Condensation risk when insulating above and below the deck
As World Governments seek to address the issues created by climate change, legislation to reduce carbon emissions is increasing the thermal performance requirements of roofs and walls. With legislation change increasing the U-values, requirements for flat roof insulation board thickness is becoming greater and greater. This increases the overall roof height and is leading to conflict between achieving the thermal performance and achieving the required threshold, upstand and balustrade heights, in both warm and inverted roof design.
One solution often proposed by specifiers is to use insulation both above and below the structural deck, but this itself leads to a number of potential issues such as taking up ceiling void space needed for M&E services, split responsibility for installation of the insulation board, the need to fix insulation to the underside of the deck and most importantly of all, the creation of condensation in the ceiling void beneath the structural deck. From a roofing manufacturer’s and roofing contractor’s perspective the biggest potential issue here is the creation of condensation, which is often mistaken for a roof leak: and none of us want that call back.
Condensation is an accepted factor in thermal performance calculations, with the creation of condensation managed through calculation to BS 5250:2011+A1:2016 Code of practice for control of condensation in buildings, but if these calculations are not run or are ignored there could be trouble ahead.
Let’s look at an example of over/under insulation to explore the risks.
You are asked to provide an inverted flat roof system to achieve a target U-value of 0.11 W/m2K. A high-performance market competitive XPS insulation such as Ravago Ravatherm XPS X 300 SL would need to be 300mm thick to achieve the target U-value. BUT the specifier tells you the maximum thickness the roof can accommodate is 140mm and that they want 100mm of foil faced PIR on the underside of the slab. This would certainly achieve the U-value, but a condensation risk analysis shows that 3 grammes of condensation is formed per m2 over January and February. In a 90m2 roof that’s 270 grammes of water in the ceiling voids; a quarter of a litre of water. That’s going to cause wet plasterboard and the roofer is going to get a call to look for a leak that isn’t there.
On the face of it, it may seem a reasonable assumption to accommodate more insulation within the structure to enable the reduction of insulation above, but in truth this can cause more problems than it solves and unless a specific condensation risk analysis is run, the risk will not necessarily be seen as an issue within a standard U-value calculation alone. As a rule of thumb, if the insulation above the deck is not twice as thick as the insulation beneath the deck there will be condensation forming.
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