Thermal performance degradation of foam insulation in inverted roofs due to moisture accumulation

A normal roofing membrane is exposed to various mechanical and climatic impacts which can reduce durability and the service life of the membrane. In so called inverted roofs the membrane is situated beneath the insulation layer - which is ballasted with gravel or vegetated substrate. Thus, the sealing is protected versus the above mentioned damaging influences. Drawback of this solution is the direct exposure of the insulation to the outdoor climate. Therefore only materials with a high vapour diffusion resistance shall be used to minimize the entry of water into the insulation. However, the long term experience shows that also XPS boards experience some moisture accumulation especially in case where the cover layer remains humid most of the time. This slows down the drying process towards the top compared to gravel ballasted roofs. Based on long term field tests of green roofs this paper provides new approaches to simulate the hygrothermal conditions in inverted roofs. The model includes the simulation of the moisture behaviour of the substrate and an estimation of the amount of rainwater remaining at the interface between the sealing membrane and the insulation boards. Applying this model allows to asses the moisture accumulation in the insulation and the subsequent increase in the material's thermal conductivity over the life time of the roof. Considering the long-term moisture behaviour helps to specify the thickness of the insulation layer required to maintain the aspired level of thermal resistance over the whole service life of the roof.