PIR Insulation over studs (not inbetween)?

[edoak]

A question about insulating stud walls.

We have a room in the roof, and the eaves space is uninsulated. I have formed an opening into the eaves space (as was sealed off), and I am in the process of rolling mineral wool insulation in between and over the floor joints to insulate the ceiling to the room below. I also plan to insulate the low-height wall / knee-wall stud partition separating the internal room and the eaves space.

I was planning on cutting and fitting PIR insulation between the joints and then taping them. For simplicity, I am now thinking it would be much quicker to fix the PIR board to the back of the studs in large sections (less cutting and measuring). This would leave an air void the width of the stud between the back of the plasterboard and the front of the PIR boards. I was thinking I could easily fill this cavity with mineral wool insulation for extra insulation.

Is this method of insulating OK? I think it should be, and if anything will be far more airtight as fewer joints and obstacles to cut around (back of plug sockets, random studs, etc.). However, I can't find any information on this, as everywhere says to cut the board and fit in between the studs. We will lose space in the eaves space, but this doesn't matter to me as will not be used and sit empty once I'm finished.

Any advice is much appreciated.

Thanks,

 

[motorbiking]

It really depends if there is an opportunity for warm, moist air to get in the void and cool, without adequate ventilation.

 

[edoak]

I suppose moist air could travel through the plasterboard, and then condense in the cavity which would be sealed in by the PIR foil-backed boards fixed to the back of the studs - but I did think by filling the space with wool might prevent this?

 

[cdbe]

Cavity batts better than mineral wool.