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Approved Document L1B - Compliance For Change Of Use To Dwelling And Related Work

Approved Document L1B provides guidance on complying with the requirements of Regulation L1 dealing with the conservation of fuel and power. It is one of four Part L Approved Documents.

Outlined below are the energy performance standards required for 'Thermal Elements'. These are walls floors or roofs that separate the heated space from the external environment or from unheated spaces such as garages. The four types of thermal element are 'new', 'replacement', 'renovated' and retained' and all may be relevant to a change of use.

We also give guidance on the specification of 'Controlled Fittings' such as windows, doors and roof lights and 'Controlled Services' such as lighting, heating, mechanical ventilation and air conditioning.

New Thermal Elements

Where a scheme involves the construction of all new elements (i.e. not replacement), the U-values should meet the standards.

Reference will need to be made to manufacturers specific 'U-Values' for construction specifications to meet relevant performance standards.

Windows, Rooflights And Doors

The standards for these have been raised in the new Part L and new methods of rating are now considered. Window Energy Rating Bands are introduced and 'centre-pane' U-values. Where the work involves the fitting of new windows, doors or roof lights in the external walls, they should meet the standards.

Required Replacement Of Elements

Any existing unit that does not achieve a U-value less than 3.3W/m² oK should be replaced with units meeting the standards.

All units must be draft proofed.

Replacement Thermal Elements

Where roofs, walls or floors are rebuilt as part of a scheme, the following thermal standards are to be met under L1B - Table 4

Renovated Thermal Elements

This is a new control introduced by L1B and requires the upgrading of performance in thermal elements where the developer proposes to renovate more than 25% of the surface area. This would typically be work to the inner or outer lining of an external wall, re-covering of a roof (flat or pitched), renovation of a ceiling, re-plastering walls, replacing a floor or its covering (screed etc.) would also be applicable.

(Refer to L1B Table A1 for a fuller list of renovation works where compliance is required)

What Standards Are Required And What If Uneconomic?

An exception to the need to upgrade performance is where the upgrade is not technically or economically feasible. The document describes a method of assessing economic feasibility by calculating the 'simple payback period' or the amount of time it would take for energy cost savings to exceed the basic cost of upgrading the thermal performance.

(Refer to AD L1B clause 72 on how to calculate)

Retained Thermal Elements

These are thermal elements that exist in the building and subject to a change of use which the designer intends to retain. L1B requires that such walls, roofs and floors be upgraded if their U-value fails to meet the 'threshold value'.

The same exceptions rules on feasibility apply equally to this type of thermal element If the upgrade is not technically or economically feasible then the designer should try to achieve the best standard that is possible.

Controlled Services (Heating And Hot Water)

The new rules aim to ensure that new systems meet a minimum standard of energy efficiency. The designer can establish the adequacy of a system by referring to the Domestic Heating Compliance Guide published by TSO and NBS.

This is a lengthy and highly detailed document giving standards for many types of heating system, fuel and controls. A typical specification for a natural gas fuelled system would be a boiler with a SEDBUK (efficiency) rating of at least 86% (i.e. condensing type) linked to a fully pumped system with boiler interlock (switches off when no demand for heating) and zone, timing and temperature controls.

There are different requirements for dwellings with floor areas over 150m².

Certification And The Commissioning Of Heating And Hot Water Systems

Regulation 20C requires that all such systems are effectively commissioned so that at completion, the systems and their controls are left in the intended working order. It also requires that a builder provide us with certificates as evidence of this on completion of the building.

Controlled Services (Lighting)

The guidance on new internal lighting systems in change of use schemes is that the number of energy efficient fittings is not less than one per 25m² of floor area (excluding garages) or part thereof or one in every four fittings whichever is the greater.

Energy efficient lighting is classed as fittings that only take lamps having a luminous efficacy greater than 40 lumens per circuit-watt. It is generally only fluorescent or compact fluorescent fittings that meet this standard. Fittings in cupboards or similar areas do not count towards the total.

External lighting must either meet the above efficiency standard or must consist of fittings that have movement and daylight sensors and lamp capacity not exceeding 150 watts.

Other Controlled Services

L1B places controls on the efficiency of mechanical ventilation systems by referring to the Energy Saving Trust's design guide GPG268 - Energy Efficient Ventilation. Fixed air conditioning systems are required to have an energy efficiency rating of Class C.

Construction Standards

Irrespective of the quality of materials used, there is a major potential for heat loss and cold bridging caused by poor standards of construction in terms of air tightness and the continuity of insulation. This is particularly critical where older buildings are converted into dwellings so it is important for the designer to ensure that the various insulated elements in a building are carefully detailed at junctions. Uncontrolled air leakage can be minimised by specifications calling for appropriate levels of sealing and the TSO Robust Details guide 'Limiting Thermal Bridging and Air Leakage' suggests suitable details for achieving this with typical methods of construction.

Historic Building Conversions

There is provision in the Approved Document to take account of the building's architectural or historic importance in applying the requirements and for listed buildings this may be mandatory. Whilst a designer should strive to achieve the best possible energy efficiency, he must ensure that measures are sympathetic to the existing fabric and appearance.