There was a time when all glazing had to do was keep the rain out and let some light in, but now it has to do more. Windows are part of the fabric of the building and the job they do in terms of thermal efficiency is becoming increasingly important. But, thermal insulation is only one of the functions of what the cognoscenti call ‘fenestration’.

Light Penetration

The principal function of fenestration is to allow light in. There are all sorts of obvious reasons as to why this is a good idea. Less obvious is the effect of inappropriate glazing — be that in terms of the area of glazing and/or its positioning. Put simply, it’s important to get the amount of natural light entering any part of the house right.

A pane of the best-quality glass will absorb about 10 percent of the useful light passing through it. Therefore, the more panes there are, the less light gets through. Added to this is the fact that the poorer the quality of the glass, the more light it is likely to absorb or reflect. In addition, we now have glass coatings designed to prevent undue heat and UV light transfer.

Sound Insulation

Contrary to popular belief, triple glazing does not necessarily reduce noise penetration more than double glazing.

Noise penetration is a factor of two things:

  • the thickness of the glass
  • the gap between the panes

Window panes of equal thickness will try to block noise of equal frequency, and noise travels more readily through a solid than it does through a gas. A double-glazed window with one 4mm-thick pane and one 6mm-thick pane separated by a 30mm gap (therefore 40mm overall) will block more noise than a triple-glazed window with three 4mm panes each separated by a 14mm gap (also 40mm thick overall).

Ventilation

There is an argument that windows should be able to open, but as we move ever closer to PassivHaus and mechanically ventilated houses, the opening window may fall into history.

The thermal performance of a window is a factor of two things:

  • the U value of the glazing and the frame
  • its airtightness

If the window does not open, then we can be fairly sure it is reasonably airtight. If it is an opening casement, then it is the quality and durability of the air seals that will determine the window’s airtightness and, ultimately, its effectiveness over time to provide an adequate thermal barrier. Obviously, this is not affected by the window being double or triple glazed.

Thermal Insulation

A U value is a measurement of the rate at which heat energy can pass through a material. So, the lower the figure the better, and zero would be great.

Good double glazing will achieve 1.2W/m², while a standard window will more likely be 1.6 to 1.8W/m². A good triple-glazed window will typically be 0.8W/m².

Take this as an example: a house of 200m² built to a reasonably good standard will use about 10,000kWh for space heating each year. Windows with a U value of 1.6W/m² will account for about 2,700kWh of that (heat lost), assuming a typical number of windows and a couple of patio doors. If, however, we upgrade to triple-glazed windows with a U value of 0.8W/m², we will reduce the heat loss attributable to the windows to (unsurprisingly) 1,350kWh per year.

If we assume that a gas boiler is keeping the house warm, then that 1,350kWh will be worth about £95 per year. If we assume that a reasonable quality set of double-glazed windows and doors will cost £40,000, then the extra cost of triple glazing will be £8,000. With an annual saving of £95, this expense will take 84 years to recover.

It is accepted that these calculations make a lot of assumptions and are more or less inaccurate, but they still put a scale on the energy cost benefits of triple glazing — revealing that the payback will be measured in decades, rather than years.

Cold Spots

One thing that triple glazing is good at, however, is eliminating cold spots in homes. Windows often have a cooler feel than the surrounding wall and the impact of that encourages people to sit further away from the window — effectively reducing the usable space in the room.

Conclusion

Building Regulations require the U value of a wall to be no more than 0.3W/m² and clearly, windows have some way to go. But, until the gulf stream dies and the jet stream finally runs amok, the UK has a temperate maritime climate — warm and wet. Although we are on a similar latitude to Canada, parts of Scandinavia and Northern Germany, we do not experience the extremes of temperatures that they have to deal with.

That means the benefits of triple glazing – ecological as well as financial – are not so obvious here. On the ecological side, if we factor in the extra CO? emitted to produce that third pane of glass, then the argument for triple glazing becomes even more shaky.

New Technologies

The demand for improved thermal and acoustic performance in glazing has led manufacturers to invest heavily in research and development.

Using a special Polyvinyl Butyral interlayer, Pilkington’s Optiphon™ is a high-quality acoustic laminated glass that offers excellent noise reduction without compromising on light transmittance or impact performance. It is ideal for properties in a high-noise environment.

Uniform Architectural Ltd has launched uni_one Termoscudo: its most energy-efficient window system so far. Termoscudo benefits from sleek aesthetic design details with additional technology that greatly improves its thermal insulation capabilities to certify it to PassivHaus standards by achieving a U value of just 0.79 W/m².

External condensation occurs when the external surface temperature of the glass drops below the dew point of the surrounding. Pilkington’s Anti-Condensation Glass is designed to delay the onset of condensation (on both double and triple glazing) by keeping the external surface warmer. This is achieved through a coating which reflects heat from the building back inside.

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