Insulating existing former cheeks

[matt-j]

I have three dormer windows upstairs in my house, and as part of renovating the whole place have stripped the inside of the cheeks back to the 100mm stud. When built they were filled with glasswool.

So, what’s the best way to reinsulate? They’re ply sheathed without any ventilation I can see, can I full fill with 100mm PIR or should I leave an air gap and go with 50mm? What’s good practice to minimise risk of condensation?

Pic attached.

 

[matt-j]

And of course autocorrect has turned dormer into dormer for me!

 

[noseall]

Fully fill the voids. Then foil tape over any gaps or joins. The larger gaps can be filled with foam prior to foil taping. If you have the tolerance around the window jambs, you should really add another layer across the studs to prevent cold bridge shadowing. Again, foil tape the joints before fixing foil backed (duplex) plasterboards.

Edited:
I've just looked at the image and you are going to be tight for any additional insulation across the studs.

We deliberately fix a 50mm timber up each side of the window aperture (when building dormers) so as to accommodate the extra insulation.

 

[matt-j]

Thanks. Agree that ideally I would want another layer over the top but they’re very narrow dormers. Squeezing insulation where possible seems like the best option.