Type of seal Three types of insulated glass unit are in general use in the UK:
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.
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.
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 –40°C. If the edge seal continues to let water vapour through, the desiccant will become saturated and the unit will fail.
The Housing Association Property Mutual’s Component Life Manual, written by Construction Audit, the technical audit arm of Building 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 £175, or on CD-ROM, on 01264-332 424.
The BPG Building Fabric Component Life Manual and BPG Building 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: Buildings - 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.