My bathroom is on the 1st floor at the back of my victorian terraced house, and as such has it’s own small sloping roof separate from the main roof.
I am about to gut the bathroom completely as part of ongoing renovations, including taking down the ceiling and replacing it.
Currently there is an ageing ceiling which appears to be made of some sort of plaster, above that there is a small gap of a few inches, and above that the roof tiles – no insulation whatsoever!

I of course want to insulate as best I can before putting up a new plasterboard ceiling.
Since space is tight and I do not want to lose headroom I was planning to use solid insulation boards (Kingspan or similar), probably 100mm thick, between the roof tiles and the new ceiling.

So my question is, is this the best solution and if so what should I do about moisture?
Do I need to use a vapour barrier to stop moisture from the bathroom from forming condensation on the insulation panels?
If so I assume it would need to go beneath the insulation, right?
And how would it best be joined where it meets the walls?

Any advice would be appreciated!

You normally need 55mm of ventilation above the foam and beneath the tiles so there is air flow to stop the rafters rotting.

Cut the sheets to fit in between the rafters and then tape over all the joints with foil tape to stop any draughts:

8 Dec 2012 14:26 by brf, on Flickr

8 Dec 2012 11:52 by brf, on Flickr

Why not just cellotex between the ceiling joists?

From the Celotex installation guide:

Celotex between rafters by brf, on Flickr

Why not just cellotex between the ceiling joists

there aren’t any, just the rafters

So the ceiling is sloping within the bathroom ❓

Sounds lots like my utility room which has the slopey celing thing going on.

If that’s the case I’d put up a false ceiling and insulate above. You can get some nice recessed lights fitted also. That’ll save all the mess of taking down the existing.

For the skeiling in my extension I used solid insulation board between the rafters (with ventilation between the board and tiles) and an extra layer inside the rafters which was then plasterboarded over. All signed off by building control so may be the correct way……

Thanks for the info footflaps. So does the silver foil and tape on those boards act as a vapour barrier? Or do you need a vapour barrier in addition to that?

there aren’t any joists, just the rafters

– correct.

So the ceiling is sloping within the bathroom?

– yes, and it slopes down to under 6 foot high on one side, so not possible to put up a flat false ceiling (at least not over the whole room anyway, maybe half of it at most.

@Rubber_Buccaneer – what did you do about the vapour barrier issue in your case?

It’s just occurred to me – is it unnecessary to use a vapour barrier if you have adequate ventilation space between the insulation boards and the roof tiles?
if so is this true even in a bathroom (which will of course have a lot of vapour)?

Use 50mm Celotex between the rafters.

I screwed battens into the sides of the rafters to make sure the cleotex sat level with the inside of the rafters and there was no air gap between it and the plasterboard and to maintain the air gap between the celotex and the tiles.

Then use plasterboard with a 15mm celotex backing on it. You’ll have 65mm of insulation, only have lost 15mm of head room (less if the lathe and platser is very thick and still have air circulation under the roof.

I don’t recall adding a vapour barrier, the insulation was foil backed though and the skeiling is in a bedroom so nothing like as much moisture as a bathroom. Also, the ventilation above the insulation means in reality I’m not worried about it.

Damn, double post.

I genuinely wouldn’t fanny about with it then . Heat loss in one of the warmest/smallest rooms in the house surely isn’t that big an issue.
Overboard with insulated plasterboard losing you 35mm then skim.
However I would fit a quality extract fan.

Cut the sheets to fit in between the rafters and then tape over all the joints with foil tape to stop any draughts:

I sealed up small gaps with expanding foam. It’s a messy job, though.

Thanks for the advice guys.

Sounds like no-one is bothered about installing a vapour barrier then…?

I’d be placing 50mm PIR board between the rafters, presuming they are 100mm deep, to leave 50mm void above them.
Then fit as thick a layer of the same to the underside of the rafters and tape the joints of this layer before you plasterboard, this works as a vapour barrier.

If you are only insulating between I wouldn’t place a vapour barrier, you’ll only get condensation forming at the joist line or where any wee gaps have been left.

Building control would probably look at ventilation to the void as well if you contacted them, simple case of ventilating the eaves but as it’s pretty old you may have this by default

just overboard with celotex backed board if you want clean and fast solution. Thicker the better but remember gains become marginal after 50mm

if old ceiling must come down go with the celotex between rafters, remembering to leave a decent air gap

just overboard with celotex backed board if you want clean and fast solution

I gotta say this is pretty tempting, as I am guessing that taking down the old ceiling will be more of a pain than it seems…

However I think I’ll have to do the proper job as I don’t want to lose any more room height, and I’m already losing about 40mm by laying a new floor.

Thanks again for all the input everyone.