Understanding U Values & Energy Ratings

Building Regulations

The uValue (measured in W/m²k) requirements are controlled via Building Regulations Document 'L' in England & Wales and Document '6' in Scotland.  In essence u Values were introduced following the Kyoto conference as part of the UK's effort in reducing Carbon Emissions (Global Warming).  The lower the u Value the less heat that is lost from the dwelling, whether it be through the windows, doors, walls or roof.

Staying wholly with the requirements of Windows, initially the acceptable u Value was 2.0 W/m²k, however this has since been reduced to 1.6 W/m²k.  IT is important to note that the u Value is for the whole window not just the glass sealed unit, subsequently referred to as the 'Centre Pane'.  Therefore it is the frame that primarily controls the u Value.  Different frame materials i.e. plastic, wood or aluminium can generate different u Values even if the same sealed unit combination is used in each.

Things were taken a stage further with the introduction of the Energy Rating System (already used on white goods) into the window industry.  Now each frame manufacturer has to have each of their profiles tested, these test results will then show what sealed unit combination is required to achieve each rating i.e. A, B or C.  As with all things there are exceptions to the rule though not many follow this link click here and it will provide in-depth information relating to Energy Ratings.

 

Centre Pane u Value

Outer Pane

Gas

Inner Pane

3

4

5

6

8

10

12

16

20

4mm Float

Argon

4mm Pilk K

3.1

2.7

2.5

2.3

2.0

1.8

1.6

1.5

1.5

4mm Float

Krypton

4mm PIlk K

2.5

2.2

1.9

1.8

1.6

1.4

1.5

1.5

1.5

4mm Float

Argon

4mm Soft Coat

3.0

2.6

2.3

2.1

1.7

1.5

1.3

1.2

1.2

4mm Float

Krypton

4mm Soft Coat

2.3

1.9

1.7

1.5

1.3

1.1

1.1

1.2

1.2

4mm Low Iron

Argon

4mm Pilk K

 

 

 

 

 

 

 

1.5

 

4mm Low Iron

Krypton

4mm Pilk K

 

 

 

 

 

 

 

 

 

4mm Low Iron

Argon

4mm KOW

 

 

 

 

 

 

 

1.5

 

4mm Low Iron

Krypton

4mm KOW

 

 

 

 

 

 

 

 

 

The above u Values are based on figures produced by Build check and are assuming the gas concentration is 90%.  The substitution with toughened glass does not alter the u Value.