How to insulate georgian bay window

[mikeymoo]

hi there

Were renovating an old house and im looking for advice on how to insulate under the bay window without causing rot on the timbers in the future.

http://imgur.com/ufr2ZkI

The above shows the area underneath the window. This is the first floor. We are putting rockwool fiber glass between the floor joists you can see. The external walls will have insulated plasterboard put on them. This leaves the area under the bay window as the only cold area so we need to insulate it.

Additionally we need to insulate above the window where the wooden supports are. It’s very important we get this right as those supports are holding up the bay window above it. Bonus pigeon in this photo!

https://imgur.com/a/jxdHvHZ

Would anyone have advice how to do this? The timbers are 170 years old so i want to make sure our insulation does not cause rot on the timbers in the future. The area underneath the window is lath and lime with timber support.

Thanks!

 

[^woody^]

This is a very common question. Have you searched the forum?

Full fill celotex, tape all joints and timber studs, or vapour check membrane on the inside.

 

[mikeymoo]

I have, thanks. So do you decide either tape the joints or put in a vapour check membrane?

What would you suggest for the area above the window? It’s 4 plans side by side so I can’t insulate between

 

[mikeymoo]

Additionally, with the Celotex should it be pressed direct against the lath and lime or should I leave a gap for air?

Apologies if these are rudamentery, I am project managing this refurb directly and very new to this

 

[^woody^]

The gist of it is that if there are air voids within a wall or roof, and a cold surface, and moist air gets in and is not ventilated, then potentially mould and timber rot can occur. So you design these things out.

No air voids against a cold surface, and no access for air/moist air.

Insulate right up against the lath and lime, seal the inner face with a single membrane, or as celotex has a foil face, use this and then use foil tape on any timber studs to get a complete barrier. One or the other

Same concept for any roof.

You are also better off fitting an additional layer of celotex across the whole internal face of the bay or roof underside, and this removed the cold areas caused by the timber studs or roof joists showing up on the internal plaster afterwards.