The selection of a suitable and practicable glazing method depends on a variety of factors such as the size of the glass, the exposure and the type of framing material and system.
Glazing and fixing techniques must comply with the recommendations of BS 8000: 1990 Workmanship on building sites Part 7: Code of practice for glazing, BS 6262: 1982 Code of practice for glazing for buildings and those of the frame and sealant manufacturers.
It is our intention to offer a general guide and introduction to the methods of glazing currently available. Diagrams are provided for illustration purposes only.
Setting blocks, location blocks and, where necessary, distance pieces must be used to provide support and adequate clearance between the glass and the frame.
The number and spacing of the blocks relates to the size of glass pane, framing system and conditions of use. They should be of a resilient, nondegradable and non-absorbent material and should be able to accommodate the thermal expansion of the frame and glass without imposing stress on the glass.
To maximise the life of a sealed double-glazed unit, the preferred frame and glazing method must not allow moisture to collect at the edge of the unit seal. Prolonged contact with water will encourage the loss of adhesion between the sealant and glass. This can result in moisture vapour penetrating through the seal into the air cavity leading to the premature failure of the unit and formation of condensation inside the cavity.
Water can get into the rebate area from a number of sources including rainwater, window cleaning operations and condensation forming inside the framing section and also on the internal room-side surface of the glass.
The glazing system should either ensure that water cannot penetrate to the edge-seal of the unit or that if it does, then it is quickly drained. There are therefore two basic glazing systems: drained and non-drained.
1. Non-Drained systems
Referred to as “solid” or “fully bedded” framing systems. The edge-seal of the unit is protected from moisture by surrounding it in glazing compound and sealant. It is important to ensure that there are not any gaps present in the sealant, which tend to trap any moisture that penetrates against the edge-seal. They are typically and traditionally used in timber frames but sometimes also in steel frames.
2. Drained Systems
In drained systems it is acceptable for some moisture to penetrate the seal but for it not to be allowed to collect in the rebate.
The use of sloping rebate platform and / or drainage holes directs the moisture away from the edge-seal of the double-glazed unit. Some systems also have holes in the framing section to facilitate the movement of air around the perimeter of the unit, thereby encouraging rapid drying.
These are known as drained and ventilated systems. They can be applied to most types of frame including aluminium, PVC-U, steel and timber.
Fully drained or drained and ventilated systems are preferred. Dry glazing with pre-formed gaskets or wet glazing with compounds or sealants may be used, provided they are fully compatible with the edge-seal, glass and framing material.
No corrosive material can be used in the glazing system or in the maintenance of the system.
Sealant compounds used must be fully compatible with the glass, the unit seal and the framing material.
For example, acid based silicone compounds must not be used against the unit seal. All sealants must be applied and used according to the manufacturer’s instructions. Insertion of gaskets on site is usually carried out under pressure and it is important that this pressure does not exceed 5 kgf per linear centimetre.
The glazing system must be designed to provide adequate clearance around the unit in order to prevent glass-to-frame contact and to provide sufficient edge cover to retain the unit in position under the design loads.
The spacer bar and edge seal of a stepped-edge double-glazed unit exposed to sunlight (i.e. visible through the upper pane) must be protected from UV radiation by foil tape or a similar shading device if not factory-fitted. It must also completely cover the edge seal of the unit to protect it from exposure to solar radiation.
Unless gasket glazing to drained and ventilated systems, particular care must be taken to protect the PVB interlayer against water ingress.
Prolonged contact with water can cause the interlayer to swell and ultimately lead to delamination. The interlayer should also be protected from contact with compounds and products containing mineral or vegetable oils and unsuitable glazing compounds such as acid curing silicones. Laminated glass should be supported on all four edges and be free from holes, notches and cut-outs to achieve its optimum performance and penetration resistance. All work on laminated glass should be carried out under workshop conditions. It should not be attempted on site. In order to ensure maximum security, a security tape or adhesive (silicone) glazing method is recommended.