Flat roof construction - thermal design and condensation

Thermal design is concerned with the flow of both heat and water vapour through the roof construction and their subsequent effect on the performance of the roof and the various components in the system. The designer therefore needs to consider the amount of insulation required to control both heat loss and condensation.

In the case of a flat roof, insulation is usually in the form of either a rigid board above the deck, or a fibrous quilt immediately above the ceiling, depending on the type of roof construction.

There are three recognised designs of flat roof construction; warm, cold and inverted.

inverted roof construction

Warm Roof

In a warm roof construction the principal thermal insulation layer is located above the structural decking, resulting in the structural deck and support structure being at a temperature close to that of the interior of the building. It is necessary to incorporate a vapour control layer beneath the insulation in order to prevent moisture vapour being forced into the insulation through thermal pressure from within the building. The waterproofing membranes are placed over the insulation to completely encapsulate it.

There is no requirement for roof void ventilation, and cold bridging through the system is easier to eliminate because there are no interruptions from the structural supports as there is in a cold roof construction.

Although this type of construction is normally continuously supported, it can also provide a stable structure directly over most profiled metal decking and is generally recommended for all flat roofs due to the simplicity of design.

Hybrid Roofs

Due to the desire to keep roof levels to a minimum, there is a growing trend when designing a warm roof to place some of the insulation below the structural deck as well as above.  This practice should be discouraged as any thermal insulation placed below the roof deck will increase the risk of condensation forming.  This type of roof construction should not be used unless a full condensation risk has been undertaken, confirming that the risk is manageable.

cold roof construction

Cold Roof

In a cold roof construction, the principal thermal insulation layer is located below the structural decking. The concept for this type of design is usually associated with roof constructions having independent ceilings to support the insulation. Adequate ventilation must be provided between the insulation and the underside of the roof deck to prevent the risk of interstitial condensation forming within the construction.

A cold roof design is not generally recommended for new build flat roof applications due to the added requirement for roof void ventilation and the difficulties in preventing cold bridging through the system.

If an existing cold roof is to be refurbished, consideration should be given to converting the design to a warm roof, or incorporating new insulation into the roofing system to improve thermal performance. However, if the latter is the case, thermal calculations will be required in order to analyse the likelihood of condensation formation.

inverted flat roof construction

Inverted Roof

In an inverted roof construction, the principal thermal insulation layer is located not only above the roof structure but also above the waterproofing, resulting in the waterproofing, structural deck and support structure being at a temperature close to that of the interior of the building. No vapour control layer is required.

This type of construction is generally only used for flat roof applications to concrete support structures due to the weight loading of the ballast that becomes necessary to restrain the insulation against wind uplift and flotation.


The designer should determine the type of insulation required for the construction by consideration of the factors listed below:

  • Required thermal performance (‘U value’) of the roof.
  • The build-up thickness that can be accommodated at roof level.
  • Imposed weight loading to the deck structure.
  • Compressive strength required (the ability of the insulation to withstand loads applied directly onto the roof system surface).
  • The level and type of traffic that the roof will experience both during and after construction.
  • Compatibility with other roofing components.
  • Required fire resistance.
  • Required acoustic performance.
  • Environmental properties.

Bauder manufactures rigid urethane foam insulation products for use within Bauder warm roof systems to provide a totally compatible system. This product has been specifically chosen for the benefits it provides, however, we are able to provide certain other insulation materials as part of the system should this be considered preferable. When inverted design is required, we can provide BauderXPS or BauderJFRI insulation along with their associated Vapour Permeable Membrane


The temperature at which air becomes fully saturated with moisture (100% RH) is called the dew point. When warm moist air meets a cold surface it is cooled, and if its temperature drops below the dew point it will give up moisture in the form of surface condensation.

The air within a building normally contains more water vapour than the external air and so therefore has a higher vapour pressure. This creates a vapour drive from the areas of high pressure to areas of low pressure and water vapour will try to escape by all available routes to the low pressure conditions outside the building.

Moisture vapour is also present in all the permeable materials of a building, including the roof construction, and as the vapour pressure inside a building is continually changing, there is a constant flow of water vapour in and out of the roofing materials. In cold weather the temperature under the waterproofing will fall and can create a zone in the roof structure where the temperatures are below the dew point. Moisture will condense in this zone to form interstitial condensation.

The incorporation of adequate insulation and ventilation in the roof void of a cold roof construction, or sufficient insulation and vapour barrier in a warm roof construction, will prevent the formation of interstitial condensation in the normal range of environmental conditions experienced.

The Bauder Technical Services Team is always available for free ‘U’ value and condensation risk calculations, or for advice on any particular project.  Just call 0845 271 8800.

Building Regulations

Minimum levels of insulation required in flat roof constructions is governed by Building Regulations. These levels are rising on an ever increasing scale as the Government strives to meet its targets for reducing CO2 emissions. Along with increased insulation levels is a requirement for attention to construction details in order to reduce thermal bridging and air leakage where the various elements of a building meet. Guidance on detail design is given by ‘Accredited Construction Details’ which is available from www.planningportal.gov.uk