Hello to all. This is my first post.

Back in October I purchased a 1930's build semi detached. After spending the winter in a really cold bedroom I have decided to do something now that I have got around to redecorating.
Being a 1930's build all it has a solid wall construction so cavity wall insulation is out of the question. I added an additional 170mm to the existing loft insulation to save energy. However most of the cold air came from around the bay window so this is where I have decided to start.

The construction is as follows. Downstairs there is a solid brick wall on which sits the downstairs bay window. On top of this there is a wooden framework that extends into the bedroom with the bedroom window on top of this. I have removed all the plasterboard from the inside as this was in need of replastering anyway and I need to be able to fit the insulation. On the outside is a layer of felt on top of which roof tiles are hung (same construction as a roof, just vertically). Now that I have removed the plasterboard you can feel a lot of air movement in the void.
A picture of the construction is available at this link: http://s11.photobucket.com/albums/a167/adon30/others/?action=view¤t=BayWindow1.jpg
and the void going down to the downstairs bay : http://s11.photobucket.com/albums/a167/adon30/others/?action=view¤t=BayWindow_Void.jpg

So basically I have a layer of plasterboard with an air gap then a layer of felt covered with tiles. No wonder it was so cold!

The plan is to fill the void from the top of the downstairs window to the bottom of the upstairs window with celotex/ kingspan then re-cover the inside with plasterboard. However nothing is ever that simple and a I now need help with a few questions I have.

1) The gap between plasterboard and the outide is 70 to 72mm. My local supplier of Celotex/kingspan has thicknesses of 50, 60 and 70mm. Now do I just go for the thickest piece that will fit or do I need to leave any sort of air gap in the void?

2) Where the bay angle changes there are two pieces of timber that are very close together (see photo). At this point it would be impossible to fill with any solid form of insulation. My plan is to purchase some cans of expanding foam and fill the gap up that way. I have no idea if this is suitable as insulation or is the best method. Any ideas much appreciated.
Close up photo of wood detailing: http://s11.photobucket.com/albums/a167/adon30/others/?action=view¤t=BayWindow_Close.jpg

3) I have various options also when it comes to refitting the plasterboard. I was thinking of going for a thicker plasterboard (12.5mm) however I can also get plasterboard with an additional 20mm of polystyrene insulation attached (30mm total insulation). Has anyone used this plasterboard? and is it any good as far as insulating properties?
I have an additional 50mm underneath the internal window ledge so the thickness is not a problem.

4) I have read in another post about fitting pvc sheeting between the plasterboard and the insulation. Would I need to consider to consider this in my project?

This is just the start of my bay window insulation. Once finished I will be tackling the void between the roof and the ceiling of the bay. One step at a time though :)
Any help much appreciated, Aidan

Thanks for the reply Tony.

Additional comment below.

1) I will go for the 70mm Celotex then and will be fitting it so that it goes between the ground floor bay window all the way to the upstairs bay window.
There will be two (out of the 10) voids where it is not possible to use a solid sheet material such as celotex/kingspan all the way down to the ground floor. Would a material such as rockwool (fibreglass insulation) be suitable. I have a little left over from when I did the loft or are there other products easily avaliable.

3) On top of the 70mm void that I will fill using celotex/kingspan are you suggesting that I fix additional insulation on top of the studwork anyway? Would I not need to fix battens to the studwork to allow for the extra insulation or would this defeat the object of the extra insulation by creating a cold bridge?
I had a look at the polystyrene backed plasterboard today and there is about 20mm of polystyrene and 10mm of plasterboard. I am assuming this can be pinned directly onto the studwork despite having polysyrene attached to the back?? This leads me to think that I could use any type of insulation at any thickness and then fix the plasterboard over the top?

xtra) I was thinking of using some of the left over rockwool from my loft to fill the bay part between the ground floor ceiling and the floor boards. Only problem is getting it in there. The floor boards are tongue and groove and haven't been lifted for 70 years.

I think air tight will be the name of the game with this one.

Finally finished this project this weekend. Just waiting on the plasterers to do there bit.

This is how I did it.
Bottom of bay:
Filled small gaps between studwork and window frame with expanding spray foam. Filled void between floors with mineral wool/ rockwool. Large gaps between studwork were filled with 60mm Celotex. On top of this a layer of 25mm polystyrene sheet was added with 12mm plasterboard. This worked out to be cheaper than buying polystyrene backed plasterboard and worked out thicker as I could also use 12mm plasterboard instead of the 9mm.

Top of bay:
On removal of the old ceiling of the bay I found the reason for it feeling so cold. A bird had decided to nest in the corner of the bay. After removing what seemed like a never ending nest I also noticed a large gap in the undertile felt where the wind was whistling in. I have since installed a new piece of felt in this area.
Aswell as this the old flat felt roof above the bay was leaking. This halted work for a couple of months whilst I waited for a roofer to replace it.
Once complete and watertight I used 80mm celotex in the roof void with 12mm plasterboard to finish.

I currently have no radiator in this room and it still feels warmer than when there was a radiator and no insulation. So it has definately made a big difference.

Thanks for the help. My next insulation project will be the living room floor. Hopefully this will prevent my feet freezing and me having to visit the doctors again.

May as well use it now that you have it and find out