Eighteen months' monitoring is reported of an experimental roof on the building physics laboratory of the Catholic University of Leuven, Belgium. The existing lightweight roof with roofing felt finish, built some twenty years before, was covered with water-repellent mineral wool slabs, of which only the edge slabs were bonded. The results showed poor water-repellent quality of the MW slab, leading to a changed formulation with better performance. Water ponding under some slabs produced moisture uptake within the slabs themselves, probably from interstitial condensation. Despite these faults, the authors suggest that the concept can be made to work, with good thermal performance. Prof. Hens is co-ordinator of CIB W40.
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