Hygrothermal measurements in internally insulated solid masonry wall before and after hydrophobization

Constructions built between 1850 and 1960 comprise a remarkable percentage of the total European building stock and are from a time when there was very little focus on energy use. At the same time, many of these buildings have facades worthy of preservation, and in these cases, the only feasible thermal insulation option is internally. Consequently, internal insulation has become increasingly popular, despite numerous cases with mold problems behind the insulation. Hydrophobization has been suggested to reduce the moisture problems as it would prevent the driving rain penetration and ensure better insulation properties, as the water content of the wall is reduced. The present paper describes a case study of a residence built in 1877. An apartment was insulated internally with a diffusion-tight insulation system. Temperature and relative humidity were measured in the indoor climate, at the intersection between insulation and masonry, as well as at the wooden beam ends. Since there were indications of high relative humidity levels (more than 85%), the whole façade of the building was hydrophobized six years after insulation, and the measurements continued. After the application of hydrophobization, a small increase in the relative humidity was noticed, although the temperature was almost unaffected. Moisture may have been trapped as a result of the integrated vapor barrier of the diffusion-tight insulation system on the internal side of the wall, combined with the possible decrease in the diffusivity of the external side of the wall due to hydrophobization. Thereby the overall drying potential of the external envelope could have overruled the possible moisture reducing effect of the hydrophobization.