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Thermal Efficiency and U-Values

What is a U Value?

A U Value is the measurement of how effective a particular material is as a heat insulator.

The formula for calculating a material’s U Value is w/m2k, which can be translated to English as ‘The energy in Watts (W), that is lost per square meter of the tested material (m2), when the external temperature is at least 1 degree Celsius lower than the internal temperature (k).”

As a result of the above equation, the more effective a material is at retaining heat, the lower its U Value, or “the lower the U Value, the better”.

What is a ‘low’ U Value?

We understand that some of you may never have researched U Values before, and therefore being told “lower U Values are better” might not be helpful. After all, what is a low U Value? 100? 10? 1?

The UK Government has set out some U Values that anyone undertaking building / construction work must legally adhere to, and these are outlined in Part L of the current Building Regulations:

Roof – 0.15 w/m2k

Wall – 0.3 w/m2k

Windows – 1.6 w/m2k

As you can imagine, these U Values only affect products / works that post-date the Building Regulations, and therefore, any original features may strongly exceed these U Values.

Some example U Values of original features, pre-Building Regulations, are as follows:

Unfilled Cavity Wall – 1.5 w/m2k

Solid Brick Wall – 2.2 w/m2k

Un-insulated Roof Space – 2.5 w/m2k

Single Glazed Window – 4.8 w/m2k

Windows and U Values

The observant among you will have noticed that the Government allows windows to have a proportionally higher U Value than walls or roof spaces.

The reason for this is very simple: you cannot add insulation to a window.

The way you improve the insulation properties of a wall or roof is by adding large amounts of an insulating material. Most likely, you will have seen the rolls of fibreglass, space-blankets, and natural and synthetic wools that large hardware stores are constantly advertising as ‘money saving DIY projects’.

They’re not lying. To add additional insulation to a wall cavity of roof is a cheap and simple job, costing around £20 per square meter for the materials, and they’re so easy to install that an amateur DIY enthusiast could do it in a long weekend. There is no excuse for modern building works to leave walls and roofs un-insulated, resulting in both wasted energy costs, and increased pollution.

Unfortunately though, there is a rather obvious issue with attempting the same project with your windows: you won’t be able to see out of them!

‘Insulating’ a Window

The insulation properties of a double-glazed window are derived from the materials used and the dead space in between the two panes of glass.

Ultimately, the glass itself is not a fantastic insulator of heat, which is why a single glazed window has such as high U Value. Resultantly, most of the heat-retaining technology that sets a Double Glazed Unit (DGU) apart from secondary glazing takes place in the dead space between the two glass panes.

Argon Gas:

The ‘dead space’ inside the DGU is not filled with standard air, which may surprise you. Standard air has a U Value of 0.024 w/m2k, which is a rather poor insulator of heat as far as gases go. Of course, though, it is vital that any gas used to fill this space is colourless, non-reactive, and non-toxic. To this end, we use Argon; a colourless, non-reactive and non-toxic Noble Gas with a U Value of 0.016 w/m2k, making it 33% more thermally efficient than standard air! An added benefit is that Argon is relatively inexpensive, meaning that the cost increase in minimal compared to the massively improved thermal performance.

 Warm Edge spacer bar:

A spacer bar is necessary to separate the two panes of glass in a Double Glazed Unit. Traditionally, these spacer bars were made of highly conductive aluminium, which meant that a lot of energy was lost through the DGU. Heat would easily pass through Glass Pane #1, be conducted through the dead space inside the unit via the aluminium spacer bar, and then escape outside through Glass Pane #2.

A Warm Edge spacer bar is made of an insulating plastic composite, thereby helping to reduce the heat that is lost through the DGU; as a result, the edges of the unit (where the spacer bars meet the glass panes) retain their warmth. It is this contrast to a typical aluminium spacer that gave this technology its name: Warm Edge spacer bars.

Low Emissivity Coating:

Our Double Glazed Units contain a transparent and completely unnoticeable Low Emissivity Coating’. This coating is highly reflective with regards to heat, which helps to prevent expensive heating from escaping outside during the winter months, and drive back the scorching sunlight during the summer months.

What does this all mean?

According to the 2014 Report, ‘Housing in London 2014: The evidence base for the Mayor’s Housing Strategy’, over 40% of the homes in Inner London were built prior to 1900 and will therefore suffer from solid brick walls, un-insulated wall spaces, and single-glazed windows. I.e., they will have the high U Values as outlined above.

Any modern building work or replacements, however, must adhere to the aforementioned Building Regulations, and as a result, by installing new double-glazing into your period home, you will be making your windows more thermally efficient than your solid brick walls!


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