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Wednesday, June 19, 2024

Specifying insulation for inverted roofs

As a manufacturer of STYROFOAM and XENERG thermal insulation,
Dow Building Solutions knows that a roof may only be 2% of the overall budget of a build. However, Dow also knows that in terms of usability, longevity and performance of a structure, a roof??s value far outweighs its cost, meaning its vital that the right decisions are made when it comes to system design, material choice and installation.


With much of Dow??s product going into large scale inverted flat roofing projects roofing manager, Richard Powell spends a lot of time on the roofs of schools, retail outlets and commercial office space discussing delivery, installation and design challenges. Powell reports that in recent months long term performance and drainage issues have been dominating his conversations, as teams tackle decisions about thermal insulation choice for inverted roofs.


He says that while considerations such as thermal efficiency requirements, product availability, ease of installation and cost come first, there are two key things to remember when it comes to specifying insulation for inverted flat roofs, and too often they are being considered at too late a stage:


??    Has the roof been designed with an inverted roof solution in mind?
??    Have all the relevant correction factors necessary for a U-value calculation been taken into account?   


With every client counting the cost of fuel ?? and so much importance being placed on meeting energy standards and aiming for energy saving ?? it??s key that such issues are taken into account in helping to allow an inverted roof system to perform against required demands.  




??    An appropriate fall must be accommodated within the roof design and that the roof must be adequately drained to prevent ponding;
??    The location, size and number of rainwater outlets should be designed in accordance with BS EN 12056-3:2000 and the deck should be without deflections or depressions in which water may pond;
??    Drainage points should provide drainage at two levels: above the insulation and at roof deck waterproofing levels.


For cost reasons, ??zero-pitch?? roofs are not uncommon. BBA Information Bulletin No 4 advises that in such cases it is particularly important to identify correct locations for drainage points and to ensure that the drainage provided is sufficient and adequate. However, it is clear that any roof should not have areas of ponding as a result of back falls – and because insulation must not be permanently immersed or submerged in water, they should be removed before insulation is installed.


Various solutions are available which help to design out deflections without resorting to the additional cost of laying additional screed ?? but such design issues should be adequately tackled at an early stage and not left until after the roof waterproofing has been chosen and installed.





European Technical Approval Guideline (ETAG) 031-1 ?? which sets out performance requirements for inverted roof insulation kits ?? advises that the declared thermal conductivity for insulation should be corrected due to the special nature of the inverted roof application and the fact insulation is being used in exposed rooftop conditions.


ETAG 031-1 states that possible water absorption over time is determined by evaluating the performance of the insulation in respect of two mechanisms for water absorption: by diffusion and post freeze/thaw. Corrected thermal values are determined by assessing the total water absorption potential ?? and it is these corrected values which ETAG 031-1 advises should be used in any U-value calculations for inverted roof systems.


ROOFMATE SL-A, the STYROFOAM extruded polystyrene product Dow offers for inverted roofs, has a closed cell structure which results in low water pick-up over time, despite the rigorous conditions on exposed rooftops. For example, tests shows that even after 300 cycles of freezing and thawing, STYROFOAM will absorb less than 1% moisture by volume, one of the reasons it continues to be specified for inverted roofs and other applications*.




Rainwater able to reach the waterproofing layer on an inverted roof will absorb heat from the underlying structure and affect the thermal performance of a roof system. Therefore, the initial U-value of a roof system must also be corrected by adding a rainwater correction factor according to Section 7 and Annex D.4 of BS EN ISO 6946:2007.


The corrected U-value of an inverted roof will be dependent on the amount of rainfall falling on the roof – which means it will be location specific – and the proportion of rainwater which can reach the waterproof layer.  


Dow Building Solutions offers a water-flow reducing layer, ROOFMATE MK, which reduces the proportion of rainwater reaching the waterproofing. Using ROOFMATE MK in combination with ROOFMATE SL-A helps to minimise heat loss due to rainwater cooling and therefore the amount of insulation required.





STYROFOAM products are CE-marked, meaning specifiers, installers and end-users can rely on the fact that declared lambda values are made in accordance with BS EN 13164:2008 and BBA datasheet No 40/10. Additionally, recent improvements to thermal conductivity of STYROFOAM-A across the product range of 0.002 W/mK – combined with the material??s high compressive strength and excellent moisture resistance ?? will no doubt continue to make ROOFMATE a popular choice in inverted roofing.


However, Dow knows that statements about product performance alone are not enough: decisions about inverted roof insulation choices need to take into account lifetime performance in real conditions on a real roof, not just the laboratory or factory ?? that??s where the true test begins.


STYROFOAM products have been used for inverted roofs in the UK since the 1960s and well before that in North America. Specifiers and roofing contractors can not only rely on that longstanding industry performance when making decisions about insulation materials but helpful advice from Dow’s technical desk when making appropriate U-value calculations which take relevant factors into account: making it easier for everyone to get it right from the start.


* All STYROFOAM products are manufactured in accordance with BS EN 13164.


®™ Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow


For more information email dbusuk@dow.com or visit www.styrofoam.co.uk.





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