Insulating around windows reveals

[Ciaransmith]

Afternoon all,

I am in the process of wood fibre boarding a bedroom and am wondering how to insulate a window.

I've used 40mm boards on the walls, but not enough room for the window. Its basically set back like the image added (not my actual window, but very similar). What's the best method here, i might be able to get 20mm in, if so do i push it right upto the window?, can wood fibre be sanded down to fit? Do i insulate the bottom and put the window cill on top?

Sorry for all the questions but want to make sure this is right

Kind regards
C

 

[fernicarry]

Its a common problem and one I'm dealing with at the moment. I've parged my reveal with thermal plaster to get it a bit flatter and I'll build up the remaining 20mm or so depth with bits of soft insulation teased apart. I'll be lining the reveal with bead board afterwards so that will hold it in place. You could build up the whole depth with thermal plaster especially if you are going for a more rustic uneven finish on the walls but its lower performing that other insulations.

You should go right up to the window frame and also fill the void under the window board. You want to make it as airtight as possible and avoid any trapped pockets of air. Again thermal plaster might be your best bet under the window board with the board bedded into it when its still wet.

 

[Ciaransmith]

I think i should be able to squeeze 20mm fibre boards around the window. Hopefully.

 

[Jahouli]

I parged with lime hemp to get it flattish then sealed around the window with airtightness tape. Then used 20mm woodfibre boards. After Plastering I used a hybrid sealant adhesive like CT1 to seal the gap between the plaster and the window frame.

I didn't insulate under the cill as such, but levelled off with lime hemp and used expanding foam tape underneath the board to create a thermal break, plus between the board and the window frame.

I went less belt and braces in the first room I renovated, using just lime hemp for insulation, but since found I still get some minor issues with cold bridging where the plaster and the oak window board meet the window frame. My house is relatively exposed and I subsequently realised I needed to pay more attention to Airtightness and reducing drafts.