National Calculation Methodology (NCM) - Scottish Government

Building Fabric
26. The U-values in the notional building must be as specified in Table 1. Taking into
account guidance in BR 443 8 , all U-values should be calculated in accordance
with BS EN ISO 6946: 2007, where the U-values calculation methods are inclusive
of repeating thermal bridges. The reference constructions conforming to these Uvalues
are identified in Table 1 by their reference identities. In addition, the general
guidance beginning at paragraph 78 must be followed.
Table 1: U-values of construction elements in the notional building
Exposed element U-value (W/m 2 K) Database
reference IDs
Pitched † roofs 0.16 236
Flat roofs 0.25 291
Walls 0.30 295
Ground and other exposed floors (see
paragraph 30)
0.25 248
Windows * and rooflights 2.2 ‡ See Table 3
External personnel doors 2.2 480
Vehicle access and similar large doors 1.5 261
Internal walls 2.0 307
Internal windows 3.85 481
Internal floors viewed from room above 1.25 355
Internal floors viewed from room below 1.23 315
† o o
A pitched roof is one with a pitch greater than 10 and less than or equal to 70 . Any part
of a roof with a pitch greater than or equal to 70 o is considered a wall.
‡
This U-value relates to the performance of the unit in the vertical plane. The U-value
must be adjusted for slope as detailed in section 11.1 of BR 443.
*
Glazed doors are treated as windows (see Table 8 below).
27. In addition, the U-values of display windows must be taken as 5.7 W/m 2 K in both
the notional building and the actual building.
28. Smoke vents and other ventilation openings such as intake and discharge grilles
must be disregarded in both the notional and actual buildings, and their area
substituted by the relevant (i.e. immediately surrounding) opaque fabric (roof or
wall).
29. Non-repeating thermal bridge heat losses for each element (including windows,
etc.) must be accounted for by using the ψ (psi) values specified in clause 6.1.4 of
Section 6. Note that the U-values as given in Table 1 do not include any allowance
for heat losses through non-repeating thermal bridges, and so the calculation tool
must make the adjustment explicitly.
8
Conventions for U value calculations, BRE, 2006.
11