Insulation Question (Warm Roof)

[ddsull]

Hi, I have a question regarding regarding a a warm roof and adding some additional insulation underneath.

We have upstairs dormer extension which is used a a home office (small room ca. 2m x 2m). The roof height is very low (probably 1.9m) so I have taken the plasterboard off to get the height between the rafters.

I was going to just plasterboard under the deck (using battens so i wouldn't drill in to the deck). However, I might lay some rigid insulation tight against the ceiling and put the plasterboard over that. I would tape the insulation (and fill gaps) so it would be air tight so might be a vapour layer.

Would I have any issues with this in terms of condensation or should I just leave as is. I read somewhere if the insulation below the deck is less than above it should be OK.


Any thoughts appreciated.

Thanks

 

[^woody^]

I don't understand what's happening to the rafters/joists if you are planning to board directly under the deck.

 

[ddsull]

There are joists on the ceiling. I've taken off the plasterboard to expose them.
There is now a 170mm gap from bottom of joist to deck above.
Plan is to add insulation (25/50mm) and plasterboard over the insulation. This will be held in place with battens which I will attach to the joists. This will give the room a feeling of more space as there will be another 100mm head room between the joists.
Hope the above makes sense.
Thanks

 

[kp00110]

SO exposed timber joists? Are you actually gaining any useable space by doing that? You will still have to lower your head to travel in the room?

 

[ddsull]

Ceiling is already off. Yeah it looks a lot better. Bottom of joist is 1.98 (gained 12mm from removing plasterboard) so no lowering your head. Will paint the joists and make a feature of them.
Any thoughts on the insulation?