Building Regulations L1B (2010 edition)
Heat Loss Calculator
(applicable only to domestic extensions)

How to use the Thermal Calculator

1. From the drawing, calculate the areas of each element and enter in the appropriate box below.
2. Enter the U-Value for the appropriate element (use manufacturers data).
3. Enter the required U-Value for the roof construction.

Required U-Values for roofs
Pitched roof with insulation at ceiling level 0.16
Pitched roof with insulation at rafter level 0.18
Flat roof or roof with integral insulation 0.18

4. An area of allowable openings is automatically calculated (25% of total floor area), but you should also enter the area of any windows or doors which, as a result of the extension works, no longer exist or are no longer exposed.
5. Read result at bottom of spreadsheet.

ELEMENT AREA U-VALUE OF ELEMENT REQUIRED U-VALUE
Total Floor Area (GF + FF + 2nd Floor)    
Ground Floor  
Roof
External Wall (inc.area of Windows and Doors)  
Windows  
Roof Lights  
Doors (greater than 50% glazed)  
Doors (other)  
Additional Openings Allowance (see note 4)