I READ Roy Johnson's comments regarding condensation in his loft (Letters, December 28).

Condensation is caused through hot air meeting with cold air and warm air always contains a lot more water vapour than cold air.

If you cool the warm air it can no longer hold the water vapours.

Example: warm air is cooled by the cold glass of the windows in a room and the excess water forms into tiny water drops on mist, this can also happen inside walls if they are porous and air is allowed to seep through and it can lead to damp walls.

While extra insulation in a house can reduce condensation on the inside of rooms and windows it can also increase the amount of damp inside walls, unless precautions are taken to prevent this, by having all the outside facing walls cavity insulated.

Of the heat lost from insulated house 20 per cent goes through the roof.

Preventing this loss is one of the easiest and the most profitable jobs. If you look at a house roof after we had frost or snow, one can see how well insulated the house is. As warm air lives in a house, the ceilings in the upstairs rooms are warmed and the ceiling with poor insulating properties (high U' value) becomes a liability.

In a two storey house with 25mm (1 inch) insulation blanket in the loft (as specified by the 1965 Building Regulations) 20 per cent of the total heat produced in the house will leak away through the bedroom ceilings and into the roof.

I have not long moved from a three-bedroom house to a flat as my children are all grown up.

When I was in the house I had a problem with condensation. I insulated all the loft space and still I had a certain amount of condensation, so I put in an air vent one on both sides of the roof and this cured the problem as it creates an air flow through the loft and keeps it dry and as for the amount of heat loss it was not worth considering.

I hope this will go some way to helping Mr Johnson's problem.

JM FAHY, Lordshill, Southampton.