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.

RHI (Renewable Heat Incentive)

To encourage the conversion of heating systems to renewable heat technologies, the government launched  the domestic Renewable Heat Incentive (RHI)  in April 2014.
This is an incentive administered by the energy regulator Ofgem E-Serve which provides financial assistance, helping people to make their homes more energy efficient . The RHI goes some way towards reaching the government’s  target  of producing 12% of the UK’s heating from renewable sources by 2020.

Renewable heat sources use naturally replenished energy rather than fossil fuels to generate heat.
Renewable heat technologies applicable to the scheme are air source and ground source heat pumps, biomass boilers and solar thermal technologies. Properties reliant on the live gas grid are likely to save the most on fuel bills.

Renewable heating technologies are much more effective in well insulated buildings.
Therefore, to qualify for the incentive, the property must first have a Green Deal Assessment carried out and any insulation recommended by the assessor will need to be installed.

The scheme is mainly used for single homes and can be taken up by homeowners, social and private landlords but is not available to new build properties other than self-build projects.

Payments for the hot water and heat generated are index-linked for inflation and made to the applicant quarterly for seven years. The amount received depends on the type of system installed and the size of the property.
However it can be up to 8.5p/kWh. A payment calculator developed by the Department of Energy and Climate Change (DECC), the Scottish Government and the Energy Saving Trust can be used to work out how much is likely to be paid.

In the first seven weeks of the Domestic Renewable Heat Incentive opening, 1,000 installations were accredited onto the scheme. Although advertising to the general public appears to be lacking.
Most people in the best position to take up the incentive don’t seem to be aware of its existence. It is only when other work is already being carried out on a property that the architect may mention this scheme.

Is it a missed opportunity to really improve the energy efficiency of the existing housing stock?
Or could it be that limited funds mean that the government can only make a token effort?