Combining elements of both cold and warm flat roof design, hybrid roofs appear to offer the best of both worlds when it comes to maximising thermal performance while limiting constructional depth. But as Daniel Bosworth Design & Technical Manager at SIG Design & Technology explains, they should be approached with caution.

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Specifiers have traditionally chosen between two distinct types of flat roof construction: cold and warm. The latter places insulation on top of the roof deck, while the former puts it below (typically between or underneath the roof joists). A third option – that is perhaps less well known – is the hybrid roof, which places insulation above and below the deck (and therefore below the vapour control layer – VCL) with no allowance for ventilation. This can improve thermal performance while simultaneously limiting the overall thickness of the roof. But how effective and reliable are hybrid roofs? What are the risks of choosing a hybrid roof? And how should architects approach their design and specification? Daniel Bosworth Design & Technical Manager at SIG Design & Technology investigates.

Where and why are hybrid roofs used?
Hybrid roofs are typically used on applications that require good thermal performance, but are subject to deck-related height restrictions, such as low-level windows and door thresholds. A hybrid system comprising 120mm of IKO enertherm ALU insulation above the roof deck and VCL, and 100mm of the same product below will provide a U-value of around 0.11 W/m²K. By comparison, a warm roof would require 180mm of IKO enertherm ALU insulation to achieve the same U-value – raising the overall roof height by 60mm.

We have mocked up a typical hybrid roof build up in the drawing below.  You can view the U-Value calculation and the Interstitial Condensation Risk Analysis here.

Hybrid Roof build up example with Armourplan membrane. Achieves 0.11 U-value. All hybrid roofs require
a project specific calculation by the manufacturer (image: SIG/IKO)

Warm roof build up example with Armourplan membrane. Example achieves 0.17 U-value (image: SIG/IKO)

Advantages and disadvantages
The principal advantage of hybrid roofs is that they can achieve lower U-values over large insulation zones while maintaining minimum 150mm upstand heights. This can be particularly beneficial on refurbishment and/or retrofit projects, where there is a need to retain existing features while improving thermal performance. They can also contribute towards a slimmer roof profile, which may be more aesthetically pleasing on certain projects.

The main disadvantage of hybrid roofs is the threat of condensation. In a warm roof construction with no height restrictions, insulation levels can ensure that no condensation occurs within the build-up. However, the presence of insulation below the deck in a hybrid roof brings the dew point down, potentially threatening the roof structure and spaces below it. Prolonged condensation in these areas can lead to damp, mould and rot. To combat this, the Single Ply Roofing Association (SPRA) recommends that hybrid roofs provide at least as much thermal resistance above the deck as below. It also recommends the use of condensation risk calculations to ensure that the dew point remains above the roof deck during service. Nevertheless, insulating between joists on a roof with a low U-value is always a potential condensation risk, and if condensation does form it is likely to be in contact with the roof joists and the deck, running the risk of rot.

A second disadvantage is the risk of the installation not being carried out to the specification, because some of the roof system is installed under the deck, or even before the deck is installed. This may make a hybrid roof more difficult to inspect and ensure correct installation. Where an accredited roofing contractor installs the whole roof from the deck up, their work is easier to inspect, but when some of the insulation is installed beneath the deck it is essential to ensure that the contractor is involved in this work and that it is also inspected.

A further disadvantage associated with hybrid roofs is that they rely on unrestricted access to the underside of the deck. Cabling, ductwork and other services located beneath the deck may preclude the fitting of and space for insulation.

When to use a hybrid roof
Hybrid roofs are best suited to low-risk, low-occupancy domestic projects. They should never be used in high-humidity, high-occupancy applications, such as swimming pools and shower rooms within sports and leisure facilities. SIG Design & Technology would typically recommend a metal-lined, bituminous vapour control layer on a fully-bonded warm roof system for moisture-rich environments such as these.

In light of the condensation risk, architects should carefully consider the building programme before specifying a hybrid roof, ensuring that all parts of the roof build up can be inspected. Consideration should also be given to the servicing strategy, as this may limit the space available for installing insulation below the roof deck. On standard timber-based hybrid roofs, the below-deck insulation is typically cut to size and slotted between the joists. Specifiers should make certain that there is an adequate attachment method for the insulation, if a steel or concrete roof deck forms part of the hybrid roof structure.

Design advice and guarantees
At design stage, architects must ensure that the depth of insulation below the roof deck does not exceed that used above (if the same type of insulation is being specified for both). Failure to do so could lower the dew point, resulting in condensation within the roof structure. Where different types of insulation are being used, the thermal capability of each must be evaluated – particularly in relation to the thickness being specified – to ensure that the underside of the roof does not outperform the topside.

When it comes to specifying hybrid roofs, SIG Design & Technology insists on a U-value condensation risk analysis (UCRA) calculation at design stage. Roofs that pass the test will indicate no annual condensation accumulation, and the manufacturer will then supply one of its high-quality roof coverings, such as Hydrostop AH-25 or IKO Armourplan. The guarantee will depend on the type of covering used (typically 10-25 years) and is generally product specific. SIG’s Design & Technology Accredited Contractor scheme (DATAC) provides assurance that the company’s roofing systems are correctly installed.

Due to the challenges which hybrid roofs place on ensuring correct design and installation, some manufacturers will not provide a full system guarantee for them. Specifiers who wish to use a hybrid roof system should therefore always consult their system manufacturer to determine whether it would be covered by their guarantee.

How should hybrid roofs be maintained after installation?
In terms of maintenance, hybrid roofs should follow the recommendations of BS6229:2003. As such, they should be inspected and cleared of any debris twice a year – or more often if they are surrounded by trees. Failure to follow maintenance guidelines can invalidate manufacturer guarantees.

Contact Details
For more information about the risks of Interstitial Condensation please read our article at