Of the two wall insulation options, exterior insulation is the most effective and the least risky, as long as a comprehensive approach is taken. This is because the entire wall can be covered up so there are fewer routes for the heat to escape. However the job is complex and expensive. The wall has to be built out and a new wall finish created. Everything on the outside of the building has to be replaced or adjusted: window reveals and sills, door architraves, roof overhangs, decorative features, gutters and downpipes, soil and vent pipes and other services.

Although exterior insulation is not appropriate for many prominent ashlar stone facades, many buildings in Cheltenham are faced in render or stucco, and there are many secondary facades that are either hidden or of little importance to the character of the town. Furthermore, some of the rear facades of Regency buildings were originally rendered, so the installation of a new rendered facade might actually be in keeping with the building’s history. It is however far from straight-forward to externally insulate a Regency building, particularly where the façade is modelled with decorative pilasters, window surrounds and other projecting elements, or where the building sits in a terrace of matching properties. As a result, external wall insulation may be inappropriate for these types of buildings.  In addition, great care has to be taken with the detailing of external wall insulation, to avoid the creation of thermal bridges (cold spots) which can attract condensation and lead to mould problems.  Therefore detailed advice should be sought from the Council and / or a Conservation Architect prior to submitting a planning application.

When installing exterior insulation on top of a traditional wall, it is important to preserve the permeability of the wall by using a moisture-permeable insulation and finish. This ensures that any moisture within the wall does not get trapped. This is especially important for the outside face of the wall because of the high risk of rainwater getting into the wall and the importance of the action of the sun in drying out the wall.

Exterior wall insulation installed to both back and side of this property has made a significant contribution to lowering fuel bills and increasing the comfort of its occupants.