Crucial consideration in calculating U-values in inverted roofs
Over the years many new XPS and EPS manufacturers have joined the externally insulated Inverted roofing market, giving designers and architects access to significant innovation and choice. The inverted insulation system protects the waterproofing with a highly non hygroscopic insulation which, installed correctly, reduces ageing from sunlight degradation, significant changes in temperature and the vagaries of weather, whilst insulating to a high level for the lifetime of the building.
Consistency in the calculation of U-values in inverted roofs is perhaps more important than in any other area of flat roofing. The performance of inverted insulation is not only affected by its declared lambda, but by its ability to perform at that level for the long term, so its structure and resistance to degradation through freeze thaw must be considered. In addition rainwater cooling of the deck will have a significant effect on the overall U-value achieved. Precipitation is obviously different in various parts of the country and this information must also be included in the calculation to ensure that the insulation system performs to the required level for the life time of the building.
Inverted Insulation manufacturers have mitigated some of these negative effects with the inclusion of a WFRL (Water Flow Reducing Layer) which, installed correctly over the insulation, moves the water away to drainage outlets at that level, negating some of the effect of that rainwater cooling the deck. This is not another waterproofing layer, it is not total protection from water ingress and being vapour permeable also allows moisture beneath the WFRL to evaporate over time. Therefore, it can correct some of the effect of rainwater cooling, not negate it.
ETAG 031 introduced a means to calculate and asses all of these issues within the U-value, which subsequently became referred to as a Design U-value calculation. This, together with BS EN ISO 6946 and subsequent BRE and BBA recommendations and updates, has been accepted as the industry standard by which all inverted insulation should be compared. It has given architects, designers and ultimately building owners a calculation that they can trust and which should deliver the required thermal value, for the lifetime of the building.