Loft Ventilation

Condensation on roofing felt
  • Description

On a nice sunny spring afternoon, take a trip up to the loft and you may discover something or two. Firstly, it can get really hot up there, but you may also discover something else, if you wear eye glasses you may find that they steam up. Why? Read on.

Modern homes are well insulated and built with controllable ventilation to make them as energy efficient as possible. Here lies the problem, when the controllable ventilation is not used properly, the humidity in the air will increase quite quickly. You may notice condensation problems occurring around your home such as condensation on windows or mould growth on cold surfaces.

So how does the air moisture content in the living room cause moisture in the loft?

Two things to remember warm air can hold more moisture than cold air and warm air rises. When this warm humid air filters through cracks in the ceiling or gaps around the loft hatch, it meets the cold air of the loft. Even where there are no gaps in the construction of the ceiling moisture will still proliferate through the ceiling to the loft. At this point the air cools and condensation forms on the surfaces within the loft.

Where no ventilation is provided the moisture in the air will condense and will eventually be absorbed in to the roof timbers. As time goes on this deposited moisture will build up and will continue to be absorbed by the timbers. Eventually the timbers will become saturated and decay will start to occur.

So why do my glasses steam up on a nice sunny spring day?

After a long cold winter a lot of water has been deposited within the loft space. When the heat of the sun in spring warms the loft, the built up moisture in the roof timbers is released into the air hence steamy glasses. If the loft is unvented this moisture will be reabsorbed back into the timbers.

If the attic is ventilated, moisture that enters the loft is dispersed by the wind and taken outside. In sunny summer days the moisture that may have been absorbed is released and the timbers dry out with the released moisture being dispersed to the atmosphere.

It has been a building regulation and a code of practice for a number of years that all lofts are ventilated to prevent this built up of moisture.

So what happens if the loft is not vented?

The consequence of not ventilating the loft is that mold will start to form on the underside of the roofing felt and the timbers will absorb the moisture. Wood in a high moist atmosphere will firstly turn black then wet rot will set in. When wet rot takes hold all affected structural timbers will need replaced, left unchecked they will start to fail and lead to collapse of the roof.

mold on roofing felt
mould on roofing felt
Mould on roofing felt starting to form
Mould on roofing felt more advanced