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I think I have a big problem – fine white mould is forming on the roof support timbers that support the tiles. These timbers are visibly damp/wet in places, just under the roof felt. There are drops that have fallen on the loft flooring.

Similar fine mould is also forming to a lesser extent on the cross timbers holding up the tile-support timbers.

We had the loft insulated with approx 12-14 ins of fibreglass in spring 08, then I put a new floor over the central roof section, over the fibreglass (i.e. not up against the eaves, where I put slatted shelving instead of flooring.

The cavity walls were insulated (polystryrene beads) at the same time.

I’m pretty sure the problem is condensation not tile-leaks – it’s over most of the roof space, and e.g. there is a drop of water on a light fitting, which has dry wood above it. The current cold weather (-7%C here last night and early morning) may not be helping, if the problem is condensation on the roof felt and timbers.

I suspect the problem may be that the wall-cavity was acting as roof-ventilation, but is now blocked with the cavity-wall-insulation. My reason for saying that is that we had a 2 storey extension built to the back of the house about 20 years ago, with 2 pitched roofs separated by a flat roof. Hence along that rear wall of the main house I assume there was no longer any eaves ventilation anyway. (there is an airbrick in the extension wall supporting the end of each pitched roof).

The house was built in the 60's, with the extension built in the 80's. There has been no previous damp/mould problem. The roof timbers were treated by Rentokil prior to the loft insulation ("fogging" method), as a safety measure (there had been some beetle infestation which I had treated myself a few months before, and was probably resolved but I wanted to be sure before the fibreglass covered over the ceiling joists.

I’d welcome any thoughts on what the problem could be and possible resolution. Also who to ask to come and diagnose it / propose a resolution (eg Rentokil???). Is there any treatment I could put on the roof timbers in the meantime (to minimise the risk of dry rot or similar? Would it help or hinder if I leave the trap to the loft open for now?

The same company did both the fibreglass-loft-insulation and the cavity-wall-insulation, so I’ll call them on Monday. But I'm concerned they may not be expert in diagnosing the problem and have a vested interest in concluding "nothing to do with us gov". I don't want to leave this mould and dampness unresolved for weeks or months while I argue the toss with them.

Many thanks
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