Insulating walls internally - damp?

[ColitisSucks]

Basically I'm wondering if internally insulating walls (with celotex and plasterboarding over) will potentially seal damp in behind the insulation and cause invisible problems.

No idea where to go for advice, as all the companies offering surveys / advice are also trying to sell insulation / cavity insulation removal etc. So clearly have a vested interest in telling me I need xyz done.

House is a 70s bungalow, has cavity wall insulation. We get mould on the external walls, it's not bad mould, just that grey/green powdery stuff that wipes straight off. We had it on some wallpaper that peeled, and it was on the outside surface of the paper (so the room side, rather than the wall side) so it's not 'in the wall' I don't think. Previous places where that's happened you could see the mould was behind the paper and had grown through it. If we put, for instance, a cardboard box against the wall for a few weeks it will get mould on it, but the wall won't?! It's quite odd.

I just feel if that mould is happily sealed in between the wall and the celotex it might just keep on growing, out of sight, and cause problems (spores / is it a symptom of damp that will cause damage? / will it eventually work its way through the new layers into the room and we're back where we started?)

Not really sure where to go for advice, so wondered if anyone has any advice of types of place to approach? Or if anyone has had similar problems and resolved them?

I have considered hiring a thermal camera once the weather is cold and the heating's on to see if I can identify any gaps in the cavity wall insulation - I know that can cause damp.

My understanding is that it's all to do with having a vapour barrier.

There is moisture in the warm air inside your house (caused by cooking, breathing) which condenses onto the cold external wall. The mould grows in that moisture. If you put insulation up which is vapour-permeable (ie not airtight) you will get a warmer room, but the moist air will just find its way through the insulation to the cold wall behind, condense as before, and you'll have mould behind the insulation.

If you use insulation with a proper vapour barrier (foil-backed usually I think, or you can use heavy-duty polythene) the moist air can't get through to the cold wall and the problem doesn't arise.

Thanks, that makes sense.

I guess, if we have a vapour barrier, the worry then is if the cavity insulation is allowing moisture to track across the cavity from outside? At the moment we could see any damp spots that appear, I don't know how I manage that risk if it's all hidden away.