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

quite quickly, as the set mortar behind it greedily draws out water from the bricks. From there, the moisture slowly penetrates deeper into the brickwork.

The comparison of the brickwork on the front and rear façades clearly shows that the front façade is much more susceptible to absorbing moisture; the rain water penetrates deeper into the façade and the moisture content reached is also much higher. On the front façade the moisture almost reaches the interior surface, while on the rear façade only the first two layers of the brickwork become damp. When comparing the initial condition with the available options for interior insulation, the 12 cm thick calcium silicate board receives considerably lower marks. The core of the brickwork behind the Euville ashlar is completely saturated all year round. Even the insulation board itself is almost completely saturated, with the exception of the last few cm of the interior surface, which can dry inside during winter. The dampness of the insulation board is accompanied by a significant loss in the insulating value of the façade's cross-section. This condition deteriorates even more when the influence of an edge joint from the exterior surface to the insulation board is considered. The moisture quickly penetrates deep into the façade and dampens the insulation board locally to the extent that the joint will inevitably become clearly visible on the interior surface of the façade. Therefore, such a solution would be unacceptable without additional measures.

On the other hand, the moisture balance in the brickwork of the rear and front façades after application of insulating plasterwork with a 3 cm layer is not significantly different to the initial condition.

The evolution of the moisture content in the façade's cross-section must also be assessed in combination with the temperature division. The front façade is shown in figure 7. At first glance, the temperature division according to the three available alternatives (i.e. the initial condition and both options for improvement of the insulation) appear quite similar. Only the depth to which cold temperatures penetrate the façade increases visibly with an increasing insulating value. This can be recognised by the increasing amount of green and blue colours in the temperature division after application of interior insulation. The increasing temperature drop throughout the entire cross-section of the façade is particularly recognisable when opting for a 12 cm thick calcium silicate board.

Based on these results, the number of frost-thaw cycles to the exterior façade were compared to the number of times the moisture content in this zone was higher than 30%, 50% and 70% respectively of the saturation degree of the Euville limestone (table 4) to evaluate the risk of frost damage to the front façade. The critical moisture content causing frost damage to the Euville limestone is however unknown, therefore these moisture contents are purely arbitrary. The results clearly demonstrate the influence of the application of an insulating calcium silicate board to the interior surface: the number of cycles in which frost occurs while the Euville limestone is wet increases significantly (+230% at w → 30% w_kr). The situation is less dramatic when applying a thin layer of insulating plasterwork. A slight increase (+50% at w → 30% W_kr) in critical frost-thaw cycles can be observed.

Exposure of facing brick to frost-thaw cycles does not necessarily lead to damage. The degree of sensitivity or resistance to frost of the facing brick in question is the determining factor here. The previous Belgian standard, NBN B27-010, used the G_c criterion for this. Although the applicability of this criterion has been questioned and certainly cannot be applied to all materials, it can still serve as a first indication when assessing the

Table 4

Overview of the number of frost-thaw cycles and the number of critical frost-thaw cycles at which the moisture content in the brickwork of the front façade is higher than the set limits (© KIK-IRPA).

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