Mildew on ceiling

[hedgepig]

Hi folks - this is my first post so hope I'm getting this right.

I have a problem on my bathroom ceiling - it's a pitched roof above, slate with no underlay or insulation. Dust and old mortar is building up in the space between the plasterboard and slates which is sitting in piles near the eaves, this is then getting damp - I think through condensation and these damp piles of dust and mortar are making mildew grow on the ceiling under these piles. I've cut into the plasterboard to remove the build up. What I really want to know is if it's advisable to try and insulate the void - to stop condensation forming in the room itself and in the void and to warm the room up? And if so how can it be done without taking down or lowering the ceiling - if I do it will it compromise the ventilation. I had thought about cutting letter boxes in the ceiling and sliding kingspan into the void? The bathroom has a window and fan for ventilation.

 

[Alastairreid]

You need a vented space between the kingspan and roof covering,

how much depends if you have felt and if it is breathable or not.

How deep are the rafters?

 

[JohnD]

why not make a hatch to climb through, and put a roll of loft mineral quilt between the joists?

 

[hedgepig]

The rafters are 6" deep, there is nothing under the slates - no felt. Think I could get inch kingspan at least and maintain the air gap

 

[hedgepig]

Sorry - I've not been clear. The ceiling and roof are pitched, once you are through the plasterboard ceiling, you've got a 6" gap, then your on the slates - so you can't cut a hatch.

 

[JohnD]

gotcha

you could take the plasterboard down and use a Kingspan or similar product which is phenolic foam slab with a plasterboard face.

 

[Alastairreid]

If you want to do it properly take the ceiling down and clear the debris away from the eaves, you can put 100mm insulation board between the trusses and 30mm over.

Reline with foil backed boards and tape and fill the joints.

A bit more work but will be warmer and mold free.

 

[hedgepig]

I don't want to drop the ceiling - it's such a dirty job in an old house. I think I'm gonna take out small sections of plasterboard between the rafters and slide sections of kingspan up into each cavity. Thanks for the help