Why triple glazing?

Green Building Store has been supplying triple glazing as standard on all its windows and doors since 2015. This blog explains why we advocate the use of triple glazing both in newbuild and retrofit projects.

An essential component of a low energy home

18. Submitted by Jo Mallows

As homes get more and more efficient the performance of individual components makes more of a difference. Low energy homes should be considered as ‘whole house’ systems, with all the individual components working together to improve the overall energy efficiency and just as importantly, the comfort of the buildings.

In terms of pure heat loss, windows and doors are very much a potential weak spot in the building fabric. So, building to the highest levels of energy efficiency we could expect the U value (thermal transmission) of the walls to be typically around 0.1 W/m2K whereas the very highest performing triple glazed windows are likely to be in the range of 0.65 – 0.8 W/m2K.  This means that heat loss through windows will be at least six times worse than through walls.  However, the picture becomes more complicated because windows also allow in free heat from the sun and it is the ‘energy balance’ of heat loss set against useful free heat from the sun when we want it that is critical to getting the best performance out of windows. 

Modelling work undertaken by the Passivhaus Institute in Germany has shown that in our climate when we improve the energy efficiency of the building, triple glazed windows give us the best overall energy balance and are therefore the most efficient option.  The energy balance of double glazing will be worse in most parts of the UK, so will undoubtedly undermine our efforts to achieve a truly effective low energy building. 

Of course, we also need to consider air leakage.  We have all experienced how cold and uncomfortable leaky old, or even new, windows and doors can be. So our triple glazed windows also need to have high levels of airtightness* to be really effective.

*We should be looking for windows certified to have achieved Class 4 airtightness to BS EN 12207

 

The physics of comfort

Although low energy homes save energy and of course reduce carbon dioxide emissions, for many people the real gain is in health and comfort.  We can save money, save energy, do the right thing for the environment and be healthier and more comfortable.  A house where there are no cold spots will mean that we are healthier and more comfortable because there should be no condensation or mould growth and no sense of cold radiating from any surface.  Human beings are particularly sensitive to this ‘cold radiance’ which affects our feeling of comfort and wellbeing.

With windows being the weak point in our building fabric they are of course also the largest cold surface in many buildings and the most significant area of cold radiance.  We will all have experienced this ‘cold radiance’ from single and double glazed windows and that all-too-common sensation of cold spots and draughts near windows.  This phenomenon explains the frequent location of radiators underneath windows to counter this effect.  Understanding comfort in buildings is a surprisingly complex subject and it is not just about cold radiance.  Cold surfaces also lead to downdraughts so that even if our windows are airtight we might still experience draughts at floor level from cold air dropping down the face of the window.  Cold draughts around our feet and legs also of course make us uncomfortable and the taller the glass surface the worse it will be.  So the large areas of glazing favoured by many building designers can potentially lead to increased discomfort.

So, how do we know what standards we need to achieve to eliminate these problems?  The Passivhaus Institut in Germany has brought together all of the research in this area and shown that we will generally be comfortable in a building if there is a difference between our ambient temperature and any surface of no more than about 4 degrees Kelvin (or Centigrade).  As we are generally comfortable at a temperature in the low 20s then we are looking for an inner temperature of around 17C or better even when it is very cold outside.  We won’t achieve this with double glazing unless we warm the inner surface by inefficiently placing a radiator underneath it.  We will achieve this with good triple glazing. 

Comfort and triple glazing

Triple glazed windows and doors are an essential component of good low energy homes and buildings such as those built to the Passivhaus standard.  As we have seen, they are good for the efficiency of the building giving us the optimum energy balance.  However, for many of us, the greatest value will come from the increase in comfort which triple glazing brings.   

 

 

Chris Herring Director Green Building StoreChris Herring, Director, Green Building Store

5th November 2020

5 responses to “Why triple glazing?”

  1. blank Tim Wilcockson says:

    Thank you for this Chris. I think you should also mention that triple glazing cuts down exterior noise especially if you live in a city or near a busy road. So aural comfort is also enhanced.

  2. blank Roz Roberts says:

    Hi Chris, This is helpful. But is triple glazing worth fitting in an old house that is too difficult/expensive to retrofit properly (ie tackling the thermal bridging areas really effectively etc). Is it that triple glazing is worth it when everything else is balanced, and a “low energy home” is not possible to achieve? (PS I’ve just moved from Golcar to a leaky old house near Brecon!)

  3. blank Chris Herring says:

    Hi Roz

    Good question which is very difficult to answer! In general if we are to meet our climate targets we need to upgrade our building stock very significantly and very rapidly to enable us as a country to ‘power down’ while ‘powering up’ on renewable zero carbon energy. This will of course also reduce levels of fuel poverty and reduce the level of deaths and illness attributable to cold, damp and unhealthy homes. Many of our homes will therefore be very significantly upgraded well before 2050 when the UK needs to be at net zero (leaving aside the complexities in that argument), even if this is not envisaged immediately. So for many buildings I would say that triple glazing makes them ‘2050 compliant’ now since we can expect good windows replaced now still to be in service in 30 years time. However if you have a building which patently cannot be upgraded then the case for triple glazing is less clear, although if it is that inefficient and difficult to upgrade there might be a question mark over its value and liveability in the future as heating costs increase/we rely more on electrification of heat in the 2030s and beyond . Leaving that bigger picture aside, you can say that for a modest uplift from double, you will have condensation free windows which are always warm on the inside (assuming you can heat the inside of this leaky old house). This will be particularly significant if you have any large areas of glazing such as large sliding doors etc which will be more comfortable to sit next to with triple glazing when it is cold outside. So, no simple answers…like many things in life!

  4. blank Chris Herring says:

    Hi Tim

    Yes you are right that typical triple glazing will have better acoustic performance than typical double. However the acoustic attenuation is largely a result of increased glass thickness, rather the fact that there are two cavities and three sheets of glass. It becomes more complex because if the sheets are all the same thickness there will be some acoustic resonance from 3 x 4 mm sheets of glass which can affect some sound frequencies and reduce the level of attenuation. Ideally for acoustic attenuation the sheets should be different thicknesses to reduce the resonance. Also worth bearing in mind is the installation: an airtight installation as in a Passivhaus will be much quieter because there are no leakage paths for air, or sound, increasing acoustic attenuation. Acoustics and windows is a huge subject in its own right, and I am no expert although we try our best to give clients the best technical advice we can.

  5. blank Callum Sword says:

    Timely search result as we embark on a (lucky for us) new build project. From “just settle for double glazing, our climate doesn’t justify triple glazing”, I’m now exploring the additional cost and any benefits – in particular how to square our preference for good ventilation (windows open much of the time) and draughts against a warm but potentially stuffy box! Hopefully a MHRV system will mitigate the ventilation / draught dilemma and allow us to enjoy a mild and fresh house interior.

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