Such a simple question, yet more complicated answer than you may think.
Think back to razor blades. Once they were cut throat razor blades like a knife, then the small blade was introduced, smaller lighter, less metal meant it could be cheaper and two edges meant double the shaves.
Then we had the slogans for twin bladed razors, “the first cuts close, the second closer still” so fast forward to today’s triple, quadruple even 5 edge technology razors, are they truly able to shave 5 times better than 1 blade, or is it just hype?
Well weird it may sound, but glazing is similar and all depends upon your view point about solar gain.
Solar gain is the free heat that comes into the room, just like standing outside when the suns rays penetrate through the glass, some of the heat comes through to warm the surfaces near the window.
Too much solar gain can be a massive problem requiring air conditioning units to cool down the excess, however some solar gain should be welcomed, because once it is trapped inside the room it can offset the need for heating.
So why is solar gain an issue?
Well there are two or three ways to measure energy loss through windows U values SAP testing and WER’s (window energy ratings).
The first, U values, have been around for many years and the same formulae are used regardless of the material, ie the U value is the units of heat lost through a window, door, roof, wall etc every hour, for every 1 degree difference in temperature.
It is easy to understand and measures the amount of heat (energy) that passes through the material being tested, the lower the number the better it insulates. However this is just about saving energy, no allowance is made for any free solar gain, just the amount that goes through the material.
If you think about it, a room with no windows but lots of insulation have a very low U value 0.2, way lower than the average window of 1.6, however it will be dark and need electric lighting, consuming energy, and the free solar gain would not penetrate the solid wall, so some glazing is needed to allow light in, negating the need for electric lights.
SAP is a method to calculate the absolute energy efficiency used in new build construction. Every size of window will have its own value, derived from the orientation within the building, the direction of the sun and if its north, south east or west facing. It will include detailed calculations combining both energy loss (U values) and solar gain. However it is very complex, time consuming to do and therefore costly, so a third method was introduced, WER’s
Window Energy Ratings, calculate the energy lost through a window (not just the glass) and allows for solar gain, and is based upon the averages in the UK, so technically no WER is 100% accurate for the precise location; they average the findings from across the UK. They are a good comparison tool to compare styles so a casement or sash window will have different values and you can see which is more efficient.
So how does this affect double or triple glazing?
Well, triple will be more efficient if you are just comparing U values, as three sheets of glass will prevent more energy loss than just two sheets of glass. However if you agree that collecting free solar energy makes sense, then just as a second sheet cuts down the solar gain of single glazing, so too the third sheet still further. There is virtually no solar gain on triple glazing.
In tests we have seen evidence that a triple glazed sash window got down to 1.4 U value (far lower than the 1.8 then regulation) however the WER fell from an A rated product to a D rated product, because of the lack of solar gain.
If triple is better than double, then why stop there, why not quadruple glazing (it does exist) or even more? Well whilst it would be very energy efficient, it would always be dark inside and the costs of electric would soon offset any energy saving costs!
Like razors the marketing people would have you believe the more the merrier.