Insulating under ceiling

[Crakkers]

Hello.

My kitchen has a side extension with a pitched roof about 25 years ago, before we moved in. Upon investigation there is minimal insulation above the plastered ceiling.

Above the "existing" / "original" kitchen ceiling is a bathroom.

The roof space above the extension is well ventilated and very drafty in winter. It looks like the ceiling under the original ceiling has been lowered by about 6 inches to match the extension. There is no insulation above this part of the ceiling.

The kitchen is cold and leaks heat like a bald head in winter!

To save pulling the existing ceiling down my plan is to insulate from below, as we have ample height. I also have a boxed in beam that is lower than the ceiling and this basically splits the ceiling in half. This is about 150mm lower and so I also hope to make the ceiling one level and flush across the whole room.


My proposal is to;

a) use 150mm loft insulation and PB over, or
b) use loft insulation plus 25mm kingspan backed PB. or
c) use 150mm kingspan backed PB - but very expensive!

Any advice would be appreciated, especially thoughts on any vapor barriers etc or condensation control needed.

Thanks in advance

C.

 

[oldbutnotdead]

If the void between kitchen ceiling and bathroom floor is airtight then there's not much point insulating that bit- heat loss to a heated room will be minimal.

The Kingspan route (150mm) will be the easiest and thermally equivalent to 300mm of rockwool or similar.
Vapour barrier- if you save yourself a little cash by fitting the Kingspan first then plasterboard you can foil tape the joins and there's your vapour barrier

If you can get into the pitched roof void then put 300mm of rockwool above the ceiling (keep eaves clear for ventilation) for the same thermal gain, much cheaper and no need to redecorate (unless the only access is to put a hole in the ceiling)

 

[Crakkers]

Thanks. Keeping the insulation and pb separate does make a lot of sense.

 

[John D v2.0]

If the kitchen side is cold it must be draughts from the outside, block those up. That void should effectively be an internal space. On the extension side the room should be insulated as normal with wool on top of the plasterboard.
Make sure you also insulate the side wall between that cold loft and the existing kitchen ceiling.

 

[Crakkers]

Thanks. Was indeed thinking of doing the outside wall. But no easy way to insulate above ceiling hence question about below.