June 2, 2015
Over the past 15 years glazing has significantly moved on from standard double glazing to special glass coatings that aid with acoustic reduction, safety, security and both solar gain and solar reduction.
One area that has seen recent development and a lot of publicity is that of triple glazing. Put simply, triple glazing is the incorporation of three panes of glass in a single unit. It is widely believed that by having an extra pane of glass, less heat is lost and the risk of noise pollution is significantly reduced, however this is not always the case as I will discuss.
In this blog we will explore the question of whether households should switch to triple glazing through exploring the benefits and possible disadvantages.
The U-value of triple glazing
The British Fenestration Rating Council (BFRC), the UK’s national system for rating energy efficient windows, defines window thermal transmittance (U-Value) as a measure of “how good a product is at preventing heat from escaping. The lower the U-value the better the product is at preventing heat escaping.”
Under Building Regulations, any new build property must have a U-value of no greater than 1.6. Currently, Building Regulations do monitor replacement windows. However the level to which the regulations are imposed does depend on the nature of the building, as those that are historic (listed) or in a conservation area do not have to meet a particular U-value but it is advised that these should improve, or at least match the U-value of the previously fitted window.
Also it is worth keeping in mind the difference between the U-value of the glass or glazing unit and the U-value of the entire window including the frame, as this can drastically alter the entire value good glazing can be let down by an inferior frame and the same is true vice versa.
Single glazed windows provide the worse U-value, with older units typically having a U-value greater than 5. Older double glazed units can have a U-value as poor as 3, whilst newer double glazed windows perform much better and comply with Building Regulations for new homes. For example, the George Barnsdale standard double glazing has an average U-value of 1.2.
Research has shown that triple glazed windows, with the correct configuration of panes and gases, can provide a U-value as low as 0.8. Because of the significant reduction of heat loss, triple glazing is widely used in those countries with colder climates, such as Sweden and Norway. In addition, low energy new builds that are designed under the Passivhaus standard, will incorporate triple glazed windows.
It is worth remembering that the window area is just one source of heat loss, the standard of glazing should match the standard of the insulation elsewhere in the house, so that the warm wrapping around the house performs consistently. If the rest of your property is poorly insulated the additional cost of triple glazing may not help reduce energy bills as heat will still escape from other parts of the property such as the walls.
The acoustic performance of triple glazing
The acoustic performance of any window is determined by glass mass and air performance (see our previous blog on the acoustic performance of timber windows for more info). Looking at glass mass specifically, the number of pieces of glass will affect acoustic performance, although varying the size of the air gaps and thickness of the panes will have a greater effect. However there, is no evidence that triple glazing performs any better than double glazing, as a lower spec triple glazed unit can be greatly out performed by a higher spec double glazed unit. This shows how important seeing performance data for the product you are investing in is.
For replacement windows, the width of the window is fixed and will limit options for gases. Argon is most cost effective but performs at larger widths (e.g. 4mm/16mm/4mm/16mm/4mm). Krypton performs well for narrower widths but comes at a price as Krypton is more expensive than Argon.
The triple glazing checklist
Research suggests that triple glazing reduces light transmission and increases the need for artificial lighting. It is estimated that triple glazing lets through 75% of sunlight, so whilst it does a great job of preventing heat loss, it actually can have an adverse effect on solar gain (i.e. the free energy from the sun that helps to heat up a property when the sun is shining). Due to the reduction of solar gain, triple glazing can actually affect the overall performance of a window unit this will be seen in the BFRC (British Fenestration Rating Council) rating given to the product.
As triple glazing is still in its infancy, there is much debate to be had regarding the difference it makes to a household’s heating bill. The PassivHaus Institute has undertaken extensive research into this – click here to view this information.
What is certain is that triple glazing is the most expensive option and it also adds additional weight to the window. Before triple glazing is specified you must be really sure that the benefits really do out-weight the cost and limitations. Below is a quick check-list that may help you weigh up the benefits:
- How good is your current glazing?
- How important is the cost or the return in your investment?
- Do you get cold spots?
- Is noise currently an issue?
- Can you actually fit triple glazing in your property?
- Do planning and conservation rules allow the change?
Thanks to Nick Welsh of Safe Gaurd for his help and advice in the writing of the blog.