L1 Target U-value Method (ADL1 paragraphs 1.16–1.26)

 “Within certain limits, this method allows greater flexibility than the Elemental Method in selecting the areas of windows, doors and rooflights, and the insulation levels of individual elements in the building envelope, taking into account the efficiency of the heating system and enabling solar gain to be addressed. It can be used for any heating system...” (ADL1, paragraph 1.16).

“The requirement would be met if the calculated average U-value of the dwelling does not exceed the Target U-value, corrected for the proposed method of heating, as determined from the following paragraphs.” (ADL1, paragraph 1.17).

The L1 Target U-value Method lays down two requirements: a Mean U-value Requirement and a Maximum U-value (Table 3) Requirement. The ‘additional requirements’ of L1 – ADL1 paragraphs 1.30-1.45, referred to in the Summary Guide as ‘Additional checks by builders’ – are, for convenience, also treated here under the heading of the Target U-value Method.

“The requirement would be met if the calculated average U-value of the dwelling does not exceed the Target U-value, corrected for the proposed method of heating, as determined from the following paragraphs.” (ADL1, paragraph 1.17).

The average U-value of the dwelling’s heated rooms is calculated from the areas and U-values of exposed and link-exposed elements. This is compared with the target U-value calculated from equation (1) in ADL1 paragraph 1.18, with the following adjustments applied, as appropriate:

·       a boiler efficiency factor f_e (ADL1 paragraph 1.20)

·       a factor of 1/1.15 for certain types of heating system (ADL1 paragraph 1.21)

·       a factor of 1.03 for dwellings with metal window frames (ADL1 paragraph 1.23)

·       an additive adjustment, ΔS (applied last), to allow for solar gain (ADL1 paragraph 1.24)

For gas or oil systems, the boiler efficiency factor f_e is the ratio of the boiler efficiency (SEDBUK or unadjusted SAP Table 4b value) to the reference boiler SEDBUK from ADL1 Table 2. For other types of system, f_e is set to 1.

The factor of 1/1.15 is applied in the case of heating systems that are not eligible for the Elemental method, as specified in ADL1 paragraph 1.2 (see under Heating Efficiency Requirement above).

The factor 1.03 is applied to dwellings for which more than half the window area has metal frames.

The additive correction ΔS is calculated from the areas of glazing facing north and south, in accordance with ADL1 paragraph 1.24. The definition of north and south-facing in this paragraph (‘± 30°’) is interpreted as meaning no more than 30° from due north or south in either the horizontal or the vertical plane. Glazing connecting a heated room with an unheated, separated conservatory is treated as external glazing for the purpose of this calculation (as in the Carbon Index Method).

"When using the calculation procedures in the Target U-value and Carbon Index methods it may be possible to achieve satisfactory solutions where the U-values of some parts of elements... are worse than those set out in Table 1. This is provided that the poorer performance is compensated for by better performance of the other elements. ... A way of achieving this would be to adopt local U-values no higher than those in Table 3." (ADL1, paragraph 1.29).

These checks follow the same pattern as the Elemental U-value checks described above under ‘U-value (Table 1) Requirement’, but apply only to roofs, walls and floors.

L1 stipulates additional requirements relating to:

Limiting thermal bridging at junctions and around openings (ADL1, paragraph 1.30-1.32)

Limiting air leakage (ADL1, paragraph 1.33-1.35)

Space heating system, zone, timing and boiler controls (ADL1, paragraph 1.36-1.41)

Hot water systems (ADL1, paragraph 1.42-1.46)

Commissioning of heating and HWS systems (ADL1, paragraph 1.47-1.50)

Operating and maintenance instructions for heating and hot water systems (ADL1, paragraph 1.51)

These requirements cannot be analysed in software but are included for completeness.