Bay window cavity insulation

3 Jun 2020
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United Kingdom
Hi All

i have looked through the forum as much as possible for advice on insulating a bay window with a cavity ,all threads seem to be about single skin bay window which helped greatly when doing the area below my bedroom window

Any advice would be much appreciated ,having taken the window board off there is a major draft coming through the cavity at the moment the only thing stopping that draft is the window board, there is random bits of expanding foam sprayed down inside the cavity not sure why its there as it doesnt seem to be doing anything the expanding foam along the top line is to help hold the down the window board

a few ideas/option

1) insulate the cavity with loose fill ,looking online this doesnt seem to be a good idea

2) knock down the internal wall build a stud work frame and fill with 100mm celotex and cover with plasterboard keeping the cavity of around 50mm allowing it to breathe cap off the top with insulation and window board

3) knock inner wall down apply dpm to inside of external wall and fill the complete void with stud work and celotex 150mm (could this prevent the outside wall drying out and cause damage in the winter months ,freeze thaw effect)

4) keep the inside wall knock off the plaster and cover with 50mm insulated plasterboard
dont want to take up too much of the roomspace

5) dont bother as the size of the wall and thickness of insulation with option 4 wont make much difference

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theres meant to be a draught to dry the cavity from any moisture ingress.
you’ve identified a problem that doesn’t exist. filling it with wool or any other porous material wont stop draught and could transport moisture in .
i’d stick a nicely mitred cill back on and seal it properly to stop the draught....or shape and bond insulated plasterboard to the top , bead the whole thing, front and top and grit and skim it all in .
Thanks for the replies so far

ive never reallly wanted the cavities filled having read about lots of problems online with it other houses in the street have had it done and have had problems

benny - not a problem as such just a cold wall and cill in winter months ,im about to insulate under the floor with celotex so whilst i was doing that i thought about the wall also
good idea with the insulated plasterboard on top though ill have look at that
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