Window frames: Materials and design
Energy
• The heat energy (operational energy) lost through a window frame during its lifetime is likely to be greater than the energy used to manufacturer it (embodied energy)
• It is important to select a frame material with the least thermal conductivity. Wood is the least conductive followed by PVC and metal.
Wood, durability and environmental impact
The choosing of wood, its treatment and maintenance are crucial in reducing a window frame's environmental impact:
• Specify FSC sourced timber
• Transport adds embodied energy. Try and source UK timber whenever possible.
• For both hard and softwoods ensure that the specification explicitly excludes the use of sapwood.
• Painting wood adds significantly to its environmental impact. Either specify a naturally durable species that doesn't need treating or select a treatment with low impact. If frames are untreated, ensure that the client understands that initial colouring will change.
• If the wood is to be treated/painted, ensure that this is done in the factory prior to site. Factory painted frames double the period before the need to repaint.
Design
• The design of the sections will have an effect on performance. Design to maximise rapid drainage, maintain dry glazing channels and locate weather seals away from wet areas.
• Keep window panes as large as practically possible. Even with wooden frames, the metal spacers between the glass panes act as cold bridges. Large panes have less perimeter length than lots of smaller panes.
And on site ...
• Be careful to avoid damage to frames on site. Ensure that they are not used as formwork in wall openings
Materials
Composites
The creeping up of energy standards in recent years has been met with a degree of ingenuity amongst the manufacturers of advanced window systems. Its long been understood that different frame materials possess different strengths and weaknesses. For example, metal windows are renowned for their durability but not for their energy efficiency, whereas for, say, softwood windows, the opposite is true. By combining the relative strengths of different materials in one product, very high levels of performance are being achieved.
Perhaps the most successful combination so far in the development of composites has been that of aluminium-clad softwood cores. Although relatively new on the scene aluminium-clad timber frames are expected to have lifetimes of in excess of 40 years. PVC by comparison is around 25 years.
FSC durable temperate hardwood (oak, larch, sweet chestnut)
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Down - cyclable as fuel |
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Low embodied energy |
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Timber sequesters carbon dioxide |
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Renewable resource |
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Low maintenance – doesn’t need painting or preservatives |
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Very durable |
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Good thermal performance |
Design life
30 years (2)
BRE Green Guide rating
Domestic; solvent borne gloss paint / water based stain; double-glazed: A+
Conductivity
Oak and similar: 0.16 W/mK (1)
FSC temperate softwood treated with plant based paint systems
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Down - cyclable as fuel |
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Low embodied energy |
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Timber sequesters carbon dioxide |
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Renewable resource |
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Moderately durable |
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Easy to repair |
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Good thermal performance |
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Moisture ingress can lead to warping and twisting |
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Maintenance is usually required (eg re-painting/staining on a 5 year cycle) |
Design life
25 years (2)
BRE Green Guide rating
Domestic;
Preservative pre-treated softwood window, double glazed, water based stain (TWAS): A+
Other solvent borne gloss paint and water based stains: A
Conductivity
Softwood: 0.12 - 0.14 W/mK (1)
Aluminium-clad FSC temperate softwood
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Wood is down-cyclable as fuel; aluminium is recyclable |
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Timber sequesters carbon dioxide and is a renewable resource |
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Requires almost no external maintenance |
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Low maintenance – doesn’t need painting or preservatives |
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Very durable |
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Good thermal performance |
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Embodied energy from Aluminium smelting |
Design life
30 - 50 years (3)
BRE Green Guide rating
Domestic; Powder coated aluminium clad softwood window, double glazed, solvent borne gloss paint / water based stain internally: D
Conductivity
Softwood: 0.12 - 0.14 W/mK (1)
Aluminium cladding: 45 W/mK (1)
Uncertified softwood treated with synthetic based paint systems
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Benefits of using temperate softwoods (see above) |
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Uncertified timber risks habitat destruction and pollution |
The additional effects of paint
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Derived from petrochemicals |
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High level of VOCs |
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High level of hazardous waste during manufacture |
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Toxic emissions during manufacture |
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Toxic during application |
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Non bio-degradable |
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Waste paint treated as chemical waste |
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Moisture ingress can lead to warping and twisting |
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Maintenance is usually required (eg re-painting/staining on a 5 year cycle) |
Design life
30 - 40 years (3)
BRE Green Guide rating
Domestic; Preservative pre-treated softwood window, doubled glazed, solvent borne gloss paint: A
Conductivity
Softwood: 0.12 - 0.14 W/mK (1)
Aluminium and coating
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Recylable and reusable (if carefully disassembled) |
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Most of the aluminium used will have recycled content to varying degrees |
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Durable |
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Very low maintenance |
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Very high embodied energy |
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Very high global warming potential |
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Non-renewable resource |
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Non thermally-broken window sections are poor thermal performers |
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The nature of some coatings can inhibit recycling – anodizing is preferable |
Design life
30 - 50 years (3)
BRE Green Guide rating
Domestic;
- Powder coated aluminium window (profile < 0.88 kg/m), double-glazed: B
- Powder coated aluminium window (profile < 1.08 kg/m), double-glazed: C
- Powder coated aluminium window (profile > 1.08 kg/m), double-glazed: D
- Powder coated aluminium window with softwood internal frame, double-glazed, solvent borne gloss paint internally: E
- Powder coated aluminium window with softwood internal frame, double-glazed, water based stain internally: E
- Powder coated aluminium window with softwood internal frame, double-glazed, water based stain internally: aluminium profile < 0.87 kg/m and timber profile < 2 kg/m: B
Conductivity
Aluminium cladding: 45 W/mK (1)
Steel and coating
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Recylable and reusable (if carefully disassembled) |
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Most of the steel used will have recycled content to varying degrees |
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Durable |
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Very low maintenance |
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High embodied energy |
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High global warming potential |
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Non-renewable resource |
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Poor thermal performance |
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If not coated, steel needs regular painting to prevent corrosion |
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The nature of some coatings can inhibit recycling |
Design life
30 - 40 years (3)
BRE Green Guide rating
Domestic; Powder coated galvanised hot rolled steel window, double glazed: B
Conductivity
45 W/mK (1)
PVC
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A small amount of recyclate is sometimes used in new frames, but most PVC, if at all recovered, is downcycled. |
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Moderately durable |
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Low maintenance – though the introduction of PVC paint might undermine the argument |
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Good thermal performance |
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Difficult to repair |
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Sections often require reinforcement |
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White PVC might show discoloration as it ages ('pinking') where lead is used in the stabiliser compound |
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Made from non-renewable petroleum resources |
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For many designers, the threat to the environment posed by the manufacture, use and disposal of PVC, renders it unacceptable. |
Design life
25 - 40 years (3)
BRE Green Guide rating
Domestic: PVC-U window with steel reinforcement, double glazed: A
Conductivity
0.16 W/mK (1)
References
1 CIBSE Guide A 1999
2 BRE
3 Building Life Plans
Downloads
• 'The environmental and economic benefits of specifying timber window frames' (World Wildlife Fund)
• 'Life Cycle Of Window Materials - A Comparative Assessment' (Napier University)
• 'Life Cycle Assessment of PVC and of principal competing materials' - (EC, 2004)
• 'Environmental Impacts of PVC Building Materials' (Thornton / Healthy Building Network, 2002 )
Window products on GreenSpec
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