No insulation (and no space for it) = damp

ian1246

MouldyOldDough said:

ian1246 said:

As others have said... insulated plasterboard is the answer.

Basically any moisture in your room is condensing on the coldest surface - the sloping bit, causing the mold.

By insulating it, it ll be less cold & therefore less condensation/mould. Remove the existing plaster, apply new insulated plasterboard - you'll loose a few inches in height but should solve the problem.

You mean on the inside of the ceiling ?

Yes - if you remove the plaster & plasterboard directly behind that will be the roof. Put in new insulated plasterboard inplace of the old plasterboard & replaster - because you'll be installing insulated plasterboard, it ll mean you'll loose a few inches of height in the sloped bit - how much you loose depends on how thick the insulation you go for!

Alternatively you could opt for Kingspan & then normal plasterboard over top - either way, make sure there is a vapour barrier & the edges are taped etc...

FreeBear

ian1246 said:

MouldyOldDough said:

ian1246 said:

As others have said... insulated plasterboard is the answer.

Basically any moisture in your room is condensing on the coldest surface - the sloping bit, causing the mold.

By insulating it, it ll be less cold & therefore less condensation/mould. Remove the existing plaster, apply new insulated plasterboard - you'll loose a few inches in height but should solve the problem.

You mean on the inside of the ceiling ?

Yes - if you remove the plaster & plasterboard directly behind that will be the roof. Put in new insulated plasterboard inplace of the old plasterboard & replaster - because you'll be installing insulated plasterboard, it ll mean you'll loose a few inches of height in the sloped bit - how much you loose depends on how thick the insulation you go for!

If the plaster is removed from the ceiling, there should be space to get some insulation between the timbers - Assuming 3x2 joists/rafters, there would be room to get 50mm of Celotex/Kingspan in there and leave a ~25mm air gap between the tiles. Ideally, the insulated PB would want to be ~115mm thick to hit the required u-value (assuming PIR/PUR insulation). But even if it were 65mm, that would be better than nothing.

If buying a sheet of 50mm Celotex to go between the timbers, I'd fix a second layer along the full width of the room, and then stick plasterboard up over the whole lot. That would keep the loss of headroom to a minimum, and reduce the condensation problem dramatically.

stuart45

Once the ceilings down you will see the rafters size. They should be bigger than 3x2 with that span, which means you can get thicker insulation in.

ThisIsWeird

MouldyOldDough said:

Simplest solution is to simply fit insulated board directly to that sloping bit, nicely mitred to suit how it butts against the wall and the flat ceiling, and screwed right through to the rafters (adhesive too, if you wish).

Better solution would be as others have said - slice along the curved join between the slope and the ceiling, and remove the existing sloping p'board first, and then see if you can also install a layer between the rafters, leaving an adequate (min 50mm) ventilation gap behind it, and then overboard with insulated p'board.

BUT, I have concerns about these stains and where they lie. Surely the timber rafters will be better insulated than just the bare plasterboard inbetween them, which will have no insulation at all? So why the 'cond' where the rafters run? I have a horrible feeling that these rafters may be damp, and that's what you are seeing coming through.

So recommendation is def to remove the sloping bits first - have a looksee, and then a fix.

MouldyOldDough

ThisIsWeird said:

MouldyOldDough said:

Simplest solution is to simply fit insulated board directly to that sloping bit, nicely mitred to suit how it butts against the wall and the flat ceiling, and screwed right through to the rafters (adhesive too, if you wish).

Better solution would be as others have said - slice along the curved join between the slope and the ceiling, and remove the existing sloping p'board first, and then see if you can also install a layer between the rafters, leaving an adequate (min 50mm) ventilation gap behind it, and then overboard with insulated p'board.

BUT, I have concerns about these stains and where they lie. Surely the timber rafters will be better insulated than just the bare plasterboard inbetween them, which will have no insulation at all? So why the 'cond' where the rafters run? I have a horrible feeling that these rafters may be damp, and that's what you are seeing coming through.

So recommendation is def to remove the sloping bits first - have a looksee, and then a fix.

That's exactly what I am concerned about - how would the rafters have got damp ?

it doesn't appear to be through the tiles - this looks like condensation - but how would condensation get to the outside of the ceiling board ?

moneysaver1978

I am not an expert but weird to see these condensation in patterns along the rafters so I would have thought it is an issue with the roof somehow rather than something caused from inside the room

stuart45

Mould often forms by the rafters on this type of ceiling. The rafters seem to hold the damp making the boards damp in those spots. You'll probably find white mould on the bottom of the rafters.

Nebbit

I have solid brick walls which cannot effectively be insulated, as they are not rendered (the external brick is a feature of the building.) After 25 years of black mould every winter, this last winter we have been running a dessicant dehumidifier for 4 hours a day. Every day it extract at least 1 litre of water. It seems to be keeping the mould at bay. 

FreeBear

Nebbit said: I have solid brick walls which cannot effectively be insulated, as they are not rendered (the external brick is a feature of the building.)

The walls can be insulated - Cork or woodfibre internally with a skim of lime. But if you have internal features that you want to retain, that complicates matters.

ThisIsWeird

MouldyOldDough said:

ThisIsWeird said:

MouldyOldDough said:

Simplest solution is to simply fit insulated board directly to that sloping bit, nicely mitred to suit how it butts against the wall and the flat ceiling, and screwed right through to the rafters (adhesive too, if you wish).

Better solution would be as others have said - slice along the curved join between the slope and the ceiling, and remove the existing sloping p'board first, and then see if you can also install a layer between the rafters, leaving an adequate (min 50mm) ventilation gap behind it, and then overboard with insulated p'board.

BUT, I have concerns about these stains and where they lie. Surely the timber rafters will be better insulated than just the bare plasterboard inbetween them, which will have no insulation at all? So why the 'cond' where the rafters run? I have a horrible feeling that these rafters may be damp, and that's what you are seeing coming through.

So recommendation is def to remove the sloping bits first - have a looksee, and then a fix.

That's exactly what I am concerned about - how would the rafters have got damp ?

it doesn't appear to be through the tiles - this looks like condensation - but how would condensation get to the outside of the ceiling board ?

How? I don't know. I'm not an expert in this - it just seemed strange to me, that's all.
If it's cold in that ceiling void - which it certainly will be in winter, with an icy blast flowing up it - then the back of the bare plasterboard will also be very cold. I'd expect that to show as wide general areas of speckled mould growth on its surface. 
Timber is a good insulator in its own right, and yet what we are seeing is that the plasterboard areas which have this 'insulation' pressed against it are the ones showing quite focused discolouration. And that discolouration is really quite intense, so I also suspect it's coming through the p'board from behind. It looks more like 'damp patches' than surface condensation. But, I obviously don't know if this is the case.
How could it happen? No idea, but it implies that it's the timbers wot are damp. How? No idea. Could a steady leak be making its way down the underside of the roof and making the rafters damp? Or, is condensation forming behind the p'board, on the timbers? No idea.
But I'd strongly suggest removing that sloping p'board as part of adding insulation to investigate - really not a difficult task, and a much better overall solution too.