Moisture settling on purlin causing damp. Common causes?

[pb2012]

Hi all,

I'm hoping for some advice on a troubling issue.

The situation is:

- house undergoing full renovation
- Current works mainly consist of full house replastering so quite a bit of moisture
- recent loft conversion carried out as part of renovation

All is well, apart from a soggy purlin which become most evident after plastering and some painting. The plaster started sucking up the moisture from the purlin.

The ridge and tiles seem ok but we'll need to get up there and properly inspect I suppose.

Judging from the picture, do any obvious problems/causes jump out and grab you??? It would seem moisture is getting down the back of the insulation boards behind the plaster, and running down on to the purlin, but I'm no expert.

Huge thanks in advance!

 

[Alastairreid]

Have you left a ventilation gap above the insulation and installed a vapour barrier?

 

[pb2012]

There is a ventilation gap above the insulation as far as I'm aware. Whether or not air is correctly passing through it I don't know.

There isn't an individual vapour barrier, other than the foil backed insulation boards themselves.

Does it look like it could be condensation forming and running down the back of the insulation boards, settling on the purlin? It sees a fair amount of moisture.

Having had more of a poke around there is definitely condensation on the back of the exposed insulation boards in the eaves.

 

[Alastairreid]

The eaves are still open to the room in the pic, moisture will still get in till you finish the walls.

 

[wwwebber]

How deep are your rafters ? and what thickness of PIR board has been installed ?. Also, is the plasterboard backed with insulation and how thick is this ?

Lastly, does your roof have a membrane ? - if so, is it a breathable membrane ?

Sorry, a few q's here - just need to ascertain what is going on etc.