Do I insulate this bay window cavity?

15 Jun 2018
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United Kingdom
Hi all,

After removing my window boards from this bay window (see pics) I have found the cavity has no insulation despite the rest of this part of the house having a circa 50mm cavity filled with fibreglass. I am wondering if I need to insulate this bay window cavity and if so what should I use? Many thanks in advance.
IMG_20200318_150315.jpg IMG_20200319_202531.jpg
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It seems like quite a thin cavity; can you get slabs that thin?
Many thanks. The middle section pictured has a wide(ish) cavity but has a few wall ties in it so putting a slab of fibreglass in would be difficult. The angled/side walls have a narrower cavity and they taper! In hindsight I should have rebuilt the side walls so they were actually parallel to the outer wall but that's not gonna happen now.

Anyways, would the the loose polystyrene bead type insulation work and can it be bought in small quantities? Or would some type of expanding foam work?
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I don't know much about this, but believe that you need to be careful not to bridge the gap with something that will carry moisture?
In the end I did manage to wedge some 50mm compressed fibreglass slab in to fill the middle section. The cavity on the angled walls are so narrow I'm not sure if they're worth filling? I could buy loose polystyrene if anyone can advise if it'd be worth filling with this, it's available to buy from ebay?
You could buy loose but unless the cavity is fully enclosed they will gradually find their way somewhere they shouldn't be. When they do houses with the injected balls they add glue so once they are in place they stay solid like a slab of polystyrene.
Many thanks for the quick response. The loose polystyrene wouldn't go anywhere as it is fully enclosed so might as well buy some and fill it, gotta be better than no insulation at all despite being fairly narrow? See picture attached...
Be especially careful that polystyrene beads cannot come into contact with electrical cables, as they react with them.

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