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The truth about triple glazing

It’s tempting to think that triple glazing must automatically be better than double glazing. It stands to reason, doesn’t it? More means better. If two panes of glass are better than one, three must be even better…  But the truth is not quite so simple.

So what is the difference?

Whilst it is true that triple glazing is generally more efficient in terms of heat and noise insulation, it is also true that it is less good at capturing solar energy. What is more, it is also more expensive and less environmentally friendly to produce. More materials and more complex manufacturing mean that any savings in energy bills will be offset by significantly greater up-front costs and environmental impacts.

The relative merits of double and triple glazing must be seen in terms of the trade-off between those initial costs and the ongoing savings that they can achieve long term.

How do we make that calculation?

From a homeowner’s perspective we need to understand how energy efficient each alternative is; we need to compare how much energy transfer is involved. However, this is not simply a question of energy loss - energy gain is also part of the story.

The nitty gritty of how this works is set out below, but if you would prefer not delve into the detail, the bottom line is that, in the UK at least, triple glazing ordinarily costs more than it saves when compared with good quality, energy efficient double glazed units.

The nitty gritty

The most obvious difference between double and triple glazing is the different level of thermal insulation that each provides: triple glazed windows are, like-for-like, marginally more efficient in this respect. But thermal insulation is not the only way that glazing affects energy transfer into and out of our homes.

The rainbow A to G Window Energy Rating (WER) used by the British Fenestration Research Council is based on an assessment of a window unit’s thermal transmission (expressed as a U-value) but it also measures how much leakage it allows (expressed as an L-value) and how well it captures energy from the sun (measured as a G-value and frequently described in terms of solar gain or WindowSolar Factor). All three measures are combined to establish the rating for any given unit. How these calculations are made is set out in a BFRC leaflet which you can access here. Since modern designs have effectively made leaks a thing of the past, the key aspects of the WER are the U-value and the Window Solar Factor.

What is a U-Value?

U is the symbol used in physics to represent internal energy. This is why the thermal qualities of all building materials are measured in U-values. The lower the U-value, the less heat is lost. For example, whilst a standard uninsulated cavity wall has a U-value of around 1.6, a regular single paned window has a U-value of around 5.8, making it an obvious point of energy loss. For this reason, Buildings Regulations now require all new homes to incorporate double glazing with a minimum U-value of 1.6.

In technical terms, a U-value is calculated as the rate of transfer of heat (measured in watts) through one square metre of a structure divided by the difference in temperature on either side of the structure. It is expressed in watts per metres squared kelvin, or W/m²K.

SafeStyle’s Eco Diamond units have been given an A certificate by BFRC. They are rated at a U-value of 1.3. Triple glazed windows typically deliver a U-value of around 0.8.

Even allowing for fluctuating fuel prices there is, sadly, no easy way to directly translate those ratings into cash savings, although the Energy Saving Trust website does offer some useful pointers. More straightforwardly perhaps, the common-sense way to get to grips with this is to ask simply whether it is worth paying extra for triple glazing if heat is going to escape more quickly through the far larger area of the walls, roof and floor of your home anyway?

What is the Window Solar Factor?

The other aspect of the comparison between double and triple glazing to take into account is the solar energy gain that modern glazing technology allows. Once this part of the all-round energy equation is accounted for the comparison looks much less lop-sided.

In the simplest terms, two panes of glass allow for more sunlight and solar energy to pass through them than three do, warming the interior more - and more quickly - and so contributing to lower fuel bills.  This solar gain can be boosted by the type of glass involved. This aspect of a window’s energy performance - its solar factor - is measured on a scale between 0 and 1.0 where a higher figure means a greater heat gain.

For example, Safestyle’s Eco Diamond Window uses two different types of glass in its construction, an inner pane of AGC Planibel A Low Emissivity Glass (.71 solar factor ) which is designed to retain energy, and an outer pane of Pilkington’s Optiwhite Low Iron Glass (.91 solar factor)  which allows 91% of the sun’s energy to pass through it.  The two work together to let as much energy in and as little energy out as possible.

In reality, the movement of the sun combined with the orientation and different shading characteristics of each individual window make it difficult to establish an entirely accurate and consistent measure of a product’s solar gain. Measures used by manufacturers are therefore purely for comparative purposes. Click here for a more detailed treatment of how solar heat gain is calculated.

No matter how efficient the glass in a window unit may be, each additional pane of glass inevitably means a reduction in the amount of solar energy and light reaching the interior. So whilst triple glazing scores well for thermal transfer this is offset by the reduced solar gain that it provides. And if triple glazing means you have to turn on the lights more often there is, inevitably, a further price to pay.

And then there’s the weather

In a climate like the UK’s which is relatively mild - at least compared with places like northern Norway and Sweden where triple glazing is more common  - this balance between solar gain and heat transfer represents a powerful argument for choosing a double glazed solution. This is even more so if the window is regularly exposed to direct sunlight, i.e. if it faces due south.

There are undeniably cases where triple glazing represents the best option. But unless you are intending to fit particularly large windows into a north facing building that is already highly energy efficient, or you have an acute problem with noise, the truth is that triple glazing is likely to be an expensively over-engineered option. The difference between an A rated, energy efficient double glazed unit and a triple glazed alternative is far less than the difference that a simple blind or curtain can make at a fraction of the cost.






Safestyle Windows Find out details below

Locking Points

The Eco Diamond incorporates hooks & mushroom cams at each locking point tested to our highest standards to give you ultimate peace of mind!

Toughened Glass

A-rated energy saving sealed units with toughened glass where required

Quality Handles

Precision manufactured from a high performance zinc alloy. They're tested to 50,000 cycles, that's more than twenty times the standard requirement!

Q-Lon Weather Seal

The Q-Lon design ensures maximum weather seal is obtained meaning no leaks and no draughts

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