Multi-layering in inverted insulation
Inverted roof insulation boards, just like warm roof insulation boards, are made using different chemicals and differing manufacturing methods. This results in no two products being the same, and in the case of thermal performance and thickness this plays a critical part in achieving best financial value for a given U-value.
For example, a board with a Design lambda of 0.036W/mK manufactured at a maximum board thickness of 150mm would require 3 layers totalling 360mm thickness to achieve a 0.10 W/m²K U-value. Whilst a board with a Design lambda of 0.039W/m2K and maximum board thicknesses of 100mm would need 4 layers. This not only increases transport and labour costs, it can make achieving the level threshold requirements of Approved Document M impossible.
This multi-layer is further complicated by inverted roof thermal calculation methods, which have to take into account exposure to rainwater through correction factor calculations. In the case of multi-layer applications, the potential for a thin layer of water sitting between the boards needs to calculated, and the accepted calculation method for inverted roof U-value calculations only includes for the effect of up to 2 layers of insulation. As there is currently no data related to correction factors if more than 2 layers are used it is not possible to prove that 3 or 4 layers of insulation can meet a given U-value requirement.
The solution is low lambda inverted roof insulation solutions that achieve the target U-value in the thinnest solution with the least number of layers, and offer other significant advantages that are not always so obviously appreciated: thinner boards mean less volume to deliver leading to fewer truck loads, fewer crane lifts, less storage area required and less labour to install. All of which can reduce overall cost on a project.
But there are also other important reasons why lower lambda XPS solutions, available in thicker board thicknesses are important. As new building regulations lead to more onerous U-value targets, thicknesses in Inverted insulation to achieve those targets are understandably increasing. QI regularly see target U-values as low as 0.10W/m²K; and even lower. In London and the South East, a low lambda board with a design lambda of 0.028W/mK, such as Ravago Ravatherm XPS X 300 ULTRA, can achieve the U-value in 2 layers of 130mm in most roof build ups. A more common 0.15W/m²K U-value can be achieved in a single board thickness of 205mm.
Align this QI@QI with our previous article on the difference between buying XPS by the m2 rather than m3 and you will be bringing together how to buy the right product at the right price for an inverted roof application, and achieve compliance.
Next month: Do you need non-combustible insulation on updtands?