A-Z of Self Build Guides A Guide To Triple Glazing

Date Published:

28/09/2009

Two layers of glass not enough for you? Trip the light fantastic with eco-friendly, super-efficient triple glazing.

By Caroline Rodrigues

The Basics

Letting in light was once the sole function of a window but no longer. Glazing is now expected to contribute towards the thermal properties of our homes and triple glazing outperforms the rest. Originally devised for the harsh winters of Canada and Scandinavian countries, triple glazing could reduce the heat loss through the windows by a third more than efficient double glazing, and the sound insulation is improved too. By 2016 new homes are intended to be zero carbon, so what are you waiting for?

Why Choose Triple Glazing?

The main benefit is in thermal insulation. As the house will stay warmer, less energy will be used in heating it – good for the environment and for your pocket. Juliet Wood of Jones Wood Architects, specialists in projects with low environmental impact, used triple glazing in her own new build home (see the large picture at the top, taken by Etienne Clément). ‘We were restricted on window sizes at the rear of our building because it overlooks a school, so designed the house with a huge, opening roof light above the living room and mezzanine bedroom area. The roof light was made in the UK and framed in steel for structural strength. Triple glazing was the best option, allowing us maximum size without excessive heat loss. Another benefit is that the sound insulation is much improved.’

Where Can I Use Triple Glazing?

Use it for windows, doors, roof lights or a wall of glass. Triple glazed units weigh more than double glazing. According to Juliet Wood, this isn’t a problem in vertical glazing, such as windows, where the walls and lintels can easily carry the weight. ‘If you’re including large patio doors, or sliding, folding doors, the system provider will adjust the frame size for the extra weight of the glazing. If you’re considering a large roof light, your structural engineer will size the joists and glazing bars accordingly.’

How Does Triple Glazing Work?

It’s the club sandwich of window design. While double glazing consists of two layers of glass with an air gap between them, triple glazing gives you three layers of glass and two air gaps. Just as with double glazing, the glass can be treated to make it more energy efficient, and the air gap can be filled with an inert gas such as argon, to help to reduce the heat loss.

What Are The Glazing Options?

Adding an extra layer of glass doesn’t make your home noticeably darker, according to Juliet Wood. ‘Each layer of glass does diminish the light transmittance through the window slightly, but the amount is negligible, especially if you’re using the glazing in a large area; the trade off is that because the thermal performance improves, you can have a bigger surface of glass.’

To improve the energy values, low-emissivity metallic coatings can be added. These reflect back some of the heat back into the house which would otherwise be lost through the glazing. As the coatings are soft - they can be made of gold – they’re put on the inner surfaces to prevent them being scratched. The glass can also be toughened, as it is for windscreens, so that if broken it forms small pieces without sharp edges, or laminated, which if damaged may crack but should hold together.

What Goes In The Gap?

Air gaps between the panes improve the thermal efficiency, but gas works better. Inert gases such as argon, krypton or xenon are used. ‘Using gas improves the thermal performance but the filling may not last as long as the unit it’s in. Gas can leak through a flaw in the seal or migrate, though new seals are very efficient,’ suggests Juliet Wood.

Which Frame Is Best?

Bear in mind at the planning stage that a triple-glazed frame will be deeper than one for double glazing. The window frame must have a good U-value, which is a measure of the thermal properties of the material. Timber is the top choice, with better U-values than metal, even if the metal has a thermal break. Juliet Wood advises: ‘Many triple glazed units come from Scandinavia, which is geared to producing ready-made timber units to a high performance. If you want the most eco-friendly option, the first choice would be FSC approved durable temperate hardwood such as oak, which has a long life. Next would be FSC temperate softwood clad with recycled aluminium. These “composite” windows are made from timber and clad on the outside with aluminium and are low-maintenance. The third choice would be FSC softwood treated with a plant-based paint system. Metal windows are also available with triple glazing but PVCu windows are not considered an eco-friendly option.’