Daniel Bosworth, Design & Technical Manager at SIG Design and Technology, discusses how flat roofs on new-build residential projects can be designed to meet the demands of the Future Homes Standard with Architecture Today’s Technical Editor John Ramshaw.

(ph: PREFA/Wolfgang Croce)

Due to come into effect in 2025, the Future Homes Standard is designed to complement the Building Regulations, ensuring that all new homes produce 75-80 per cent less carbon emissions than those built under the current regulations. An interim target of reducing carbon emissions by 31 per cent is set for next year and will be delivered through changes to Part L, which will come into force in June 2022.

Maximising the performance of the building envelope will be critical in achieving these stringent new targets. So, what can be done with regards to the design and specification of flat roofs? What types of construction build-up and materials make the best choices? And what roofing details are critical to understand and resolve? Daniel Bosworth, Design & Technical Manager at SIG Design and Technology, in conversation with Architecture Today’s Technical Editor John Ramshaw, answers these questions and more.

Private house in Chichester, West Sussex, designed by Bayside Architectural Design. The flat roof design comprises two elements: a raised green roof, which sits over the swimming pool and is supported by a bituminous membrane, as well as smaller lower roof linking the main house with the pool (ph: Adam Coupe)

How should architects approach the design/specification of thermally efficient flat roof structures?

At present, the best starting point for the roof fabric on a new-build domestic project in England is a U-value of 0.11 W/m²K. For refurbishment or extension work on existing houses, the recommended figure in England is of 0.18, but specifiers should really aim higher than this if they want to greatly improve the thermal performance of the roof. One potential issue relating to the betterment of U-values is the finished warm roof height. As insulation thicknesses increase so do achieved U-values, however the relationship is not linear i.e. you achieve less performance per millimetre of insulation the thicker the insulation becomes. This is shown in the graph below.

A typical flat roof with a U-value of 0.12 will require around 200mm of PIR insulation, so roof build-ups can quickly become fairly deep. This can have a knock-on effect in terms of achieving adequate door and window upstand heights, particularly where new roofs abut existing structures. In cases where roof depth is restricted, thermal modelling software can be used to boost U-values in other part of the building envelope, such as the walls and windows, to make up for any shortfall.

SIG Design and Technology’s sister company SIG 360 can run SAP (standard assessment procedure) calculations for domestic projects. This is a free service if the sale of the insulation is guaranteed or is chargeable if not. Projects are examined on a case-by-case basis, and the company can also arrange for airtightness testing. In general, early engagement with SIG’s technical team is recommended to avoid potential pitfalls and maximise the overall performance of the roof.

Designed to achieve a U-value of 0.18 W/m²K, the main flat roof is insulated with 150mm of Foamglas insulation laid on a plywood substrate (ph: Adam Coupe)

What is the best type of roof build-up for thermal performance?

Without a doubt it’s warm roof construction in line with the recommendations of BS 6229:2018 (flat roofs with continuously supported flexible waterproof coverings). Unlike cold roofs, where the insulation is located between the joists, warm roofs facilitate a continuous, unbroken layer of insulation across the whole area above the deck. This is more efficient as it eliminates thermal bridging through the roof structure. Added to this, cold roof ventilation is often poorly designed and executed leading to interstitial condensation.

What type of insulation is preferable and why?

There are a number of different types of insulation available, each with its own advantages and disadvantages. Vacuum-insulated panels (VIPs) provide the best thermal performance while limiting the overall thickness of the roof build-up. A VIP insulated roof will be around half the depth of a standard PIR insulated structure to achieve the same U-value. The main draw back with VIPs is that they can be easily damaged on site, which can harm their insulating properties. For this reason, some manufacturers supply them as ‘cassettes’ wrapped in rigid PIR insulation, or they can be site installed with a layer of PIR above and below the panels.

PIR insulation is frequently the go-to option for reasons of cost-effectiveness, thermal efficiency, and availability. Furthermore, SIG can discuss the proposed roof system in relation to the SIG systems tested to BroofT4 fire rating. Rigid mineral wool boards have the added performance benefit of non-combustibility but are less thermally efficient and heavier than both PIR and VIPs. Foam glass board has a very high compressive strength but can be expensive. Like mineral wool board, it is non-combustible, but often has a combustible tissue-based facing.

Designed by Rick Mather Architects, the roof build-up on London’s Centre Point House comprises IKO Ultra-Prevent waterproofing, tapered insulation, an IKO self-adhered vapour control layer, IKO primer and a concrete deck (ph: Terence Smith)

What about airtightness? Can and should this be designed into the flat roof?

Absolutely. SIG flat roof systems are fully sealed, using a combined air and vapour control layers (AVCL). This should always be sealed using the recommended method for each product and connected to AVLCs set in the roof upstands and abutments to ensure the building has continuous, unbroken airtight layer. Jointing laps need to carefully designed into the scheme and care taken to ensure product compatibility between adjoining AVCLs.

Are there other key details to consider in terms of improving U-value performance?

Insulation and airtightness are without doubt the most important factors, but specifiers should consider the entire building rather than just the roof. Obviously, the roof design should be as good as possible, but it is important to examine the performance of the glazing, walls, doors and other fabric elements to optimise the overall U-value.

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