The elemental method for buildings other than dwellings

In the elemental method, compliance is demonstrated by specifying:

• Maximum U-values for walls, floors and roofs and rooflights

• Maximum area-weighted U-values for windows, personnel doors and rooflights

• Maximum combined area of windows and doors (as a percentage of exposed wall area)

• Maximum area of rooflights (as a percentage of roof area)

• Minimum performance standards for all energy consuming building services

• Design details to meet air leakage standards and to avoid solar overheating.

With regard to windows within part L2 the following should be noted:

• Display windows, shop entrance doors and similar glazing are exempt from the maximum U-value standard of Table 3.1, but not from the maximum area standards of Table 3.2

• The terms rooflight and roof window are synonymous

• For the purposes of calculating maximum areas, dormer windows in a roof may be included in the rooflight area.

Maximum U-values are shown in Table 3.1.

With the exception of vehicle access doors, Table 3.1 is the same as Table 2.1 for dwellings, and so the illustration of these U-values in Fig. 2.2 is still applicable. Also, the same comments apply with respect to unheated spaces and metal frame windows.

The area-weighted U-value of the windows, doors and rooflights can be calculated from the formula:

Table 3.1 Maximum U-values for the elemental method.

Exposed element

Maximum U-value, W/m2K

Comment

Pitched roof, insulation between rafters

Pitched roof with integral insulation

0.20 0.25

Any part of a roof having a pitch of 70° or more can be considered as a wall

Pitched roof, insulation between joists

0.16

Flat roof

0.25

Roof of pitch 10° or less

Walls, including basement walls

0.35

Floors, including ground floors and basement floors

Windows, personnel doors and roof windows, glazing in metal frames, area-weighted average for the whole building

Windows, personnel doors and roof windows, glazing in wood or PVC frames, area-weighted average for the whole building

2.00

A roof window may be considered as a rooflight. The higher U-value for metal frames allows for extra solar gain due to greater glazed proportion

Rooflights

Vehicle access and similar large doors

2.20 0.70

Applies only to the unit, excluding any upstand which must be insulated

where AW, AD and AR are respectively the areas of windows, doors and rooflights

UW, UD and UR are their U-values.

The maximum areas of windows, doors and rooflights depend on the building type, and are given in Table 3.2.

Table 3.2 Maximum areas of windows and rooflights.

Building type

Maximum window and door area as % of exposed wall area

Maximum rooflight area as % of area of roof

Residential buildings where people temporarily or permanently reside

30

20

Places of assembly, offices and shops

40

20

Industrial and storage buildings

15

20

Vehicle access doors and display windows and similar glazing

As required

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