Cradle to Grave: The comparison of window life cycles
ed version of a VELFAC RIBA CPD article – to read the full article visit RIBAcpd.com
Windows play a significant role in buildings and are available in a wide range of designs and frame materials. This article looks at uPVC, aluminium, timber and composite aluminium/timber frames in terms of environmental impact, energy consumption during manufacture, and cradle to grave lifecycle. It also considers the expected durability, maintenance requirements, and economic benefits of each frame design.
- uPVC manufacture requires fossil fuels and chlorine gas; uPVC can withstand repeated recycling but this process can be complex
- Aluminium production generates dangerous pollutants; aluminium can be recycled repeatedly
- Timber is defined as renewable, although environmental concerns have increased the use of timber from sustainable forests
- Aluminium / timber – the energy used to produce the aluminium sash is offset by the low energy timber frame.
- uPVC and aluminium frames are generally low maintenance, although aluminium can corrode in exposed locations so is usually coated or anodised
- Timber frames require more maintenance than other materials
- Aluminium / timber frames are very low maintenance as the coated aluminium sash protects the timber frame.
The Institute for Building and Urban Design at Heriot Watt University1 gives the expected service life for windows as:
- uPVC - 25-35 years
- Timber - 56-65 years
- Modified timber - 68-80 years
- Aluminium / timber - 71-83 years
Note: The expected lifespan of aluminium windows is expected to be the same as (if not better than) aluminium / timber windows.
- uPVC windows have good thermal resistance, with U-values between 1.5 W/m2k and 0.8W/m2k
- Aluminium windows have low thermal resistance, unless provided with thermal breaks, with U-values between 1.9W/m2k and 1.0W/m2k
- Timber windows are the most energy efficient, achieving U-values between 1.2W/m2k and 0.7W/m2k
- Aluminium / timber windows are similar to timber windows, achieving U-values between 1.6W/m2k and 0.8W/m2k.
Manufacturing energy consumption
Research by Napier University, Edinburgh2, shows that the amount of energy required to manufacture a standard 1.2m x 1.2m window results in the following embodied energy measures:
- uPVC - 2980MJ
- Aluminium - 6GJ
- Timber windows - 995MJ
- Aluminium / timber - 1460MJ
Weathering and environmental impacts
Napier University3 also studied the comparative performance of frame materials when exposed to different environmental impacts:
- PVC samples were unaffected by humid conditions but deteriorated significantly during temperature/humidity and UV testing (the latter resulting in severe discoloration)
- Uncoated aluminium exhibited corrosive effects under humid and high temperature conditions; powder coated and anodised samples were unaffected
- Small timber samples exhibited warping and crevice-opening when exposed to moisture and temperature but complete timber window units with proper surface treatments were not affected in the same way; UV testing resulted in little discoloration
- Aluminium / timber samples did not deteriorate under any testing conditions; coated aluminium did not corrode and protected the timber underneath.
- An emphasis on sustainable design is increasing; in 2009, The International Energy Agency4 noted that the world’s energy demands had doubled in the past 40 years, with increased use of fossil fuels having a severe impact on climate.
- In 2016, the UK domestic sector accounted for 29% of final energy consumption; 37 million tonnes of carbon are produced UK-wide annually, with buildings accounting for over 40% of UK carbon dioxide emissions.
- As 250,000 new homes are needed every year, reducing emissions is now vital - improved energy efficiency is considered the cheapest, cleanest and safest way to do this.
- Window systems should embody sustainable design and manufacture in order to limit environmental impact and extend product lifecycles.
- By understanding these issues, specifiers and contractors can make informed window specification decisions, balanced against project specific requirements such as building location, acoustics, or M & E.
- Dr G.F. Menzies. “LIFECYCLE ASSESSMENT OF TIMBER, MODIFIED TIMBER AND ALUMINIUM-CLAD TIMBER WINDOWS”, Institute for Building and Urban Design, Heriot Watt University, Edinburgh. Page 34.
- M. Asif BSc MSc, A. Davidson BSc and T.Muneer PhD DSc CEng MlmechE FICBSE. “LIFE CYCLE OF WINDOW MATERIALS - A COMPARATIVE ASSESSMENT” Napier University, Edinburgh. Page 9.
- Ibid. Pages 10-13
- IPCC 2007