New series with Knauf: insulating domestic cold flat roofs

New series with Knauf: insulating domestic cold flat roofs

A new series from Knauf Insulation will look at what products are required for the right application. Using insulation correctly is important to achieve the required performance, and meet customer and regulatory standards. In the first of our technical columns, Bradley Hirst, Technical Services Manager from Knauf Insulation explains what you need to know about insulating domestic cold flat roofs.

Thermal performance

If there is no product or U-value specified on the architect’s drawings, or if you want to substitute the insulation, take advice before you start construction. Thermal performance is stipulated by the building regulations so if you use the wrong product, you will not achieve the intended performance and building control may not sign off the roof.

Lambda values determine the thermal conductivity of an insulation material (the lower the lambda value, the better), but it’s not the only factor that affects the performance of insulation.

Get a good fit

During installation, it’s important to ensure there are no gaps that will allow heat to escape. Glass and Rock Mineral Wool are easier to install correctly than other insulants such as rigid boards because they knit together and maintain a friction fit between joists as the timber contracts and expands. Eliminating air gaps not only ensures the desired thermal performance is achieved, but will also maintain fire safety and acoustic performance.

Fire safety and noise

Many homeowners are concerned about fire safety, so using insulation classified as non-combustible under the Euroclass reaction to fire classification system, such as Glass or Rock Mineral Wool provides reassurance that the insulation will not contribute to the development of a fire or flame spread.

If the property is near an airport, the local authority may stipulate an acoustic requirement too. But if they don’t, it’s worth considering insulation that is excellent at absorbing sound such as our OmniFit Slab 35 which reduces environmental noise and the drumming effect of rainwater on roofs.


On cold flat roofs, ventilation is as important as insulation, otherwise, interstitial condensation will form, which could lead to mould growth. There must be a minimum 50mm ventilation gap above the insulation layer, so factor this into the build-up. If head height is an issue (for example, on some refurbishment projects), consider installing a warm roof, where insulation is installed above the roof deck.

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