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Dow Building Solutions - Specifying insulation for inverted roofs: getting it right from the start

As Roofing Manager for Dow Building Solutions – manufacturer of STYROFOAM™ thermal insulation – I know a roof may be only 2% of the overall budget of a build. By Richard Powell, Roofing Manager, Dow Building Solutions

However, I also know that in terms of the usability, longevity and performance of a structure, a roof’s value far outweighs its cost, meaning it’s vital that the right decisions are made when it comes to system design, material choice and installation.      

With much of our product going into large scale inverted flat roofing projects I spend a lot of time on the roofs of schools, retail outlets and commercial office space discussing delivery, installation and design challenges – but in recent months long term performance and drainage issues have been dominating my conversations as teams tackle decisions about thermal insulation choice for inverted roofs.

Of course considerations such as thermal efficiency requirements, product availability, ease of installation and cost come first.  However, besides these requirements, there are two key things to remember when it comes to specifying insulation for inverted flat roofs, and too often I see them being considered at too late a stage:

1.   Has the roof been designed with an inverted roof solution in mind?

2.   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. 

Ensuring adequate drainage

Our position at Dow is in line with BS 6229:2003: that an appropriate fall 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.

We all know that 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.

Thermal resistance

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.

ROOFMATETMSL-A, the STYROFOAM extruded polystyrene product we offer 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 cooling

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.

Taking a holistic view

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, we know 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 our 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. 


For technical support or help calculating a U-value contact our Technical Services Team on Literature and a stockist list can be found at For more details contact Dow Building Solutions by email on and one of the account managers will be in touch.

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

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

Readers' comments (1)

  • My preference would be for a sprayed polyurea (not Polyurethane because of apllication restrictions) over cut to fall expanded foam insulation panels. Dead simple in practise and providing huge heating cost savings and negligible maitenance for the life of the building.

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