With the cold weather that we are experiencing, the need for our homes to be well insulated is important. Not only to keep us warm but to reduce both our fuel bills and carbon emissions.

Long term, without adequate ventilation, condensation could be a major problem as the moisture, trapped within the loft space, could cause damp and mould growth inside the building and in extreme cases rot the timber.

Cold Roof Construction (Insulated directly above ceiling level)

The requirement for ventilating a traditional cold loft space is that air should enter the loft at the lowest point on one side of the roof and exit on the opposite side of the roof. Heat from inside the building rises through the ceiling forming water vapour that rises through the quilt or fibrous insulation, when it meets the cold air in the loft some of the moisture is absorbed. The quantity of absorption will depend on the temperature.

Warm air can absorb more water vapour than cold air. If the air layer directly above the insulation is constantly changing it stands a good chance of absorbing most or all of the water vapour and take it outside through the low level ventilation.  The warmer the air in the rooms below the insulation, the higher the amount of moisture that can be absorbed. The cooler the loft the less moisture can be held before condensation forms. Consequently during cold weather the risk of condensation forming is greater.

Airflow through the loft can discourage condensation.  Most days we have a slight wind causing a positive wind pressure on one side of the roof and a negative pressure on the opposite side. This is normally adequate to take the water vapour outside the loft space. When conditions are extreme the amount of water vapour increases, low level ventilation cannot disperse it all and condensation occurs.  

When it is very cold or there is no wind (or both) and the building is giving out more heat, high level ventilation is required. Continuous ridge ventilation would be the preferred method. This allows for the warmer air to rise and disperse the water vapour outside via the ridge vents. If ridge ventilation is not possible then tile vents will work. However, they must be put as close to the ridge as possible; if they are halfway up the roof then water vapour will become condensation at the level of the vents.  Air should not be allowed to enter at the ridge only.  Eaves to ridge ventilation is recommended, as it will cope with both wind and no wind situations.

When upgrading your insulation make sure you leave a minimum of 50mm air gap at eaves level around the loft space. If there is no low level ventilation, there are products available to fit into existing soffits and eave vents ensuring a permanent air gap is maintained.  You may also wish to consider upgrading any water pipes or tank insulation inside the loft space as, because the loft space will be colder, there is a increased possibility that they could freeze.  You also need to make sure that loft access hatches are sealed.

For further advice please call Steve Lawless at Elliotts Insulation and Drylining on 02380 623960 and he will be happy to discuss your specific needs on an individual basis.