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What is a U-value?
 
A U value is a measure of heat loss. It is expressed in W/m2k, and shows the amount of heat lost in watts (W) per square metre of material (for example wall, roof, floor etc.) when the temperature (k) outside is at least one degree lower.

The lower the u value, the better the insulation provided by the material.
 
 
Building Regulations - England and Wales
 
New Buildings (ADL 1A & ADL 2A)
The main requirement of Approved Documents L1A and L2A is that buildings meeta defined CO2 emissions target. The roof, wall and floor U–values required to meet this target will depend on the design of the building, orientation, heating system etc. To ease the process, Kingspan Insulation has undertaken analysis to give the ‘best starting point’ U-values, for specifiers to work from in getting their designs to comply. They will be almost exactly what is required for some buildings and short of what is required for others. These ‘best starting point’ U-values are shown here.
 
Existing Buildings (ADL 1B & ADL 2B)
Approved Documents L1B and L2B give specific U-value requirements for newly constructed elements and retained/refurbished elements. They apply to all works, regardless of whether the works relate to an extension, conversion or renovation.
These required U-values are shown here.
 
Building Regulations - Scotland 
 
New Buildings (Section 6 – Energy)
The main requirement of Technical Handbooks Sections 6 is that buildings meet a defined CO2 emissions target. For domestic, this target is generated by a set of 2010 notional buildings, which also offer a simplified elemental approach to compliance. If a building is constructed in accordance with the parameter values that define the notional building, then it will automatically comply with the CO2 emissions target. The ‘notional’ U-values for roofs, walls and floors are shown opposite. For non-domestic, the approach is the same as that described overleaf for ADL2A and ‘best starting point’ U-values are shown here.
 
Existing Buildings (Section 6 – Energy)
Technical Handbooks Sections 6 give specific area-weighted average U-value requirements for newly constructed elements and altered/refurbished elements. They apply to most works*, regardless of whether the works relate to an extension, renovation, or conversion e.g. of a whole unheated building, loft or garage. For existing dwellings, there are differing requirements for newly constructed elements, depending upon the thermal efficiency of the existing building*. The required U-values are shown here.