combined flat & pitched roof insulation

2 Nov 2007
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United Kingdom
Just need to check a few things before attempting to insulate our kitchen roof space.

We have just discovered it is not insulated at all. The kitchen is the old Victorian out house buildings - converted probably about 20 years ago. We've just gained access to the adjoining neighbour's out house sheds and, because there is a gap in the top of the party wall we have found there is no insulation in our roof space. The gap is big enough to get through so we obviously want to insulate asap.

The roof is original pitched with width added by a flat roof. Looking through the space you can actually see right through to the flat roof space and far wall. The joists are uneven and the area is not large (about 10 metres sq) with the flat roof taking up about 1x4m. All that lies between the roof space and the kitchen is the ceiling plasterboard. I am concerned about ensuring ventilation. I am planning the following - please could someone tell me if I'm on the right tracks?

To use aluminium thermal foil as a vapour barrier throughout.

To increase the insulation in the flat space with polystyrene insulation sheets (it'll also make it easier to get the foil in as I can slide it through when attached)

To top up in the pitched area with blanket insulation.

Do I need to use a vapour barrier throughout?
Do I need to leave gaps anywhere or will the party wall gap provide sufficient ventilation?
Is blanket insulation easy to cut - if so how? Will scissors do it?
should we brick up the party wall after we've insulated - just concerned about nesting vermin - do they like blanket insulation!!?

Any additional thoughts/tips would be very welcome - I've not insulated a roof space before.

Many thanks for your time.
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