Bay Window Roof Insulation - Advice Please

[Flinstone]

Hi there!

I have a 1920s bay window with sloping roof atached to my lounge.

The original lath & plaster ceiling in the bay (1m deep x 2m wide) deteriorated so I removed it. Since there was no insulation fitted I would like to insulate the bay ceiling as efficiently as is sensibly practicable before replastering.

My idea was to put 50mm Celotex friction fitted between the 50mm ceiling joists which are 300-400mm centres. And because I will not be able to access the roof space once the plasterboard is up I thought I would put ordinary 150mm insulation wool across and over the joists first. The slated roof is underfelted by the way.


So my questions are:
1. First, is Celotex a reasonable idea to put between the joists?
2. Ditto the idea of putting ordinary insulation across and over the joists too?
3. Would the additional insulation over the joists create a potential moisture problem for the timbers?
4. When considering the previous question would it make sense to add a vapour barrier underneath e.g., foil-backed plasterboard?
5. Celotex advise a 50mm air gap above the board. Obviously in my case there is plenty of air space above it before it meets the roof, but does this indicate a need for 50mm clearance on the actual joist timber, i.e., should not have Celotex full height of joists?
6. The Celotex board in Wickes (because it is sold in small sections) does not indicate which type it is. Is it all the same or should I specify a particular type at, say Travis Perkins?
7. I have read bits 'n pcs about timber acting as cold bridges. Would this presumably be negligible for such a small roof?
7. Would 12.5mm over 9.5mm plasterboard be recommended for additional insulation?

I can post a pic of the exposed interior of the roof if it helps anyone.

Thanks and regards,

 

[CyberStGeorge]

Don't need to go to the expense of Celotex.
Put as much wool insulation as you want acroos the top of the ceiling joists.
2x 100mm will do the trick.
The roof needs air circulation.
Unless your felting is breather felt or you va tile vents it is better to use normal fibre insulation than Celotex anyway.

 

[Flinstone]

Thanks for the reply.

As it happens I grabbed some Celotex insulation equivalent from Xtratherm at the weekend so I would like to use it.

Is there a way to identify if the felt is breathable? Assuming I don't have either that or tile vents, I think there will still be air circulation above the ceiling - plus the insulation yet to be added - only because air can be felt passing across the back of the hand in the roof space.

I'm interested in your comment about the fibre insulation being advantageous over Celotex in a non-ventilated space. Why would that be please? (I had thought at the time that the Celotex would limit moisture passing thru compared to the wool).
Do you have any thoughts about thermal bridging by the joists - is it a non-starter for consideration (Celotex/Xtratherm go on about it a lot)?

Thanks again,

 

[CyberStGeorge]

Today with new regs Celotex will refer to thermal bridging. You must bear in mind that if you have no Soffit under your guttering, ie between the wall/window and the fascia board your guttering is probably connected to, I say probably because your guttering could be attached to the radter ends, you have no constant flow of air.
You will get air circulating because of tiny inlets between the tiles and felt ends etc.
1920's houses did not commonly use a space as described, although it is not unknown as many of todays buildings have their construction based on 1920's regs and inventions.
As for whether or not you have breather felt, it is unlikely if your roofing was re roofed in the last 10 years. It has been re roofed as you do have felt.
The most common felt is a black bitumen type. It could be black plastic with small squares on it., can be silver on the underside.. Breather felt is a material like cotton in construction usually blue or green on the outside and white on the inside. Will not go into makes. There are many types.
Basically the Celotex you purchased can be put between the rafters and/or over the top also. Do not worry as you are doing a belt and braces job.
Just leave a good size void above.

 

[CyberStGeorge]

Forgot.
The reason why I pre to use fibre as opposed to Celotex is simple.
The fibre is not sandwiched between foil.
If you get condensation your central heating will help the condensation to evaporate.
If Celotexing between ceiling joists then allow a small gap either side of the joists 1-2mm is enough.
Put small nails in to hold it up while plasterboarding.

 

[Flinstone]

Thanks,

If Celotexing between ceiling joists then allow a small gap either side of the joists 1-2mm is enough

Sorry if this seems a daft question but does this refer to the creation of a vertical gap between each joist and the Celotex (to allow the passage of warm air into the roof space), or a horizontal gap above and below the Celotex (to allow ventilation in and around the top & bottom of the joists)?

Regards,

 

[noseall]

all timber framed buildings have an internal vapour barrier.