How to … Get the best out of double glazing

Double and triple glazing is here to stay – recent changes to the energy conservation requirements of the Building Regulations have seen to that. To meet the thermal insulation standards, all new developments and most window replacement work will need double glazing as a minimum specification.

You may think this is no bad thing, particularly for developments in noisy urban settings and given the state of the British climate, but you may want to consider the fact that double or triple glazing will not last as long as single glazing. Not withstanding the odd cricket ball or football, single glazing will normally last the life of the building whereas double glazing, or insulated glass units, as they are known in the industry, commonly fail in less than five years, although they can last in excess of 20 years if specified well.

The problem
Insulating glass units generally fail because too much moisture penetrates the space between the two panes of glass, causing misting of the glass.

There are many reasons why water vapour gets into the insulating glass unit.

A typical pattern is that moisture gets past the glazing sealant or gasket and lies next to the edge seal of the unit. The moisture then gets drawn into the insulating glass unit through the edge seals, because they are permeable, or around an edge seal that has separated from the glass.

The failure of insulating glass units due to misting has been a particularly common problem in timber frame buildings. However, that does not need to be the case.

What to specify
Not all insulating glass units are equal. For instance, dual-seal units generally perform better than single-seal units.

Ideally, insulating glass units to the standard BS 5713:1979, should be specified. For even higher-quality units, specify those kitemarked to BS 5713.

You can check units for a British Standard number and kitemark symbol, as they are stamped on the spacer bar that separates the panes of glass.

You should also be aware that insulating glass units will soon be produced to a more stringent European standard, BS EN 1279.

To minimise the risk of moisture coming into contact with the edge of the insulating glass unit, specify a drained and vented glazing system. This will involve a more substantial window frame; aluminium and plastic window frames have drained and vented glazing systems as standard. Such systems are increasingly being used for timber window frames rather than fully bedding the units in sealants.

What to look out for on site
Many of the problems associated with insulating glass units are caused during installation on site, especially where a fully bedded glazing system is used – this is a system where the edge of the unit is completely surrounded with sealant.

It is essential to use setting and location blocks to hold the insulating glass unit firmly in place and maintain clearances (3mm all round for units up to 2m long, though 6mm should be used at the bottom if drained and ventilated).

The blocks must be at least 25mm long and the same thickness as the unit. If used on sloping rebate platforms, they should be wedge-shaped. They should be inert and durable and are usually made of plastic – bits of cardboard will not do.

Equally important are distance pieces, provided between the face of the glass and the bead or rebate upstand (50mm from corners and at 300mm centres). Loadbearing glazing tapes may also be used.

Sealants should be applied evenly and should fill the entire space between the rebate and unit; there should be no voids, which is easier said than done.

One way to avoid site installation issues is to specify factory-glazed windows, but consider the handling hazards associated with large and heavy windows.

Be prepared
These comments are merely guidance to good practice. With careful consideration of materials and design solutions, many problems that could be faced down the line can be avoided from the onset. Building LifePlans, a sister company of Building Performance Group, specialises in risk management and latent defects insurance, and offers consultancy services based on robust component durability data.