Insulation for bathroom roof

[seank]

My bathroom is cold in winter and very hot in summer. I cut an exploratory hole in the ceiling and found there is no insulation, just TILE->FELT->70mm RAFTER->10mm PLASTERBOARD. This part of the roof is angled at about 35 degrees, beneath the eaves of the back of the house.

I am wondering if anybody knows of a high performance rigid insulation I could install if I take down and replace the plasterboard. I realise that as things stand at the moment, if I keep a 50mm air gap beneath the felt, I only have room for 20mm insulation (in addition to the thickness of the plasterboard). Sheets of polystyrene seem the obvious solution, but I'm not sure whether they could be described as 'high performance'.

I assume I would need to attach a vapour barrier, either separately or as foil-backed plasterboard. But would that serve any purpose if the insulation was rigid and watertight?

Look forward to reading anyone elses thoughts

 

[bathjobby]

You could always insulate underneath the felt between the rafters as they do in loft conversions, and then polystyrene horizontally between rafters before fixing new plaster board.

 

[Nige F]

Kingspan is what I used in my chalet bungalow when I reboarded some skeilings helps having a son and mates who are spreads

 

[salem2000]

You could always insulate underneath the felt between the rafters as they do in loft conversions, and then polystyrene horizontally between rafters before fixing new plaster board.

I wouldn't use Polystyrene as this can degrade cables. Use kingspan or Celotex panels. I believe you will need to leave an air space between the the felt and the insulation for ventilation of the roof space.

Salem.

 

[seank]

Thanks nige and salem! I will check out kingspan and celotex. But I am curious, how does polystyrene degrade cables? You mean electricity cables do you?

 

[salem2000]

Yes, electrical cables. A chemical reaction can occur when in contact with Polystyrene, this can degrade the cables insulation.

Salem.

 

[bendylow]

why not leave the old ceiling where it is and overboard with thermal backed plasterboards,50mm drywall screws will hold them in place,just make sure you hit the joists with the screws,it's quicker and cleaner than taking it all down.