The pros

• The extra pane of glass provides another barrier against freezing winter weather, making your home warmer
• Triple glazing can also improve summer comfort by reducing over-heating.  The solar heat gain (g-value) for a well-specified triple-glazed window is about 20% less than double glazing.
• A triple-glazed unit can result in a 35% reduction in heat loss compared to double glazing. You may no longer need radiators under windows and the peak load of the heating system is reduced.
• Triple glazing can normally get the best Energy A+ rating
• Lower heating bill.
• The extra pane of glass provides additional strength and security, making it harder to break into. The middle pane is usually toughened.
• Sound insulation is marginally better compared to a standard glass double-glazed unit.

The Cons 

• Triple glazing is heavier and more expensive than double. The cost can be almost twice as much in some cases.   
• While there is a strong argument for installing triple glazing in countries where average winter temperatures are well below zero, such as Norway and Sweden, the UK has a warmer climate with an average winter temperature of 7 degrees Celsius. Double glazing is perfectly adequate.
• Triple glazing uses more materials and has higher fabrication energy cost/environmental impact. It increases CO2 emissions embodied in the windows by between 40-50%.
• Triple glazing with standard glass can stop as much noise getting out as in – turning a room into an echo chamber.
• Soundproofing windows use laminated glass as the glass is more flexible and able to absorb sound. It also needs a gap of at least 100mm between the internal and external panes.  The best sound insulation comes with secondary double glazing with acoustic glass.

The cost/benefit argument

Example

For a five-bed 250 m sq house, triple glazing (standard white PVCu windows and doors) could cost around £10,600 compared to £7,460 for double glazing – this is a difference of £3,140, according to Tim Pullen, an expert in energy efficiency in residential homes and author of ‘Simply Sustainable Homes.’ Estimated energy savings of 922kWh – saving around £50 per year if it’s supplied with a gas boiler. Over 20 years, this adds up to £1,000. The annual savings are small – and the payback period long (62 years).  

U values

U values measure how effective a material is as an insulator. Thermal performance is measured in terms of heat loss.  Traditional windows with just a single pane have a U value of more than 5. Modern double glazing has hugely improved performance. Building Regulations today insist all new windows should have a U value of at least 1.6 and 0.3 for walls.  Windows are the weak spot in the overall thermal efficiency of a building. The ultra-energy efficient PassivHaus standard require triple-glazed windows with a U value of no more than 0.8.  Windows have to meet the standard of insulation in the rest of the house (walls, floor and roof).  

Do triple glazed windows have better energy efficiency ratings?

The Windows Energy Ratings system follows a rainbow label similar to those found on fridges, freezers and washing machines with ratings between A+ (the best) to G (the worst). Building Regulations require all new windows to be at least C-rated. The rating is calculated taking into account the window’s overall thermal efficiency, air leakage and solar gain (how much heat from the sun passes through the window). The top A+ rating includes both double and triple glazed windows.

“Triple glazed windows have three sheets of glass but aren’t always better than double glazed windows. To choose the most energy efficient window look for the BFRC rating,” says the Energy Saving Trust. BFRC stands for British Fenestrations Ratings Council. 

And finally…

There are many building professionals who argue that triple glazing simply isn’t justified in a climate like the UK. It costs more and probably won’t make much difference to your heating bill.  While a double-glazed window is perfectly adequate, an extra pane retains a little bit more heat and will make your property feel warmer. Unless you are building your home to Passivhaus standards or living in the Scottish Highlands, you are probably better off choosing a modern, super-efficient double glazing which is A+ rated.

When considering double glazing, one option many people forget to take into consideration is curtains.  A curtain pulled across a window at night adds an extra layer of insulation and so further reduces heat loss. An even better solution is to have a roller blind and a curtain.  In summer, a roller blind is useful to keep the heat out but still let the light in.  In winter, the combination of a roller blind and a curtain helps to make insulation even more efficient.