ARGH Mould on inside of roof

Rather than insulating the roof, I suggest you look at any breaches in the ceilings allowing warm moist air from the house into the loft, seal those, then increase the ventilation through the loft.

Alternatively, you may have a leaky roof.

Owain

Are you sure it's mould? What does it look like?

I had white furry stuff all over the inside of my last house - but it turned out to be salts leaching out of the wood...

Jim

Almost certainly due to lack of ventilation up there I'd have thought; that wants sorting pronto else the condensation you're seeing is going to cause rot. Do you have (eg) air gaps at the soffits (ie bottom of the roof, above the house walls)? Or tile vents? Or airbricks through any endwalls?

David

Agree with others- ventilation. Had the same problem when builders doing a conversion failed to put in ventilation. White mould over most of the timbers. They put in some air bricks and the problem gradually cleared. It has never returned.

Peter Scott

Sounds like a combination of both poor ventilation and exessive water vapour entering the loft. Seal up all holes into the loft with silicon, where pipes and cables enter and install a sealed loft hatch. If you can, lay a poly vapour barrier over the loft floor (can be stapled down against the joists) and seal edges with tape, then lay insulation over this. This will also reduce fuel bills too.

Mould can be one of a number of varieties, with differing remedial action required.

Depending on what type it is, you may have to treat it or even replace timbers if it is something like dry rot, and not just add insulation and ventilation

However, normal white salts are often mistaken for mould or fungus in roof timbers

dg