Roof insulation in garden office -not enough room for airgap

[rmwebs]

Hi all,

As you've possibly seen from a few of my other posts, I'm building a garden office. The structure is up (it was a 44mm log cabin made by Shire) and I've got my insulation (50mm celotex for walls, 100mm for ceiling, 50mm already down under the floor).

The issue I've got is that the roof bearers are only 12cm in height, thus leaving me with only 2cm air space once the 10cm celotex is in place.

2cm is obviously not sufficient for a 'cold roof' system, so I've now got to look at something along the lines of a 'warm roof' retrofit.

I've lined the inside of the roof with a breathable membrane, covering the joists at the same time.

Would I be right in thinking I can put the celotex right up against the inside of the roof, between the joists and then fix the plasterboard (with a small 2cm gap between the plasterboard and celotex)? I'm assuming I will then need some sort of ventilation for the inside of the room?

I've read a few posts where ^woody^ has chipped in and from what I can tell, one option would be to have the 100mm celotex right up against the roof, and then fill the 20mm gap below it with quilt insulation, meaning the that above the plasterboard, there will be no air at all, hence no condensation. Have I got this right?

Any info or advice would be appreciated.

Cheers,

Rick

 

[rmwebs]

Just to update, the 100mm celotex does actually fill the entire hole, so I wont need to add any extra insulation in there by the looks of things.

Obviously I'll need some ventilation in the actual room, I'll likely go for a small vent above the door or something. There is actually a slight gap around the door, so maybe that'll be enough. I guess it'll be a bit of trial and error.