Loft Insulation

[36TEAK36]

Hi, I live in a standard 1930's semi with a pitched roof. For many years now I have noticed that in winter i get a damp looking patch around the edges of my bedroom ceiling. It does not actually feel wet, but there is a faint black mould all around the edges.This damp is only along the edges that are the 3 external walls of the house. All the upstairs ceilings in the house are curved at the external edges, and it is these curved areas that have the mould. There does not appear to be any problems with the roof tiles, so no leaks anywhere. I checked the loft insulation and it is only about 100mm thick wool type insulation, with standard loft board over the top, and it is probably very old. I would like to remove the existing insulation and re-insulate my loft floor with Kingspan Insualtion, but am not sure what thickness to get. I am choosing the Kingspan insulation as my loft joist heights are only slightly over 100mm high, so I dont think it would be ok use a 270mm wool type insulation as I would then need to compress it down to 100mm so that I can put the loft board back on top. Can I just buy 100mm Kingspan and cut to size? Would it matter that the insulation would then touch the plaster of the ceiling below? Also, is any able to advise how Kingspan effectiveness compares to standard insultaion? For example, would 100mm of kingspan be equivalent to 270mm of standard insulation?

Any advice would be most appreciated.

[Old_Geezer]

Damp ? Mould ? I'd suspect felt/roof despite your check. Not going to be caused by insulation IMO. Maybe the flashings. Get an expert to give it the 'once over'.

[sinderella]

Could it be condensation?

[garyfordwebber]

do you have a bathroom that links with the room?

[buildersmate]

This is a classic problem and it is caused because the edges you describe have less (or zero) insulation. So they are colder than the other areas such as walls or ceilings, and moisture from the air in the house condenses on them more readily. Try to get some Cellotex/Kingspan down the rafters at the edges.
Yes, you cut the foam to shape with a fine toothed saw.

[The Builder]

Also ........... I guess that a 1930s house has been re-roofed at some time. Quite possibly it was done with old type non-breathable felt. If so, your problem (apart from the poor insulation at the perimeters) is a lack of roof ventilation.

Any roofer can fit slate vents for you to give a decent crossflow.

The way to check is to have a look at the underside of the slates. If you see tar type felt, then it's possibly unventilated. If you can see the underside of the slates, then you are ventilated.......... and I'm wrong about the lack of ventilaton. ;o)