Annex 6.N Conversions
6.N.0 Introduction
6.N.1 Conversion of unheated buildings
6.N.2 Conversion of heated buildings
6.N.3 Conversion of historic buildings

annex 6.N

Conversions

6.N.0 Introduction

The criteria for when conversion occurs are outlined in Section 0. However the guidance for energy efficiency measures is split into 3 further categories of building and these are unheated, heated and historic.

A building that was originally designed to be unheated in most instances has the greatest void to fill in terms of energy efficiency. It is inevitable that heating such buildings will adversely affect energy efficiency and because of this, the most demanding measures are recommended for conversions. In some instances, it may even be more worthwhile demolishing these buildings and rebuilding with new construction.

In the case of a building that was previously designed to be heated, the impact on energy efficiency as a result of the conversion, may be either negligible, none whatsoever or in some circumstances even an improvement. In view of this, a less demanding approach is recommended which at the same time still ensures that some overall improvements are being made to the existing building stock.

With historic buildings, the energy efficiency improvement measures that should be invoked by conversion are less straightforward. The remaining number of these types of buildings in the country is finite. The majority of them have visual features that are not only worth preserving but the industry of today can have difficulty in replicating such construction. For these buildings the least demanding measures apply, however it is a rare occurrence that no improvement in energy efficiency can be made.

6.N.1 Conversion of unheated buildings

Where conversion of a building that was previously designed to be unheated is to be carried out, it is appropriate to treat the building as a new non-domestic building and follow the guidance given in 6.0 to 6.8. This category includes buildings with heating rated at a maximum of 25 W/m² of floor area and installed for the purposes of frost protection.

6.N.2 Conversion of heated buildings

Where conversion of a building that was previously designed to be heated is to be carried out, it is appropriate to examine the insulation envelope and the building services and upgrade as follows:

1. The average U-value of a roof should not be worse than 0.35; and
2. The average U-value of any wall or floor should not be worse than 0.7 (except where zero thermal transmittance occurs, see 6.0.6); and
3. Where the windows, doors or rooflights are being replaced, the average U-value of the replacements should not be worse than 1.8 for those with plastic or timber frames or 2.0 for those with metal frames; and
4. The total area of windows, doors and rooflights, should not exceed the percentage values in Table 2 to 6.2.1; and
5. Thermal bridging need only be taken into account where the U-value of an existing element of building fabric is being improved. For guidance on condensation see Section 3: Environment; and
6. Limiting air infiltration need only be taken into account where the U-value of an existing element of building fabric is being improved and then only to those components which are being altered; and
7. Where an extension is formed and/or alterations are being made to the building fabric or the building services at the same time as the conversion, the guidance given in Annex 6.H should be followed.

6.N.3 Conversion of historic buildings

There is no specific guidance given here on this subject. The general heading of ‘historic buildings’ is further sub-divided. These sub-divisions are ranked in order of importance. Consequently each case will have to be dealt with on its own merits.

In some instances, only an improvement in the heating system will be possible, but where this occurs, the most energy efficient measures within that area should be sought. Best practice should be adopted, rather than just good practice. For example, review the carbon intensity of the heating system and install an efficient condensing boiler or even consider CHP if the building is large enough and provided there is sufficient demand for heat. Plant that uses biomass may be particularly appropriate for a historic building in a rural location.

Any improvements to the fabric insulation of the building will often depend on whether or not the installation work can be carried out using a non-disruptive method. For example, insulating the ceiling of an accessible roofspace. In certain cases, buildings are given historic status because of the features that exist on one particular facade and in these cases, it may be possible to make some improvements to other less critical elevations or areas.

In all cases, the ‘do nothing’ approach should not be considered initially. Innovative but sympathetic and practical solutions on energy efficiency, which are beyond the scope of this guidance, can often result in an alternative package of measures being developed for a historic building. Consultation on such matters at an early stage with both the verifier and the development control officer of the relevant local authority is advisable.