Air holds a certain amount of moisture as invisible vapour. Condensation is caused when the moisture in the air comes into contact with a cold surface and condenses at the dew point (the temperature at which the air becomes saturated). The increasing levels of insulation, impermeable cladding materials and other new methods of construction has led the potential for condensation to rise.

There are two types of condensation: surface condensation and interstitial condensation.
Interstitial condensation is when the the dewpoint is within the structure. This will often be on the warm side of relatively vapour resistance layers.

Surface condensation is when the the dewpoint occurs on a surface and is often noticed when mould or moisture appears on the surface of a wall or window. In many cases, the problem can be solved by simply washing off the moisture or mould or providing adequate mechanical and/or natural ventilation. However, condensation can cause more serious problems, for example:

  • the build up of mould spores may lead to significant health problems.
  • condensation on a window pane may drip down and rot the frame.
  • condensation collecting on roof rafters may lead to degradation of the timber structure.

Architects should consider this condensation risk at the design stage and minimise potential problems by specifying suitable ventilation as well as ensuring surface temperatures are kept high by providing adequate insulation and heating. The ideal ventilation system would extract internal moisture laden air from activities such as cooking, drying clothes and hot showers, to the outside, and replace it with external air.

This could also utilise a heat recovery system for optimum temperature control and energy saving. Approved Document F gives the minimum standards required for the background, purge and extract ventilation of buildings.

Interstitial condensation can be controlled by considering the vapour resistance of the materials used in construction, by ventilating air spaces in suspended floors, cold roofs and cladded timber-framed walls and by the use of vapour control layers.

Vapour control layers provide a physical barrier against moisture in the air reaching the internal structure. They can be in the form of a polythene membrane, foil backed plasterboard, taped insulation or a suitable coating applied to the internal surface of an element. Vapour control layers should be positioned on the warm side of the insulation. Any pipes, electrical fittings, etc. which pass through the vapour control layer should be kept to a minimum and taped and/or sealed.

Thermal bridging can cause condensation to collect on cold spots on areas such as wall-floor junctions, roof eaves and areas around window and door openings. With careful design of the insulation at these vulnerable points, condensation risk can be reduced.

As condensation can substantially reduce the thermal performance of insulation and a building’s structural integrity, a Condensation Risk Analysis can be undertaken at the design stage to assess the likelihood of condensation problems. Many of the insulation manufacturers offer this service.
A Condensation Risk Analysis can predict the risk of interstitial or surface condensation by analysing the components of the building’s elements, the order in which they assembled, the use of the building and its geographical location.

Architects often query the use of trickle vents conflicting with the air tightness required in new buildings. Professionals argue that by providing homes with such little air leakage the condensation problem is exasperated. Although trickle vents must be provided and must be controllable, to work affectively they may need to be open for the majority of the day, making a mockery of the need for an air tightness 10 m³/(h.m²) at 50 Pa required by the building regulations.