Bay window board insulation

[Secret_Sipper]

We are in the process of buying new uPVC windows that are of a better quality and are all the same style that's in now.

Unfortunately whoever put ours in last had cowboys doing it and has made a right mess with the bay window. So much so it has sagged within 5 years)

I believe the window sits on the window board whi
When replacing the bay 've asked the window fitter to replace the steel supports with thicker wooden supports.

My question is, would it be alright fixing insulation board (kingspan/celotex) directly to the underside of the window board and box it in or would I need to batten the insulation off the board to create a gap?

I'm also unsure what thickness I would need for it to be effective. Anything is obviously better than we have now (which is non)

To add, we will be having a pitched mono roof put over the bay and porch which should direct any rain onto guttering.

 

[Adam182]

Direct fixing to the underside will be fine you wouldn't want to leave a gap in this case as it would leave potential for the cold air to get inbetween and render the insulation pretty useless, in an ideal world the insulation would be on the inside but in this case I imagine that isn't an option.

Hope this helps.

 

[Secret_Sipper]

I know it might seem silly replying to threads that are self explanation, I've always been of the "I'd rather ask" kinda person. Thanks.☺

Unfortunately, no. The wall its sat on has cavity wall insulation but there really is just a 20mm thick piece between the inside and outside the underside of the bay window.

Thanks again

 

[Adam182]

I know it might seem silly replying to threads that are self explanation, I've always been of the "I'd rather ask" kinda person. Thanks.☺

Unfortunately, no. The wall its sat on has cavity wall insulation but there really is just a 20mm thick piece between the inside and outside the underside of the bay window.

Thanks again

For the little time it takes It's absolutely worth asking these questions just to be sure regardless of how self explanatory they might seem.

To answer the thickness question I would just go with whatever you can fit under whilst still making it look decent. Personally I would aim for 75mm.