ROY Johnson (Letters, December 28) has condensation forming on the underside of his roofing felt.

Condensation forms when warm air comes up against a cold surface. Examples are breathing on a mirror, cold tap only condensing when a hot bath is run and steamy car windows.

The underside of his roofing felt is cold, especially in winter, and the air coming into the loft from his house is obviously warmer. This is true even in well insulated lofts as no amount of insulation can prevent all heat loss.

If the roofing felt was not there he would not have the problem as the air from outside would be blowing through the minute gaps in the roof tiles and airing the loft continuously!

Now that he has roofing felt the air inside the loft is static and the fresh outside air has no way into the loft. Very recently roofing felt manufacturers have produced breathable roofing felt which helps but on its own will not prevent condensation.

I think that Mr Johnson's problem would be solved by introducing additional ventilation to his loft.

To summarise there are four main ways of eradicating condensation in lofts with non-breathable roofing felt: 1. Soffitt ventilation 2. Vented slates or tiles installed in the main roof areas 3. Vented ridge tiles 4. Ventilation by air bricks to gable ends (if applicable) If anyone needs advice on this matter I will be happy to give it and offer a free quotation for any work required.

ROY KINGDON, Kingdom Building Services, Sholing, Southampton.