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What’s the difference between a Hot and Cold roof?

A commonly asked question we get, is what is the difference between a hot and cold roof? There are typically two ways in which to insulate a roof. These are, simply put – a warm roof, and a cold roof.

Cold roofs rely on insulation being laid between the roof joists with an air path of at least 50mm between the top and the underside of the roof deck.

This void has to be ventilated sufficiently well to ensure that any moist air which gets into it is blown away before it condenses.

Whilst not recommended, in some cases ventilation is avoided by inserting a breathable membrane between the insulation, and the structure.

In refurbishment projects, ensuring adequate ventilation is very difficult to achieve, particularly given the increased depth of insulation now required under building regulations. Therefore, it is very infrequently used.

Warm roofs are formed by laying a rigid insulation board on top of the roof structure. It is laid over a vapour control layer and the waterproofing is laid in top of the insulation.

The vapour control prevents any moisture in the hot air rising from within the building from reaching the condensation point.

As well as providing all the usual benefits of good insulation, this system also keeps the roof warm and at a more constant temperature, thus reducing thermal expansion of the structure, which can put a great strain on roof membranes.

Warm roof insulation allows the entire roof structure being insulated, ensuring a more energy efficient structure, however the downside is that it does increase height to the roof structure and where roof doors, low lying windows and other details are present, warm roof conversions might not be suitable.