Which timber for framing internal dormer window?

[Tirglas_7]

Hi

Not sure if this is the right forum, apologies if not

I'm retrofitting 150mm of rigid insulation into the dormer rooms in my house. I have it in 50mm and 100mm sheets. On the vertical areas the timbers aren't very deep so will only accommodate 50mm between and maintain an air gap, and then im Overboarding with 100mm on top.

Only place I can see it being an issue is around the window where I'd have nothing to secure the plasterboard to.

So plan is to attach 100mm timbers to frame out all along the edge of the window in a box that mimics the current frame, but then gives me a solid base for the pb to attach and for the new increased windows sill to sit on.

I'm aware of the need for acclimatisation etc, but haven't got a clue on what type of timber would be best/most cost effective/least likely to warp or bow?

Also the timber up there isn't treated as far as I can see (80's build) should I be using treated timber?

Apologies if that is too much waffle, many thanks for reading

 

[foxhole]

3x2 timber stud , already kiln dried.

 

[Tirglas_7]

many thanks foxhole, i'll get on to my local timber yard

i have a couple of kreg pocket hole jigs, so i'm planning on using pocket screws to attach them to the original timber. Do you think these would be ok? or is there another method you would use?

edit, would you use treated or sawn kiln dried?

 

[foxhole]

No need for treated internally, just screw through , all we be hidden by plasterboard?

 

[Tirglas_7]

Yes all will be covered,

Sorry to sound thick, but are we talking screwing at angles in, or some 100mm plus screws (I have some 150mm for the insulation +pb) directly through?

Thanks again