Up until the late 1970’s we predominantly used open cavity construction in the UK.  Air bricks were installed at low level to allow air to pass into the space under the timber floor and into the cavity walls.

The tops of the cavity walls were left open so air was allowed to flow to and from the cavities into the roof space.

This all meant that cold air passed all around the outside of the living areas.

However, because the construction was so generally so open, the thermal performance was extremely poor, with little or often no, insulation installed.  In these situations, condensation was not usually a problem.

Since the early 1980’s we have been closing cavities at the top and using cavity liners to prevent ventilation into the cavities.

At the same time we have steadily increased the level of insulation in the roof and now include it in the cavity walls.

Also, in more recent years we started to better understand the thermal benefits of more air-tight construction.

This means that controlled ventilation of both the living space and structure becomes all the more important.

How condensation occurs:

Condensation is the conversion of water vapour to water – which occurs when a surface is cooler than the dew point temperature.

Dew point is the temperature at which the air is fully saturated with water vapour.

Air can hold a certain amount of water vapour, depending upon its temperature – the warmer the air, the more it can hold.  Therefore, the risk is that as warm air passes from the living area into the cold area, it cools and loses its ability to hold so much water vapour. It then deposits the water vapour as condensation on cold surfaces.

An example: In a two-storey house with a floor plan of 100 m2, there is a total of 420 cu metres of air, which, at 20 degrees, could potentially hold up to 8.4 litres of water vapour.

If this warm air passed through the ceiling into the roof space and cooled down to 10 degrees, it could then only hold 3.4 litres of water vapour.  So 5 litres of water would be deposited somewhere.

It may not be as dramatic as that in practice, but this serves to illustrate the potential risks.

Eaves ventilation
Air gap at eaves or low level; e.g. over the fascia or within soffit.  Can be a continuous gap, or soffit or tile vents set at regular spacing to give equivalent of the required gap.

Ridge ventilation
Air gap at ridge or high level.  Can be a continuous gap through dry ridge system, or tile or ridge vents set at regular spacing to give equivalent of the required gap.

Cold roof
When the insulation is laid over a horizontal ceiling with a large space above; e.g. an uninhabited unheated loft space.

Warm roof
When the insulation is installed above and/or between rafters, with either a small gap or no gap above the insulation; e.g. where there is a habitable room in the roof space.

Impermeable underlay
Also referred to as ‘high resistance’ (HR).  Allows little or no passage of water vapour through its structure.  The technical definition of an impermeable underlay is one where the vapour resistance is more than 0.25MN s/g.

Vapour permeable underlay
Also referred to as ‘low resistance’ (LR).  Allows the passage of water vapour though its structure.  The technical definition of a permeable underlay is one where the vapour resistance is not more than 0.25 MN s/g.

Vapour resistance
The vapour resistance of a material is a measure of the material’s reluctance to let water vapour through.  The vapour resistance is quoted for a particular thickness of material and is normally measured in MN s/g (Mega-Newton seconds per gram).

Air and vapour control layer (AVCL)
A single layer or membrane designed to restrict air movement and resist the passage of water vapour through a structure such as a ceiling. To be effective the vapour resistance should be greater than 200 MN s/g.

(as defined in BS 9250)

‘Air open’ roof tiles and slates
Most concrete and clay roofing tiles are regarded as air open and will dissipate water vapour passing through the underlay into the atmosphere .  If the roof covering is not air open then the batten cavity will require low and high level ventilation.

Well sealed ceiling

• A well sealed ceiling is designed to limit the passage of air through its structure by avoiding construction gaps, particularly at wall/ceiling junctions and those around pipe and cable penetrations.
• Loft access hatches must not be located in rooms where large amounts of moisture may be generated, such as kitchens and bathrooms.
• Air leakage through an access hatch should not exceed 1 m3/hour at a pressure difference of 2 Pascals.
• Total air leakage through downlighters should not exceed 0.06 m3/hour/m2 at a pressure difference of 2 Pascals.
• Recommendations for the construction of a well sealed ceiling are given in BS 9250: 2007.