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September 2015


We recently upgraded our 1980 dormer bungalow house. We fitted triple glazed windows and doors and used external insulation.

The house is now lovely and warm but items stored in the attic are getting wet due to condensation and we have water staining on the rafters.

The pitch is very low on the exposed southwesterly side and it appears that incorrect tiles were used in building the house.

The old sarking felt was replaced with a modern one as the laps on the old felt were not correct. There is eaves ventilation.

We asked a dampness expert to visit. He suggested that we now use extra roof battens to allow more air into the attic. This is going to involve putting up scaffold, taking off the roof tiles and fitting extra battens etc and will be very expensive. Can you suggest any other solution?


From your description it sounds like you have taken good steps towards upgrading an energy inefficient property to modern standards of comfort and energy conservation but in so doing have pushed the limit to which such buildings were designed to be heated and ventilated.

Retrofitting energy efficiency into old properties challenges the old structure to a point where the building physics cannot cope with the new regime unless the new forces at work are properly understood and dealt with.

You have increased the vapour pressure in the house by sealing off draughts with new windows and pushed up the internal temperature without controlling how the vapour moves through the structure; that water laden warm air cools at any step change point and reverts to water as the condensation you are seeing under the sarking felt.

As ambient conditions vary into the winter the condensation point changes and your symptoms might worsen if condensation begins to form in the insulation, potentially damaging the structure or, as I have witnessed, bringing down ceilings with the weight of water.

In new construction a vapour control layer or VCL is fitted to prevent such a problem. This would line the ceiling behind the plaster on the warm side of the insulation in the form of a plastic sheet, fully sealing the property by joining all elements to keep vapour warm.

Having sealed the windows and fitted external insulation you are restricting the flow of air into the house and depleting air quality by reducing oxygen levels as you breathe and further increasing vapour pressure through normal living activities such as bathing and cooking. This is an unhealthy situation that needs urgent action.

In new construction a ventilation system will be installed to ensure healthy air quality and reduce vapour pressure. Some systems also recover part of the heat from the outgoing air and sophisticated units use heat pump technology to recover all the heat and can provide hot water too.

To resolve your problem it may be possible to line your existing ceilings with a vapour barrier and new plaster board and skim but this will be a messy operation and might prove inadequate at junctions. A heat recovery ventilation system should be easy to install in your bungalow’s loft space and will reduce vapour pressure, create healthy living conditions as well as saving you energy. Your local building surveyor can advise.

Fergus Merriman is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland.