Window frame Materials & Design

FSC durable temperate hardwood (oak, larch, sweet chestnut)
FSC temperate softwood treated with plant based paint systems
Aluminium-clad FSC temperate softwood
Uncertified softwood treated with synthetic based paint systems
Aluminium and coating
Steel and coating
PVC

 

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)

oak

Pro

Down - cyclable as fuel

Pro

Low embodied energy

Pro

Timber sequesters carbon dioxide

Pro

Renewable resource

Pro

Low maintenance – doesn’t need painting or preservatives

Pro

Very durable

Pro

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

painted wood

Pro

Down - cyclable as fuel

Pro

Low embodied energy

Pro

Timber sequesters carbon dioxide

Pro

Renewable resource

Pro

Moderately durable

Pro

Easy to repair

Pro

Good thermal performance

Con

Moisture ingress can lead to warping and twisting

Con

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

aluminium clad wood frame

Pro

Wood is down-cyclable as fuel; aluminium is recyclable

Pro

Timber sequesters carbon dioxide and is a renewable resource

Pro

Requires almost no external maintenance

Pro

Low maintenance – doesn’t need painting or preservatives

Pro

Very durable

Pro

Good thermal performance

Con

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

Pro

Benefits of using temperate softwoods (see above)

Con

Uncertified timber risks habitat destruction and pollution

The additional effects of paint

Con

Derived from petrochemicals

Con

High level of VOCs

Con

High level of hazardous waste during manufacture

Con

Toxic emissions during manufacture

Con

Toxic during application

Con

Non bio-degradable

Con

Waste paint treated as chemical waste

Con

Moisture ingress can lead to warping and twisting

Con

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)
 

Read more: Paint: Health & the Environment

 

Aluminium and coating

anodised aluminium

Pro

Recylable and reusable (if carefully disassembled)

Pro

Most of the aluminium used will have recycled content to varying degrees

Pro

Durable

Pro

Very low maintenance

Con

Very high embodied energy

Con

Very high global warming potential

Con

Non-renewable resource

Con

Non thermally-broken window sections are poor thermal performers

Con

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)

Read more:  Aluminium production & environmental impact


   

Steel and coating

powder-coated steel

Pro

Recylable and reusable (if carefully disassembled)

Pro

Most of the steel used will have recycled content to varying degrees

Pro

Durable

Pro

Very low maintenance

Con

High embodied energy

Con

High global warming potential

Con

Non-renewable resource

Con

Poor thermal performance

Con

If not coated, steel needs regular painting to prevent corrosion

Con

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)


Read more:  Steel manufacturing & environmental impact

 

PVC

pvc window

Pro

A small amount of recyclate is sometimes used in new frames, but most PVC, if at all recovered, is downcycled.

Pro

Moderately durable

Pro

Low maintenance – though the introduction of PVC paint might undermine the argument

Pro

Good thermal performance

Con

Difficult to repair

Con

Sections often require reinforcement

Con

White PVC might show discoloration as it ages ('pinking') where lead is used in the stabiliser compound

Con

Made from non-renewable petroleum resources

Con

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)

Read more: Polyvinyl chloride (PVC)

 

References


1 CIBSE Guide A 1999
2 BRE
3 Building Life Plans

 

Window products on GreenSpec
 

 



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