Insulating un-felted pitch garage roof

[CptKernow]

I'm looking to insulate my 1950s detached garage. I thought I'd start on the roof by fixing some 100mm Celotex directly on top of the joists.

My main concern is about ventilation / moisture issues, which I have to say I'm not too clued up on. From reading around there seems to be lots of advice (often conflicting) on how to do this properly.

So, the main question is do I need a damp proof barrier / sheet between the roof and Celotex? Or is the fact it is fairly well ventilated (having no felt) mean this is not needed?

Cheers,
Chris

 

[John D v2.0]

So, the main question is do I need a damp proof barrier / sheet between the roof and Celotex? Or is the fact it is fairly well ventilated (having no felt) mean this is not needed?

It's not needed if the ventilation is good, but it won't hurt. You can seal all the gaps around and pipes or writes that go through the plasterboard and it will help and probably be easier.
Celotex will be a pretty good vapour barrier if you tape all the joints, so I'd recommend doing that.

 

[CptKernow]

Thanks for the reply John. Think this is what I'll do.