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18 May 2017

Maximising Thermal Performance in Roofs

Flat roof construction can normally be categorised as either cold roofs or warm roofs, depending on the position and efficiency of the principal thermal insulation layer. In a warm roof, the structure and substrate stay warm, below a rigid board insulation. With a cold roof, the substrate and structure are above the insulation and will become cold (or hot) to equalise or match the external temperature.

Warm roof design is mandatory in European flat roof practice, while New Zealand has traditionally accepted the cold roof solution. This is rapidly changing as we realise the benefits of warm roof design and construction.

The warm roof system provides a continuous insulation layer, while the cold roof is often bridged by the structural elements. Cold roofs provide a cold surface which risks interstitial condensation forming on the structural elements, potentially causing unseen damage. A cold roof is therefore required to be ventilated to enable damaging moisture-laden air to escape the roof cavity for the sake of the structure. It is worth noting that ventilation is required not for the membrane, but for the structure. In many flat roof designs this is achieved with ventilation domes.


Left: Typical Cold Roof showing cold-bridging & ventilation dome.
Right: Typical Warm Roof with continuous insulation.

However, the ventilation is not well proven or regulated. How moisture performs in a closed space is not predictable. The vents through the roof are less than ideal and in some cases can also allow cold damp air to be drawn inside the structure. Furthermore, the R-Value in a warm roof design is likely to retain its thermal value, whereas the insulation in a cold roof can be degraded by the invisible impacts of interstitial condensation and moisture to the insulation material. Most insulation products have reduced performance when wet or damp.

Installing the same R-Value insulation product into a warm or cold roof will result in very different construction R-Values. The disadvantages of a cold roof system are clearly demonstrated when comparing the two methods in Design Navigator.

R value Table

Warm roof construction has the principal thermal insulation layer placed above the structural deck and immediately below the waterproofing membrane. Locating the insulation above the structural deck is desirable, as the insulation is laid in a continuous plane and the stated R-Value of the insulation product is therefore achieved.

The structure and substrate is protected from thermal extremes and will remain stable and last longer. In a warm roof configuration, a vapour barrier layer is recommended to achieve the highest performance standards.


Nuratherm Warm Roof System (see full details here).

Because the insulation is kept dry between the membrane and the vapour barrier, it will retain the R-Value performance. In contrast, cold roof insulation products often degrade over time. Normally insulation finishes at the top of a wall. With a warm roof, the designer must ensure that the wall insulation is specified to extend to the base of the roof substrate.

There is the view that certain regions in New Zealand aren't suited to a warm roof system. Some believe that they are unnecessary in Auckland and the Upper North Island as those areas have a temperate, sub-tropical climate. However, often people forget those areas also have the highest average relative humidity levels in the country. Therefore, they are prone to condensation build up in the roof cavity.

The Nuralite Warm Roof System performs in all climates and conditions. Nuralite considers it the future of roofing, with the system helping to create warmer, drier homes and achieve higher thermal performance in buildings.

nuralite warm roof system in action

View more information on Nuralite, including contact details.
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