is an understandable perception that low U-value is better, but if the embodied carbon in manufacturing triple glazing outweighs the savings from lower emissions when the window is in use, then it is not necessarily the right solution.

Whilst the research from IE and CE does not fully take into account the Life Cycle Analysis of different window types, it has certainly raised interesting questions to support the double/triple glazing debate and gives strength to the energy balance philosophy. Not since the energy crisis of the 1970s has the debate on the number of panes in a window been so relevant and so important.

Energy balance

A window’s energy balance is the difference between the amount of heat from sunlight the window transfers to your home and the amount of heat that escapes through the window.

This can be represented as follows:

In order to benefit from the energy balance approach, it is important to understand the three main properties of glazing:

  • U-value – thermal heat loss
  • g-value – solar heat gain
  • tv value – daylight admittance.

Using only the U-value of a window ignores the benefits from the other properties and often gives a distorted view of the value of triple glazing.

By focusing on energy balance instead of just U-values, this will help to change the perception that improved U-value is the only way forward. With a good energy balance, solar gain will help to heat the inside space and thus reduce heating costs and effectively reduce CO2 emissions thereby increasing the climate payback potential. With good daylighting properties, this will reduce the reliance on artificial lighting and thus also reduce energy required for lighting.

Ultimately, this will support the argument that low U-value is not always best value and that the concept of climate payback should apply to products as well as finished buildings.

Climate payback

Using data from the recent IE and CE research, the operational carbon saving of a ‘typical’ (1770 x 1200mm) timber triple glazed window over a double glazed window is around 2.6kg CO2/year.

As it takes an extra 51kg of CO2 to manufacture the triple glazed option, this would result in a climate payback period of around 20 years. As sealed glazing units currently have a life expectancy of 20 to 30 years [4], this hardly seems appropriate for some new build projects and is certainly not appropriate for renovation projects when using high performing windows in a building fabric which generally will have less energy efficiency than a new build.

Also, by maximising the concept of climate payback we can look at the extra energy saved from reduced heating costs due to good solar gain and reduced electrical lighting needs from good daylighting design which will increase the operational carbon saving potential and not only pay back the embodied carbon for the manufacture of the window product, but can also pay back for the

CO2 emissions caused by other materials in the construction that are less able to support climate payback.

High performing double glazed windows have an important role to play with regard to energy balance and climate payback, and will support the need to make effective solutions affordable.

Affordability

The cost of triple glazing is understandably higher than double glazing for obvious reasons – more glass, extra layer of gas (and type of gas in some cases), higher cost of manufacture in handling more components etc. For a solution to be given the consideration it deserves, it must be affordable from the point of capital outlay, throughout its operational life and ultimately its removal and disposal.

High performing double glazing has the potential to tick all of the right boxes and when we consider that it is feasible to provide a double glazed window using appropriate installation products in a recessed application to achieve an installed U-value of 1.1 W/m2K, this makes for a more cost effective solution.

As a comparison, the PassivHaus specification requires that roof windows should achieve an installed U-value of minimum 1.0 W/m2k. So what real difference does a 0.1 U-value actually make?

Fabric Heat Loss Calculation

Calculating the heat loss through a building element requires three values to consider:

1. Area of element (m²)

2. U-value of element (W/m²K)

3. Temp difference inside and outside of building (∆t)

This gives the following calculation: Area (m²) x U-value (W/m²K) x Difference in Temp (∆t) = Fabric Heat Loss (W)

Therefore, for 1 m² of glass x 0.1 of U-value x 11° temperature difference*

(*using met office data [5] on 2012 mean UK temperature of 9° outside and assumed constant 20° inside):

1 x 0.1 x 11 = 1.1W (per hour)

Then 1.1W x 24 hours = 26.4W (per day)

Then 26.4W x 365 days = 9636W (per year).

This calculation shows that for every 0.1 U-value difference in 1m² of glass, the heat loss changes by 9636W (or 9.64kWh) per year.

Using this heat loss calculation, we are able to determine both the energy lost and the CO2 created by the energy loss per square metre of roof window glazing for a 0.1 U-value difference between high performing double glazing and triple glazing. (See table below).

heat loss calculation

Energy Performance

If we assume an average of 3m² of roof glazing per dwelling, this equates to 15kg of additional CO2 in total for electricity and only 6kg additional CO2 in total for gas, when using high performing double glazing instead of triple glazing.

This small amount of additional CO2 output can easily be compensated for in other areas of the construction and more cheaply than the extra cost associated with triple glazed windows and should be considered at design stage when developing the building performance strategy to ensure the correct solution with regard to performance, environmental impact and cost.

We can also calculate the extra cost associated with increased energy loss using high performing double glazing instead of triple glazing. Space heating using electricity (14.5p per kWh −

January 2014): 9.64kWh x 14.5p = 139.78p − say £1.40 per square metre of glazing per year. Multiply this by the average 3m² of roof glazing per dwelling, this equates to just £4.20 per year.

Space heating using gas (4.5p per kWh − January 2014): 9.64kWh x 4.5p = 43.38p − say 43.4p per square metre of glazing per year. Multiply this by the average 3m² of roof glazing per dwelling, this equates to just £1.30 per year.

As the majority of homes in the UK are heated using gas, this creates an excellent case for the energy balance argument. Even if there is a migration to using electricity over time due to dwindling gas resources, the argument is still sound. Therefore, for the next section on cost, we will use the cost figure for gas central heating.

Cost

If we continue with the assumption that the average loft conversion/new build house in the UK has 3 m² of glazing, then this is equivalent to approximately 3 x VELUX PK10 windows (942 x1600mm).

If you purchase 3 x high performing double glazed VELUX GGL PK10s (–60 pane), this will cost £1452 (full list price Feb 2014). If you purchase 3 x triple glazed GGL PK10s (–66 pane), this will cost £1812 (full list price February 2014).

Therefore, 3 x triple glazed windows cost £360 more than 3 x high performing double glazed windows of the same size.

Currently, glazing is considered to have a maximum life expectancy of 20 to 30 years [4].

This means that over 20 years, the 3 x triple glazed windows will cost an extra £18 per year (20 x £18 = £360).

The energy performance calculation above shows that with triple glazing, you only save £1.30 per year in energy costs at today’s energy rates over double glazing (using gas price – January 2014).

That is a difference of £16.70 out of pocket per year. If energy prices double every 5 years for the next 20 years, you will save a total of £98, which averages out at £4.90 per year. That is £13.10 out of pocket per year.

As a comparison, when using electricity for space heating over 20 years with energy prices doubling every 5 years, you will still only save a total of £315, which averages out at £15.75 per year.

Still £2.25 out of pocket per year.

If the windows last for 30 years, the 3 x triple glazed windows will cost an extra £12 per year (30 x £12 = £360). If gas energy prices double every 5 years for 30 years, you will save a total of £13.70 per year with triple glazing. This example at best provides payback at £1.70 per year.

Changing Perceptions

The use of triple glazing has a place in future sustainable design solutions and steps have been taken to reduce the environmental impact of its manufacture.

In general however, the energy performance of the window (especially in renovation projects) should be measured by the energy balance and not just the U-value alone.

The challenge therefore is to develop collaborative partnerships to influence key decision makers in order to ensure that designers and builders do not take a ‘tick the box’ approach in order to create the right results in software simulations and calculations. If we can encourage the project team to look at the bigger picture using Life Cycle Analyses of products and materials to develop the most appropriate solution for a project, then this supports the need to move away from the ‘one size fits all’ mentality and creates a more flexible template for future design solutions, with the opportunity to put Active House [6] principles at the heart of the design process.

References

[1] Inspired Efficiency was set up in 2005 to challenge conventions and assist in implementing energy solutions that are realistic, pragmatic and valuable.

[2] Circular Ecology was inspired by the fields of ‘circular economy’ and ‘industrial ecology’ and provides environmental consultancy and research services for footprinting, Life Cycle Assessments and training.

[3] Embodied carbon is the amount of CO2 released in the manufacture and transportation of a product. Recent research has shown that timber triple glazed windows have a carbon footprint of around 60% higher than double glazed windows.

[4] German Ministry of Buildings report 2001

[5] www.metoffice.gov.uk/climate/uk/2012/annual/averages.html

[6] www.activehouse.info/

[notice]This article was originally produced by Paul Hicks, Sustainability and Design Manager for VELUX, for the CIATs AT magazine, Issue 110, which you can download here, and is reproduced here with permission[/notice]

SpecifiedBy Editor

4 comments

  • Fine if Velux want to argue each project should be considered in its own context. But it reads like a long winded argument in favour of double glazing & falls into the trap of considering CO2 & energy in isolation. There is no mention of comfort, health or mould risks associated with lower performance windows (or rooflights) in high performance building fabric.

    • Good points Elrond, thanks for the comment. Generating discussion and debate is what it’s all about!

  • As professional glaziers we always recommend high performing double glazing to our customers and I agree that it is the “more cost effective solution” and any feedback we receive on it is always extremely positive.