The third in 精东影视鈥檚 series of whole-life costs for materials focuses on double-glazed units. It is compiled by 精东影视 Performance Group to help specifiers.
Factors affecting durability The rate at which water vapour leaks past the edge seal and hence the durability of the unit depends on several factors:

Type of seal Three types of insulated glass unit are in general use in the UK:

  • Single seal This comprises an aluminium spacer bar filled with desiccant with a perimeter seal material such as polysulphide mastic that has a high resistance to moisture vapour. Single-seal units are generally less durable than other types.

  • Dual seal This is similar to the single-seal unit but has two seals of different materials with different properties. The primary seal material has a high resistance to water vapour but less good mechanical properties such as adhesion and flexibility. The second seal material has a lower water vapour resistance but better mechanical properties. Dual-seal units generally have better durability characteristics than single-seal units.

  • Extruded butyl tape This is relatively new in the UK and consists of a solid butyl compound section that has good adhesion to glass and a high resistance to water vapour. The compound also contains a desiccant and an integral corrugated aluminium strip that gives mechanical strength.

    Butyl tape has much lower thermal conductivity than aluminium spacer bars. This means that durability is enhanced because there is lower risk of sealant edge stress and internal condensation. Initial indications seem to show that the durability of units made this way is equal to or greater than that of dual-seal units.

    Presence of edge tape

    Some units have their edges sealed with an aluminium foil tape. The tape facilitates handling and protects the seal material from ultraviolet light. Tapes can accelerate deterioration of seal materials by forming a channel at the bottom edge of the unit in which water can collect.

    Glazing systems

    The glazing system used should fully support the unit and prevent water collecting and coming into contact with the bottom edge. This is best achieved by using a drained and ventilated system that can be used with all types of frame material, including timber. This allows moisture to drain away before it has a chance to affect the edge seal.

    The other glazing system that is used extensively in the UK with both timber and steel frames is a fully bedded system. This relies on differing vapour permeability characteristics of different types of sealant. However, even careful application of the sealants can result in air pockets that fill with water, leading to deterioration of the edge seal and early failure of the unit.

    Exposure to ultraviolet light

    Exposure to ultraviolet light can lead to the glazing gaskets becoming brittle and the failure of some edge sealants. Glazing rebates should be deep enough to protect the edge seal from ultraviolet light. The 18 mm depth recommended by the British Standard and the Glass and Glazing Federation is often not offered as standard by some frame manufacturers.

    Durability tips

  • The performance of drained and ventilated glazing systems should be checked regularly and drainage and ventilation paths kept clear of debris.

  • Units that are British Standards Institution Kitemarked should be specified. Surveys have indicated that these units have less than half the level of failure as units that are not Kitemarked.

    Types of failure

    Edge seal failure Failure of double glazing units is characterised by the formation of condensation on the inner surface of the glass at usual service temperatures. This is almost always caused by the edge seal letting in water vapour.

    The desiccant (typically silica gel crystals) in the spacer bar is there primarily to absorb the water vapour that will be present during manufacture at ambient levels of humidity and to reduce the dewpoint within the enclosed space to below usual operating temperatures. BS 5713 requires the dewpoint in newly manufactured units to be no higher than 鈥�40掳C. If the edge seal continues to let water vapour through, the desiccant will become saturated and the unit will fail.

    Further information

    The Housing Association Property Mutual鈥檚 Component Life Manual, written by Construction Audit, the technical audit arm of 精东影视 Performance Group, provides insured lifespan assessments for more than 500 building components. It is updated twice a year to reflect industry feedback and changes to standards and codes of practice. Published by E&FN Spon, it is available in loose-leaf format, price 拢175, or on CD-ROM, on 01264-332 424.

    The BPG 精东影视 Fabric Component Life Manual and BPG 精东影视 Service Component Life Manual will be available from E&FN Spon, in loose-leaf and CD-ROM formats later in 1999.

    BS Handbook HB 10141: 1997: 精东影视s - Service Life Planning: Part 1: General Principles is available from the British Standards Institution on 0181-996 9000.

    For further information, contact Gary Moss at BPG on 0171-240 8070.