Vapour barriers & insulation

[Feltwell]

One of the things "to do" whilst revamping one of our attic rooms as a bedroom for my daughter is to improve the insulation. We insulated between the rafters when we did the roof, but were limited as to what depth of insulation we could put in - plus you get a "cold bridge" effect through the rafters. So much cutting and jointing was required that the insulation is bound to be compromised in places.

Hence, I'm now planning to underclad the sloping ceilings with more insulation, board and then skim. The original ceiling remains but gets covered up - unfortunate but if I want to improve the insulation there is no alternative.

Do I need to worry about a vapour barrier? Presumably that is there to stop any moisture getting through to a cold surface and condensing - so more of a worry on a bathroom or kitchen than a bedroom.

Also, I need to improve the insulation on the dormer window cheek walls. The dormer is a rather a nice feature of the room, with an arch-topped sash window (and you can see the horrible water heater from my other post!)

The external roofline runs across the cheek wall of the dormer like this:-

The area above the red arrow is tile-hung on the outside, which leaves hardly any room for insulation behind - it must cause heatloss to the room:-

So again the plan was to clad the wall internally with insulation, plasterboard and skim - would a vapour barrier be advisable?

As far as overall "breathability" in that room goes, there is hardly any external wall - the only bit is the small area below and around the window, and the two tile-hung cheek walls to the dormer as above.

And finally - elsewhere in the room are some cupboards in the eaves, which when you access them they go straight onto the bare brick of the inside of the solid 9" external walls. Again insulation would be good - they pick up heat from the room below. I could try to insulate against the room (so forming a "cold" cupboard) but it would be easier to insulate against the external wall and have a "warm" eaves cupboard. Once again - vapour barrier? Form a gap with battens between the brick and the insulation? We're not talking much wall here - about 2' high, of a total external wall of over 30' height.

I'm very conscious that none of this is breatheable, but in this situation I don't see that it needs to be. Opinions?

 

[MdB]

What sort of insulation are you thinking of using? If you are not worried about breathability then I would use Celotex or similar as its performance is twice as good as anything else. If you tape it up (as you should) then you have an impervious solution so there is no need for any vapour barrier.

 

[AMc]

Agreed tape the joints in the celotex/kingspan/ecotherm with aluminium tape and you've created a vapour barrier.
If you're thinking of using insulation backed plasterboard - which we used - then you might consider a dpm behind, but again once the pb has been installed, skrim taped and skimmed I would think there would be precious little moisture movement to worry about.
I would create warm cupboards to minimise a cold void with a potentially leaky door into the warm room. Should be warmer and minimise the risk of condensation inside the cupboards.

I don't envy you trying to insulate around that window - by the time you've got 50mm or so of insulation and 9mm of board on the cheeks I'm not sure you'll have space to do anything at all. I might be tempted to accept it is a weak point along with the window and concentrate on the places you can do more?

 

[Feltwell]

Yes, it is the Celotex/Kingspan type foil-faced solid foam boards I was going to use. I'm in 2 minds on if to go for the plasterboard faced insulation, or fix plasterboard seperately - if seperate then taping the joints would be easy, if combined then as AMc says a DPM behind is the only way to properly seal which I'm not keen on doing. I do take your point though, by the time you've taped the plasterboard joints and skimmed I wouldn't have thought much moisture would get through.

There is just enough room to have 50mm of insulation + board on those cheek walls.

What about insulation direct onto the external walls, inside the eaves cupboards? In effect it's like dry-lining. Batten the walls to give a small gap behind the insulation?

It's a lovely house with it's complicated roof structure and all the nooks and crannies up in the attic - but it certainly doesn't make it simple to insulate the roof!

 

[Penners]

Batten the walls to give a small gap behind the insulation?

Yes, and a vapour barrier to be on the safe side.

nooks and crannies up in the attic

If you think that's difficult, try having crooks and nannies up there.

 

[AMc]

Well we didn't put vapour barriers in behind our insulation. We battened then insulated between with 25mm then applied insulation backed plasterboard (50mm with 10mm) fixed to the battens (not the walls) as the joins don't coincide you have almost no chance of vapour movement from the inside out. As condensation is the major issue upstairs and a warm wall will not condense I wouldn't worry too much - you'll get all your condensation on that single glazed window I reckon. That said a layer of polythene fixed to battens before you apply insulation/board won't cost much or take too long either but it may be a pain to fit.
Insulation + board will probably allow you to fiddle more with the installation than the combined product, but it will obviously take more time where you have large flat surfaces.

 

[Penners]

a layer of polythene fixed to battens before you apply insulation/board won't cost much or take too long

Quite so. As I said above, it's therefore probably worth the minimal cost and time, just to be on the safe side.

 

[Feltwell]

Thank you all for the help and advice!