Insulating a bay window

littledug · 24 Apr 2016

The plaster was loose underneath the bay window in my 1940s semi. So once I'd ripped out the lath and plaster I wanted to the opportunity to insulate the bay before it is replastered.

There appears to some kind of felt exposed now (see pics below).

So it would be great to get some advice on how to do this.

I was just thinking that I'd get some 50mm Celotex (or similar) and place that between the joists (joists are 50mm) then just fill any tiny gaps between angled joists with expanding foam.

Do I need to tape over all the joints where the Celotex meets the joists?
Do I need to worry about air gaps for ventilation, thermal bridging etc?
Is is best to add another thin layer of Celotex over the top to cover the joists before adding the plasterboard?
Is it best to use insulated plasterboard on top of the Celotex?
What type of insulated plasterboard - foil backed or just normal 12.5 or 9.5mm?

Any advice would be fantastic.

Thanks!

noseall · 24 Apr 2016

Here's one we did earlier.....
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We fitted 40mm between studs which left an air gap (25mm) on the cold side. We foamed and taped the joints.
We fitted a further 50mm across the studs and foamed and taped.
Then fitted 12.5mm foil back boards and skimmed. Not ideal but a lot better than it was before

noseall · 24 Apr 2016

You will likely need to add a new window board to allow for the new depth of celotex etc.

littledug · 25 Apr 2016

Thanks Noseall!

What is the best way to secure the 2nd layer of Celotex?
Also same question for the Plasterboard?
Any particular type of fixing e.g. Drywall screws?

Cheers

noseall · 25 Apr 2016

littledug said:
Thanks Noseall!

What is the best way to secure the 2nd layer of Celotex?
Also same question for the Plasterboard?
Any particular type of fixing e.g. Drywall screws?

Cheers

I used 90mm drywall screws. Skeleton fixed the celotex with some 70mm batten clouts then fixed through both the plasterboard and celotex into the studs.

Doggit · 25 Apr 2016

Nice job Noseall, but you didn't need the foil backed plasterboard, as nothing will get through the foil on the celotex. Beyond that, very impressive.

noseall · 26 Apr 2016

Doggit said:
Nice job Noseall, but you didn't need the foil backed plasterboard, as nothing will get through the foil on the celotex. Beyond that, very impressive.

I like to use the word 'robust'.

littledug · 26 Apr 2016

noseall said:
I used 90mm drywall screws. Skeleton fixed the celotex with some 70mm batten clouts then fixed through both the plasterboard and celotex into the studs.

Thanks, but to a bit of novice no idea what "skeleton fixing" is or what "70mm batten clouts" are?

noseall · 26 Apr 2016

littledug said:
Thanks, but to a bit of novice no idea what "skeleton fixing" is or what "70mm batten clouts" are?

Skeleton fixing is one my terminologies for "pop just a few fixings in because the plasterboard fixings will do the real job of grabbing the lot..."

70mm batten clouts are all I had available. Basically galvanized nails for fixing roof battens. Anything will do as long as it holds the stuff in place long enough for you to get the plasterboard on and fire the longer screws through the lot.