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THE ENERGY FUTURE OF EXISTING BUILDINGS IN BRUSSELS: BETWEEN PRESERVATION AND PERFORMANCE

exterior insulation, cavity wall insulation and interior insulation.

Regarding quality and performance, exterior insulation is without a doubt the best option; cold bridges are avoided and the supporting structure is protected against changes in temperature and moisture content. However, because the original façade is lost, this is often not an option for listed monuments. If such an alteration were permitted, this solution would provide better protection for valuable interior elements because the interior volume is better protected.

Buildings with an uninsulated cavity wall can be insulated retrospectively. But as cavity walls have only been used since WWII, this approach is also often not applicable to monuments. The most common practice for improvement of the insulation value of monument façades is therefore the placement of interior insulation. Interior insulation strengthens the impact of cold bridges and as a consequence is less efficient and of course leads to the loss of the original interior decoration. Furthermore, this solution too is not always applicable to historical heritage.

Applying interior insulation to monument façades also carries with it risks in terms of keeping the exterior façade intact. Interior insulation does, after all, have an impact on temperature division and moisture balance in the cross-section of the façade, particularly when no cavity wall is present and the façade forms the direct separation between the interior and exterior environments. The temperature in simple brickwork tends to vary gradually between the interior and exterior surface (fig. 4a). However, after applying the interior insulation a large temperature difference will occur across the insulation layer, making the interior surface of the façade warmer and the parts of the walls on the exterior of the insulation layer considerably colder (fig. 4b and 4c). As a consequence, the depth of influence of frost on the exterior surface of the façade increases.

Moisture balance is determined by the balance between the precipitation on the façade, possible condensation in the wall and the evaporation from both the interior and exterior surface. Drying is influenced by the temperature division in the cross-section of the façade. A higher temperature in the façade without thermal insulation will promote drying. Conversely, after the application of interior insulation the moisture content in the wall will increase, thus increasing the risks of frost damage (increased by the decreased temperature near the exterior surface) and biological attacks to the exterior surface.

There are two ways to insulate the interior: methods that form a water- and condensation-tight layer on the interior surface to keep out the moisture; and methods that make use of capillary active materials. Methods that form a water- and condensation-tight layer make use of watertight and water resistant insulation materials such as rock wool, polyurethane, etc. in addition to applying a

Fig. 3

Distribution of heat losses to building section according to the initial situation (left) and the situation after improvement of the insulation according to Solution A (right). (Daidalos, 2011).



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