insulation and vapour barrier help

[leighton92]

I'm not a builder I'm a leccy hence the probably daft question but I've got a few bits on insulation I'm not sure of

loft - , not for a bedroom but its going to have a velux. staircase and carpets etc. its basically a clean room for keeping stuff in, its got 3 sloping sides so not much use for anything else

the rafters are 3x1. I'm planning on covering the rafters with a plastic sheet. then fastening 80mm celotex below them followed by plaster board. is this the correct position for the vapour barrier?

the loft floor joists. is it ok to use isowool type insulation in these without any vapour barrier? will I experience any condensation problems?

im also insulating a vaulted kitchen ceiling. ive fitted 100mm celotex inbetween these rafters, is it correct to cover the celotex with a vapour barrier before boarding in this case?

any help would be appreciated. thanks

 

[tomfe]

Vapour barrier should be on the warm side, so between your pb.
You don't need a vcl between floors however if your loft is unheated then it might be a good idea.

 

[CJRatch]

In the loft if your celotex is going below the rafters and not inbetween foil tape all your joins and for belt and braces fit foil backed plasterboard.

The kitchen ceiling should have 100mm inbetween with minimum 25mm fitted below the rafters. Again foil tape these joins and fit foil backed plasterboard for belt and braces.

Done this way on both you'll have a perfect Vapour Barrier and with correct ventilation above you'll never have to worry about condensation in the roof space.

You wont need a VB between the ceiling and loft unless your loft gets excessively cold but with you insulating the loft rafters and maybe running a small heater in winter you shouldn't need to.