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risk for frost damage. The G_c factor is calculated using the following formula:

G_c =-14.53-0.309 a + 0.203 S

Where a is the water absorption coefficient of the material, expressed as saturation percentage per time unit in S^0.5, and S the water accessible porosity, expressed as a percentage of the total volume. The experimentally determined G_c values based on the illuminated samples are recorded in Table 5. The standard dictates that the G_c value for vertical façade elements in solid brickwork must be less than -1 and less than -2.5 for horizontal façade elements. Therefore, depending on the exact value, both the joint mortars and the façade parts in Euville display a possible risk of frost damage according to the aforementioned criterion. Take note that Euville limestone is generally considered to be frost resistant and that the WTCB is more likely to list guideline values between -4.4 and -5.5 for the G_c factor. The Euville limestone results are more varied, which may indicate the use of lesser-quality materials , leading to a risk of local frost damage.

It is important to note that for the evaluation of the materials’ sensitivity to frost, only possible frost damage to the materials is taken into account. Damage resulting from the combination of materials in the masonry, such as a joint being pushed outwards for example, is not taken into account in this criterion.

CONCLUSION

The evaluation of the available options to improve the insulating quality of the exterior façades of the former veterinary school in Anderlecht shows that the risks of possible frost damage to the façades brickwork increase significantly, particularly to the front and side façades, which have an ashlar in Euville limestone, and to a lesser extent to the rear façade, which is mainly brickwork. Therefore, the construction of the historical brickwork has an important influence on temperature division, moisture content and the accompanying risk of frost damage to the ashlar, regardless of the type of interior insulation.

The initial decision to reduce the primary energy consumption of the building from approx. 400 kW/m² to 188 kW/m² by improving the insulation to the shell of the building entailed the application of a 12 cm thick insulation board based on calcium silicate to the interior surface of the exterior walls. The study shows that this type of approach leads to a marked increase in the risk of frost damage to the ashlar, particularly to the façade sections in Euville limestone, which are already sensitive to moisture. Moreover, there is a risk of edge joints becoming clearly visible on the interior surface, caused by water transfer through the joint and the insulation board. A 3 cm thick layer of insulating plasterwork, resulting in a somewhat higher energy loss through the exterior walls, allows for a substantial gain -in terms of possible frost damage- in comparison to the original proposal for improvement of the insulation. An increase in the risk of frost damage in comparison to the current situation remains, though it is less significant.

To assess the actual risks of frost damage, the G_c criterion for the materials according to an old Belgian standard was compared to on-site observations of the damage.

An evaluation of the sections of the front and side façades showed considerable damage to the limestone (fig. 8). However, the primary cause of the observed damage to the Euville limestone has more to do with the formation of gypsum as a result of air pollution than with frost damage. This in and of itself confirms the general assessment of Euville as a frost-resistant limestone. The formation of the gypsum crust does however influence the pore structure close to the stone's surface, thus increasing the risk of frost damage over time. Therefore, totally eliminating the risk of frost damage is impossible.

The possible risks are, however, connected to the amount of rain which penetrates the wall. It is above all the uppermost sections of the walls (which are more exposed to rain) that are particularly susceptible to this. Based on the study, the decision was made to diversify the application of interior insulation: on the ground floor and first floor it was opted to follow the original proposal and apply insulation, namely 12 cm thick calcium silicate boards. On the top

Table 5

Overview of the experimentally determined G_c factor for the various façade materials (© KIK-IRPA).

72 | Risk analysis for applying interior insulation in historical buildings: a case study of the former veterinary school in Anderlecht