Dry Lining a room, adding insulation, the correct way

[markwm]

I'm re-doing a large bedroom in a old house (2 skins of bricks on external walls, no cavity).

No major damp issues in the bedroom, just the odd bit of condensation on the chimney breast. The ceiling is in bad condition, it's bowed and taking it down is not an option as I don't wish to have 10 tonnes of dumped on my head. Timbers are fine, just old are deformed. Lowering is no issue is ceiling are very high.

So the plan is to lower the ceiling around 10 inches, add some loft insulation in the new ceiling void. Plasterboard with 9.5mm.

External walls I plan to strip the wallpaper, exposing the old lime plaster then dryline them using 3x2" timber. My questions revolve around the insulation and ventilation in these dry lined walls.

1. What's the best 50mm insulation to use in between the battens? rockwool, kingspan or polystyrene?

2. How crucial is a vapour barrier? I know I can use foil backed boards but they work out quite a bit more £££. If I use polythene sheet/vapour barrier stapled to the battens, do I just overlap join the sheets with regular double sided tape?

3. This is the main thing I'm concerned about. Do I need ventilation behind the newly created dry lined walls? Should I add a couple of vents in the plasterboards of the external walls? If so, why? and also if I do just how can air circulate behind the Plasterboard anyway as it will be crammed full of insulation?

thanks in advance for your replies.

 

[r896neo]

Don't use 9.5 boards, go for 12.5 as its marginally dearer but much better on a ceiling.

1. Rockwool Kingspan and Polysterene are the three common insulation types and frankly all very different. If you have any rooms above this one then go for rockwool type as it has a sound insulating quality too. If no room above kingspan is by far best as far as u value goes but is much dearer.

Vapour barrier would be a good idea in a ceiling but as for the walls the theory goes that completely filling a void with insulation leaves no room for vapour to condense so... there is no need to ventilate it but it should have a vapour barrier

 

[noseall]

It may be necessary to vent the ceiling void into the loft space. A few holes in the original ceiling will do.