Your questions: Should I get triple glazing?


GW asks: Can you tell me whether there is a huge difference warmth wise between double and triple uPVC glazing?

Ian Mckay of Lewes-based sustainability consultants Deeper Green responds:

Great question! As a general rule of thumb, the carbon versus cost benefit of upgrading from double to triple glazing is strongest when the glazing in question is a few decades old and is getting close to the end of its design-life anyway.

On the other hand, if you have five-year old PVC-U glazing with good quality double glazed units (say around 25mm deep) then I would find that hard to justify. The cavities of double glazing are filled with special gases like argon or krypton and this helps to reduce convected heat loss across the cavity.

Over time however the seals tend to go, the gases dissipate and you will get internal condensation. That is a clear giveaway that the unit has failed and is not working optimally. It is relatively cheap to just have the glazed unit itself replaced. However, if the double glazing is only, say, 12mm to 15mm in thickness, it is not going to be particularly high performing whatever state the unit is in.


Assessing efficiency

A way to assess the thermal efficiency of different types of glazing is to look at its U-value. The U-value is the rate of transfer of heat through a structure. The units of measurement are W/m²K. The better-insulated a structure is, the lower the U-value will be.

I have compiled some reference U-values for different types of glazing:

Type of glazing U-value range
Single 4.8 to 5.8 W/(m²K).
Double 1.2 to 3.7 W/(m²K)
Triple below: 1 W/(m²K)


The above ranges of thermal efficiency will be affected by how deep the cavity is, what type of gas is used in the cavity, what special thermal reflective coatings have been applied to the glass and the thermal efficiency of the frame itself.

From these values you can see that it is really worthwhile swapping single glazing for best quality double or preferably triple glazing as it is up to four or five times more thermally efficient. However, if you already have pretty decent double glazing of, say, 1.5 U-value, then you are only going to get a fractional improvement with triple glazing.

In short, if you are in the market for new glazing, consider going straight for triple glazing.


Installing glazing

When it comes to the fitting of units, ask the installer to use good air tightness taping and/or sealant. Just using a bit of expanding foam, doesn’t really do the job and you will be left with unwanted cheeky little draughts and needless heat loss in cold weather. Getting an air-tight installation makes a massive difference. Make sure they are a FENSA registered installer.

Interestingly, if your property suffers from excessive solar gain (in other words it gets a lot of sunshine!), you can ask for solar control glazing and you will see a marked reduction in unwanted solar heat gain on the elevations concerned.

One other thing to mention is whether or not to get trickle vents fitted to the windows. Trickle vents allow a little bit of crucial background ventilation which is particularly useful in cold weather. However, if you want a super low-energy house and have enough money to invest then go for a mechanical ventilation system with heat recovery. If you do so, then don’t get trickle vents on your windows.

Got a question to ask us? Email
or write your question in the Comments section below



  1. Jill Goulder

    Very useful advice, Ian – many thanks. May I also mention to readers the option of magnetic-strip secondary glazing (which can also be used as triple glazing on an existing double-glazed window)? Here’s a short video about it, and here’s a PDF that gives more info: Unlike some double glazing, it can even be used in listed buildings.
    Trying to save the planet, one window at a time!

  2. Andrea Jones

    Great article on deciding about triple glazing and great tip Jill. I work with a lot of householders who can’t afford to re-do their windows, or live in conservation areas where they are not allowed to! So it’s good to be reminded if this happy and simple solution to making their homes more energy efficient. Thanks! Andrea, of Eco Works Hove.


Submit a Comment

Your email address will not be published. Required fields are marked *