Insulation in bay window framework?

[Patroller]

Hi all. I have recently removed all of the internal plasterboard underneath our upstairs bay window. Since the double glazing was fitted several years ago there was a little bit of settlement and cracks appeared where the wall ended and the wooden bay framework started. I have now fitted heavy duty straps to stop any further outward movement (although it may not have moved any further anyway, I wanted to be sure.)

Now that I have this void and have replaced / strengthened the timber framework, I am wondering what sort of insulation to use, if any? Obviously the outer membrane is not air tight and I imagine the air circulation in the void would dry it out should any water penetrate the outer tiles and membrane. Would it be best to leave this as a void or pack it out with something? If so would I risk damp being transferred to the internal plasterboard if water did penetrate the outside?

I have attached a picture.

Thanks for all advice.

 

[Patroller]

Hi again, can anybody advise as I am hoping to start tomorrow?

Thanks.

 

[freddiemercurystwin]

What is the grey board behind the studs? Is the grey board damp at all?

 

[tony1851]

Looks like traditional bitumen felt.

@OP just put a couple of inches of kingspan between the studs (flush with the inner face) and then plasterboard over.

 

[Patroller]

Thanks for your advice.

My plan now is to pack out 'fairly tight' with kingspan or similar rigid insulation and use regular plasterboard over the top (which is what was there before) before skimming with plaster over the lot.

I have seen other people advising a vapour barrier behind the plasterboard. Would this be a good idea? I understand that it would help stop damp penetrating the plasterboard should it ever get wet in there (no sign at the moment after heavy rain). I am torn between the modern method's of insulation etc and the old fashioned 'leave it bare and open' so that airflow naturally keeps everything dry. So is packing it all out with various materials potentially going to hinder damp drying out should it ever find a way in, or helping condensation to form?

So am I better sticking with the solid insulation cut to size and filled between the joists, with plasterboard over the top and a final plaster skim over the whole lot...

Or

Doing all of the above but also putting a vapour barrier directly behind the plasterboard?

Many thanks for all replies...

(and yes, the grey material in the photo is the felt).[/b]

 

[theprinceofdarkness]

Putting a damp proof membrane (actually a Vapour proof membrane) behind the plaster board is a must. What happens is that the room is full of warm slightly damp air. As this air filters through the plasterboard towards the outside, it gets cooler. At some temperature the water vapour cools enough for the water to appear as condensation, i.e. drops of water, this will run down the inside of the outer skin causing rot and marking the plasterboard. It is called interstitial condensation.
Frank

 

[TheVictorian]

I'd put kingspan or similar between the timbers and then tape over the timber with 100mm aluminium tape to provide a vapour barrier, saves messing with any other products.

 

[Patroller]

Thanks for both posts. So I can either use the aluminimum tape or a roll of vapour barrier, such as the following from screwfix?

http://www.screwfix.com/p/vapour-barrier-green-300ga-2-5-x-20m/12869

Is it just a case of trying to keep it taught as I place the plasterboard over the top of it and work my way around the bay in sections? I take it I should try and keep the vapour barrier as one complete section all the way around? Or just cut it into sections and fix to plasterboard sections before I screw them on? Would multiple layers of it be good if I have enough to spare?

Sorry for all the questions!

 

[freddiemercurystwin]

If you use Kingsan or similar it will have a foil face, so just foil tape over the studs to create a single barrier.

Or use the one you linked to and just staple it to the studs, obviously leave a little slack to allow for the curve. a single piece laid in one sheet thick will do.

 

[Patroller]

Thank you, your help is appreciated. I can make a start now.

 

[MidsTW]

As others have said! I did mine last year... WIP picture below.

View media item 90319
Use the thickest you can.... 100mm will probably be okay. And don't leave any small gaps have fun!

 

[MidsTW]

Just looked at your original image more closely... Are you going to be having a window sill?

If you are it might be worth boxing out a bit further too... You can then screw a window board down and have more insulation below!

 

[Patroller]

Thanks both. I am not planning on creating a larger void for more insulation purely because we are quite short of space. I will pack it out as best I can, unfortunately the joists are at angles and therefore it might be difficult to really pack it out tight but I will try. I may have to help to secure it in place using tape. I also don't want it butt up against the outer membrane.

As for the window sill, it is currently in sections on my floor waiting to be put back on. It is very shallow, but being a bedroom we have no real need of much depth to it.

Thanks again!

 

[MidsTW]

How deep Is the timber? You can get 50mm celotex which will work. You can cut it slightly oversize and it will compress when you push it in... And you can always shave a bit off with a craft knife of the angle is against you.

 

[Patroller]

It is 3", so I could get 50mm celotex or similar in there, if I can find a supplier nearby.

Cheers.