Vapour control layer between plasterboard ceiling and fibreglass insulation in roof void, or not?

[SJRSJR]

Hello

I’m getting part of a single story roof re-slated and while the tiles are off I thought I would beef up the 100mm fibreglass insulation which is currently rolled out between the joists, lying directly on the back of the plasterboard ceiling of the utility room and bathroom below.

I’ve read I need a vapour control layer between the insulation and the plasterboard. I’ve also read I don’t as the skim coat and paint act as a barrier.
Which is correct?!

I’ve only lived in places where the insulation was in direct contact with the plasterboard and I thought I would just be adding another 200mm on top of the existing 100mm.

There will be ventilation between the top of the insulation and the tiles/roofing membrane but this is new to me.

If I should do it, is it just a case of rolling it over the joists and putting the fibreglass back on top?

Any help would be appreciated. Thanks

 

[SJRSJR]

I should add I’ve got down lights in these rooms!

 

[noseall]

Hello

I’m getting part of a single story roof re-slated and while the tiles are off I thought I would beef up the 100mm fibreglass insulation which is currently rolled out between the joists, lying directly on the back of the plasterboard ceiling of the utility room and bathroom below.

I’ve read I need a vapour control layer between the insulation and the plasterboard. I’ve also read I don’t as the skim coat and paint act as a barrier.
Which is correct?!

I’ve only lived in places where the insulation was in direct contact with the plasterboard and I thought I would just be adding another 200mm on top of the existing 100mm.

There will be ventilation between the top of the insulation and the tiles/roofing membrane but this is new to me.

If I should do it, is it just a case of rolling it over the joists and putting the fibreglass back on top?

Any help would be appreciated. Thanks

VCL goes directly behind the plasterboard. The roof space should be vented.

 

[SJRSJR]

Thanks, it will be vented.

The plasterboard is already up so do you drape it over the joists and tack it in place.

What about downlights? They have insulation guards but is there a fire risk?