Case study

Penwith Housing Association in Penzance, Cornwall, was formed in 1994 to take over the local authority housing from Penwith District Council to make it possible to gain access for funds to upgrade the entire stock. This consisted of a mix of 1940s houses with solid concrete block walls and post-war cavity built homes. The 1940s examples had a SAP rating of 1 and an NHER of 1.1. The application of external insulation, additional roof insulation and double glazing raised this to SAP 26. However, the crucial BEPI rating was raised to 97, i.e. close to Building Regulations standard current at the time. The addition of gas central heating raised the SAP to 76 which dramatically illustrates the effect of fixed appliances to the SAP value.

Being of concrete construction and rendered there was no problem as regards changing the appearance by overcladding. The technique involves applying a render to provide an even and smooth fixing surface to the rigid insulation panels. The panels then receive a waterproofing finish. A mesh is applied to the insulation to provide a key for the external waterproof render which is finished with pebble dash (Figure 10.1).

External cladding has a number of consequences. For example, carrying it round window reveals means that the window frame size is reduced. Roof eaves and verges have to be extended and rainwater/soil vent pipes have to be modified to take account of the deeper eaves.

It is necessary for the insulation boards to receive a finishing coat. In the case of most insulants the finish should offer total waterproofing. A polymer-based render is the most reliable in this respect. This is an adhesive render with an alkali resistant glass fibre mesh as reinforcement. Applied in one or two coats it offers a choice of finishes, for example:

• textured renders in a range of colours;

• roughcast, also called harling or wet cast.

It is also possible to use cladding which includes

• lightweight natural stone aggregate;

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