Bay Window Cavity - how to insulate ?

[Nogsx3]

Below the bay window of the main bedroom I've discovered a cavity (see photo) whereas the rest of the house is brick. This drops down to the top of the ground floor bay bay window.

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• Nogsx3
• 19 Feb 2011
• 1

It gets very cold in the bedroom and now I know why ! I was thinking of using Celotex 75mm PIR and replace the plaster with dry lined plasterboard?

Do I need to add a plastic sheet vapour barrier as well ? If I cut the PIR as tight as possible for the gap do I need to attach / bond it to the timber frame ?

Thanks

 

[Trimmer82]

Hi Nogs

Cutting the 75mm in tight should be fine, though there is no harm tacking a nail or two either side to keep it wedged in.
The vapor barrier is always a good idea.

Late reply though, hopefully you've already closed that gap up

 

[Nogsx3]

Thanks for the reply and no its not filled in yet !

The gap varies in depth by 5cm and the edges of the wood in the bay are angled at about 30 degrees. Cutting Celotex to fit tight would be impossible so I've decided to use DriTherm (glass) cavity slabs. I'll cut to rough size, it will compress a bit so I'll get a good snug fit with no air gaps. Then vapour barrier the lot. The plaster board needs to be replaced so I'll use foil backed I think.

At either end of the bay is a small gap, about 5mm between the wooden upright and the brick wall. I'm not going to be able to fill that easily and impossible to vapour shield. I was thinking use some spray foam carefully to just fill the gap, leaving plenty of time to expand before plasterboard goes up. Any better solutions ? Near the top of this gap either side I can feel a slight movement of air so need to do something with it.

Stupid question, I do all that, how do you attach the plasterboard ? If I use screws I breach the vapour barrier... not sure what to do... ?

 

[hammerswinger]

Yes I would go with the expanding foam for the 5mm gap.

I wouldn't worry about the screws going through the vapour barrier, the hole they make will not make any difference.

 

[Nogsx3]

Thanks ... job for the weekend then !

 

[shippobabe]

New boy on the block.
Re' the insulating of bay window cavities - can rockwool be packed into the cavity having gained entry from the bedroom?
Got the same problem (condensation - mild) as the other lads but don't see any mention of the use of rockwool or fibreglass materials? Would appreciate any thoughts.

 

[noseall]

Regards the bay - it would do no harm to run a sheet of celotex across the studs prior to plasterboarding to help prevent cold bridging.

I understand space can be an issue but even 25mm can make a difference.

Just an example, we are currently installing 100mm celotex between studs and 70mm across the suds to satisfy current reg's. If you use a foil sandwich insulation board as well as foil tape, foam or mastic on any joints or gaps then VCL will be achieved.

 

[Nogsx3]

I used DriTherm cavity slabs (glass mineral wool) as the gaps I was filling were so uneven in depth and width. Anything rigid would have left big gaps and impossible to cut. I put in a vapour barrier, foil tape etc and used lined plaster board just for good measure. Had no problems all winter, all dry and made a massive difference.

 

[shippobabe]

I used DriThern cavity slabs (glass mineral wool) as the gaps I was filling were so uneven in depth and width. Anything rigid would have left big gaps and impossible to cut. I put in a vapour barrier, foil tape etc and used lined plaster board just for good measure. Had no problems all winter, all dry and made a massive difference
.

Thanks for that Nogsx3 that's the road methinks I'll go down 'cos I believe that's what I'll find when I open it up - same as you. Take it you/ applied the vapour barrier towards the outside before you inserted the DriTherm slab/
Thanks again for your help.

 

[Nogsx3]

Outside wall, dritherm, vapour barrier + foil to stud frame, foil backed plaster.

I cut the dritherm so it was tight fit, if you leave it too wide it crincles up and leaves gaps. Any uneven gaps, I used thin strips and filled best I could. The stud frame was far from straight, up to 5 cm difference gap in a 1.4m range! Then the depth of area varied from 65mm-75mm. I used 75mm dritherm so depth wise its a good fit. I read all the guides on dritherm site, some building regs and this seemed like the best option for me. I did get a couple of quotes, they just wanted to stuff loft insulation in, no vapour barrier and normal plaster.....hence I did it myself.

 

[shippobabe]

Outside wall, dritherm, vapour barrier + foil to stud frame, foil backed plaster.

I cut the dritherm so it was tight fit, if you leave it too wide it crincles up and leaves gaps. Any uneven gaps, I used thin strips and filled best I could. The stud frame was far from straight, up to 5 cm difference gap in a 1.4m range! Then the depth of area varied from 65mm-75mm. I used 75mm dritherm so depth wise its a good fit. I read all the guides on dritherm site, some building regs and this seemed like the best option for me. I did get a couple of quotes, they just wanted to stuff loft insulation in, no vapour barrier and normal plaster.....hence I did it myself.

Hey Nogsx3 ease up or I'll be sending for you! Thanks again you've all but done it for me, I'll get back when I get round to doing it. Checking out material suppliers now and getting ready to put hammer through t' wall.