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Conservatories and Sunrooms - Pilkington K Glass, Low E Glass, Suncool Glass.

What is the U Value of a typical Double Glazed sealed unit?

The following table shows some typical U values for vertical insulating glazing Pilkington Insulating Units and the effect of using K Glass, Argon and varying cavity widths.

The lower the U Value - the better the insulator. U-value is the measurement of heat transfer through a given building material, glass, etc. In the UK all U Values are quoted as W/m²K.

  Cavity Width
Glass Specification 12 mm 16 mm 20 mm
Optifloat/air/Optifloat 2.9 2.7 2.8
Optifloat/argon/Optifloat 2.7 2.6 2.6
Optifloat/air/Pilkington K Glass 1.9 1.7 1.8
Optifloat/argon/Pilkington K Glass 1.6 1.5 1.5
Overall width of Insulight Unit (mm) 20 24 28

U values based upon 4mm Pilkington Optifloat thickness. (NOTE: Optifloat is the name given by Pilkington to "normal" float glass)

Other Information:

Figures determined in accordance with the requirements of BS EN673: 1998 for `normal' exposure conditions. U values are rounded to the nearest 0.1W/m2K as described in clause 9 of the standard. The total overall U value of a window can be calculated taking into account the insulation of the glazing, the unit spacer and framing. 

We often get asked what is the best cavity width to use in a double glazed sealed unit. As can Click here for a brochure requestbe seen above there is verylittle difference between a 16 mm and 20 mm cavity.

You will also note that adding Argon Gas alone has very little effect in improving the U Value. Only when the Argon is combined with Pilkington K Glass is any significant improvement made.

Our contractors offer a wide range of glass options


Pilkington are only one of many glass suppliers world-wide. We are only giving their products so much prominence here because of their large market share in the UK.
If you live in Australia we suggest you have a look at the Pilkington - Australia web site at
Different parts of the world quote U Values in different ways. In the UK all U Values are quoted as W/m˛K. The lower the U Value - the better the insulator. U-value is the measurement of heat transfer through a given building material, glass, etc

For more information, please visit:

Four Season Sunroom | About Pilkington Glass | An Important Question | Values and Insulation | arm-a-Glass™ | Types of Solar Glass
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