Double Glazing

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 aluminum, 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 is typically done on-site.

Glazing method
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 amendments plus those of the frame and sealant manufacturers and any recommendations featured in the Glass Guide 2000.
It is not the intention of this section to provide method statements on the various techniques of glazing, but to offer a general guide and an introduction to the methods of glazing currently available.
Diagrams are provided for illustration purposes only.

Glazing blocks
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, non-degradable and non-absorbent material and should be able to accommodate the thermal expansion of the frame and glass without imposing stress on the glass. Before glazing, check that the clearances are appropriate for the thickness and type of glass. If the glass is in direct contact with the framing materials, it could lead to breakage. Care should be taken to avoid blocking drain holes or slots in drained framing systems.
Further details are given in BS 6262: 1982 and BS 8000 part 7: 1990. A new European Standard prEN ISO 14439 currently in preparation will advise on the function, requirement, materials and dimensions of glazing blocks.