Energy Efficiency

Thermally Efficient Windows

Up to 20% of your home’s heat can be lost through your windows. That’s why it’s important to make sure your new ones are as energy efficient as possible.

As a homeowner, you may think, “How am I supposed to know what is energy efficient and what isn’t?”

To simplify things, lets start by talking about how a window is made up. Every part can affect its energy rating…

heat transfer, multi-chambered profile, energy efficiencyThe frame is the best starting point. uPVC is a natural insulator, so it’s one of the most energy efficient materials for a window frame. In a Liniar window, multiple chambers hide inside the frame, acting as barriers and making it harder for the heat inside your home to be transferred through and escape outside. The drawing on the left shows the Liniar EnergyPlus system, with the colours representing heat and cold – with red being the heat from your home and purple the cold air coming from the outside.

Glazing also plays a huge part in energy efficiency. Double glazing is still the most popular choice, although triple glazing is increasing in popularity in the UK.  Double or triple refers to the layers of glass that are used back to back in your window. There is a wide range of types of glass that can be used, and you may hear the term “low emissivity glass or low-e glass”, which has an invisible coating to reflect heat back into your home

Whatever glazing you choose, one thing they all have in common is a spacer bar in between the panes. This is the long continuous strip of material that is sealed to the glass to separate the panes. Spacer bars are typically made from aluminium or a low heat conductive material – you might hear these referred to as “warm edge spacers.” A spacer does what it says on the tin, but not only spaces the glazed panels, it helps prevent the transfer of heat too. The space created between the panes of glazing is often filled with gas – usually either air, argon or krypton.

Ok, so you’ve got a frame, glazing, spacer bar and gas. But how is an energy rating calculated?

The bots at the BFRC (British Fenestration Ratings Council) calculate a windows energy rating based on

  • U-Value (of the glass Ug & the frame Uf)
  • the solar heat gain or ‘G Value’ (of the glass)
  • air leakage rate of all the components of the fitted window.


The U-Value is something that is quite often talked about and something you might see when talking about energy ratings. A U-value is the measure of the heat loss in watts (W) per square metre (m²) of material when the temperature (K) outside is at least 1 degree cooler – (complex stuff) you will see it written as W/m²K!

When the u-value has been calculated it is combined with the solar gain of the glass and the window is given a numerical BFRC rating – the WER, or Window Energy Rating. To make things simpler, this is converted to a band, which now runs from A++ to G. The rating is displayed on a lovely rainbow label – this may look familiar, as similar labels should be on the back of your fridge, freezer and other white goods.

I’m reliably informed by my tech-bot friends that the rainbow label used to start with A+ in the purple band at the top – but from 1st October 2015 this year, when the BFRC bots realised that windows (and the bots behind them) were getting very clever at keeping heat where it should be, they created a new category *drum roll* the A++ category! Luckily, Liniar windows were already super efficient and so the system could achieve A++ well ahead of the new band.

So! To summarise…

The closer to having a WER of A++, and the lower the U-value is, the more energy efficient the window or door is, meaning lower fuel bills for human-bots.

Finally, if any fabricator, installer or even homeowner-bots are curious, I will let you in on a secret! There is a nifty little tool on the Liniar website called the WER Calculator. One of our Liniar tech-bots has made a video explaining how to use it… you can have a play around and find out different combinations and energy ratings for your windows.  Have fun! I like to have a play when I have my afternoon cuppa!

Until next time,

Mark II


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