Annex 6.H Alterations and extensions
6.H.0 Introduction
6.H.1 Alterations to building fabric
6.H.2 Extensions
6.H.3 Alterations to building services

annex 6.H

Alterations and extensions

6.H.0 Introduction

This Annex relates to existing buildings.

Extensions

In the case of extensions, the majority of the construction will be new-build and seldom will there be the need to build to a lesser specification than for a completely new building. At the interface of the existing and new construction however, it is appropriate to build to a slightly lower specification to allow the transition to occur. An example of this would be the use of proprietary metal ‘wall starter’ ties where existing brickwork stops and new cavity blockwork begins. It will still be necessary to ensure that the other building standards are met with regard to the transitionary construction.

Alterations

Historic buildings

The situation for alterations is somewhat different. It is more than likely that the existing construction will be from a different era, in building regulation terms. In many instances it will be appropriate to consider each building on its own merits. Some of the guidance given in this section is written in specific terms, but in certain cases (e.g. historic buildings), it may be necessary to adopt an alternative package of energy efficiency measures which are appropriate to the amount of alteration work being undertaken.

6.H.1 Alterations to building fabric

Reference should be made to the guidance on extensions (6.H.2) for alterations that involve increasing the non-domestic building floor area and/or bringing parts of the existing building that were previously outwith the insulation envelope into the heated or cooled part of the building.

Alterations to the insulation envelope of a non-domestic building should be considered using the guidance in the following paragraphs:

Infill of small openings

The infill of an existing opening of approximately 4 m² or less in the building fabric should have a U-value which matches at least that of the remainder of the surrounding element. In the case of a wall or floor however, it should not be worse than 0.70 W/m²K and for a roof, no worse than 0.35 W/m²K.

Infill of large openings

The infill of an existing opening of greater area (than approximately 4 m²) in the building fabric should have a U-value which achieves those recommended in Table 1 to 6.2.1. Another way would be to follow the guidance in the paragraph above, but compensate for the energy efficiency deficit by improving the overall U-value of other parts of the insulation envelope.

Insulation envelope formed from internal elements

Where the alteration causes an existing internal part or other element of a building to form the insulation envelope, that part of the building (including infill construction) should have U-values which achieve those recommended in Table 1 to 6.2.1. This will most likely occur where a building or part of it is demolished as a phase of the alteration work. Another approach would be to follow the guidance given for "infill of small openings" above, but compensate for the energy efficiency deficit by improving the overall U-value of other parts of the insulation envelope. Where this occurs at a boundary with land in different occupation, no upgrading is necessary if the element is a wall that is exclusively the property of the adjoining building.

Windows, doors and rooflights

Where windows, doors and rooflights are being created or replaced, they should achieve the recommended level of U-value specified in Table 1 to 6.2.1. For secondary glazing however, an existing window, after alteration should achieve a U-value of about 3.5 W/m²K.

Percentage areas of openings

Where additional windows, doors and rooflights are being created, the overall total area (including existing) should not exceed the percentage areas given in Table 2 to 6.2.1.

Solar gain

Where additional glazing is being created in the external building fabric, the impact of any solar gain should be considered in relation to any mechanical ventilation or air conditioning system in the building.

Reconstruction of elements

Where the build-up of an element forming part of the insulation envelope is to be altered or dismantled and rebuilt, the opportunity should be taken to improve the level of thermal insulation. Table 1 to 6.2.1 gives benchmark U-values and in many cases these can be achieved without technical risk, within the constraints of the existing construction. It is recognised however that certain constructions lend themselves better than others as ‘candidates’ for upgrading. A building that was in a ruinous state should after renovation, be nearly able to achieve the level expected of new, construction. It is inappropriate for a building, which is currently occupied and its heated or cooled status is not being altered, (e.g. unheated to being heated) to have its rooms vastly reduced in area or height, unless the building owner intends that these changes are to be made. Similarly, fitting external thermal insulation should not cause excessive enabling alterations, unless the building owner intends that these changes are to be made. Consideration should also be given to the other building standards and the impact that they will have when upgrading thermal insulation. There are not that many cases however, where after an alteration of this nature to the insulation envelope, a roof cannot achieve an average U-value of 0.35 and in the case of a wall or floor, 0.70 W/m²K.

Limiting thermal bridging and air infiltration

When alterations are carried out, attention should still be paid to limiting thermal bridging at junctions and around openings (see 6.2.4) and also limiting air infiltration (see 6.2.5). As far as alterations are concerned, it should be acknowledged that it is only appropriate to consider the work that forms the alteration and also the impact of that work on the existing building.

6.H.2 Extensions

Where a non-domestic building is extended, the new building fabric should be designed in accordance with either the Elemental Method, the Heat Loss Method or the Carbon Emissions Calculation Method.

It should be noted that the Carbon Emissions Calculation Method is a holistic approach and the building services systems will need to be taken into account.

The merits of each method when considering new buildings are set out in the preamble to 6.2.1, 6.2.2 and 6.2.3, but for extensions the following should also be taken into account:

Elemental Method

a. When using the Elemental Method, Table 1 to 6.2.1 (or Annex 6.P) gives the U-values that should be achieved. When calculating the maximum permitted area of windows, doors and rooflights for an extension to a building, the percentage areas in Table 2 to 6.2.1 should be applied only to the walls and roof of the extension.

Heat Loss Method

b. With the Heat Loss Method, zero thermal transmittance should be considered at the interface of the existing thermal insulation envelope and the extended part of the building. This is not appropriate where the situation described in 6.0.5 applies.

Carbon Emissions Calculation Method

c. The Carbon Emissions Calculation Method can be used for the extension alone, where the energy efficiency of the building services systems can be considered in isolation from the existing non-domestic building. In other cases, the entire building, as extended will need to be assessed. In application, this approach will only be suitable when detailed knowledge exists of the original construction and the building services systems in the existing building.

Alterations that extend the insulation envelope

Alterations that involve increasing the non-domestic building floor area and/or bringing parts of the existing building that were previously outwith the insulation envelope (or where the insulation envelope would be expected to exist) into the heated or cooled part of the non-domestic building should be considered in the same way as extensions. Examples of such work are, changing a roofspace, part of an unheated warehouse or a deep solum space into office accommodation:

• in the case of a roofspace, this will usually involve extending the insulation envelope to include, the gables, the collars, a part of the rafters and the oxters, as well as any new or existing dormer construction. The opportunity should be taken at this time to upgrade any remaining poorly performing parts of the roof which are immediately adjacent to the alterations, for example, insulation to parts of the ceiling ties at the eaves;
• in the case of an unheated warehouse, this will usually involve extending the insulation envelope to include, the existing floor, perimeter walls and the roof/ceiling to the new office area.

In the case of a deep solum space, this will usually involve extending the insulation envelope to include, the solum/existing floor and perimeter walls to the new office area.

First time heating or cooling

6.H.3 Alterations to building services

Where the building was previously not designed to be heated or cooled, and heating and/or cooling equipment is introduced into the building (excluding heating of 25 W/m² of floor area, for frost protection purposes) the guidance given in 6.2 to 6.8 should be followed.

Entirely new or complete replacement systems

Where an entirely new or replacement building services installation is installed in an existing heated or cooled non-domestic building, including an extension to the building, the guidance given in 6.3 to 6.8 should be followed.

Part replacement of systems

If a building services installation is being replaced in part, the guidance given in 6.3 to 6.6 should be followed, and where possible 6.7 and 6.8 but only as it affects the new or replaced components of the installation. Such alterations should not allow the installation as a whole to be downgraded in terms of energy efficiency or be compromised from a safety point of view. It is recognised that some alterations to building services, because they are done on a piecemeal basis, will not result in optimum energy efficiency being attained for the entire system. Where this occurs, the person responsible for the commissioning of that part of the system should make available to the owner and occupier, a list of recommendations that will improve the overall energy efficiency of the system.

High carbon intensity heating systems

a. Where an entirely new or complete replacement installation of a heating system with a higher carbon intensity is unavoidable (see 6.3.1 and 6.3.2), an evaluation of the U-values of the existing insulation envelope of the building should be made. The elemental values that are obtained should be improved by dividing them by 1.15 and rounding down the revised figure to 2 decimal places. For example: an existing wall U-value of 0.70 will become 0.60 and an existing roof U-value of 0.35 will become 0.30. The resulting improvement should then be effected to the building fabric.

ACMV in office buildings

b. Where an entirely new or complete replacement mechanical ventilation or air conditioning system is installed in an office building, the Carbon Performance Index (CPI) referred to in 6.6.2a. should be at least equal to that of any existing system but not less than 90.

ACMV in other non-domestic buildings

c. Where an entirely new or complete replacement mechanical ventilation or air conditioning system is installed in a non-domestic building (other than an office building), the total specific fan power or SFP (i.e. the design power of all fans in the distribution system divided by the design ventilation rate through the building) of not greater than 1.5 W/litres/second.

Building logbook

d. The building logbook referred to in 6.8.1 should be amended to take account of the alterations to the services. Where such a logbook does not exist, (e.g. due to the age of the building), the record of the alteration work carried out, including any recommendations to the dutyholder, should form the basis for a logbook.