Installing PIR insulation between wonky donky joists Information, help and advice on many architecture, self build and design problems and issues

[DIY_Luke]

Hi there!

I'm trying my hand at insulating one of the rooms on the top floor of my house that will become a small office. I'm in the process of undertaking the challenging job of fitting PIR between the pitched ceiling joists, and the joists are a little bit tipsy!

My concern is that because the joists are a tad wonky that when the insulation is in the front of the PIR boards won't meet on the same plane, and then for when I cover them with plasterboard there will be some small gaps underneath. Someone suggested I put a thin sheet of insulation over the top of the PIR and the rafters to avoid bridging, but will that be enough?

Thanks in advance for your time!

Regards,
Luke

[stoneyboy]

Hi diy_luke,
Screw 50x25 battens to the underneath of the rafters, use packing pieces to give an even surface across the rafters.
Regards S

[DIY_Luke]

Hi S,

Thanks for your guidance! I assume i'd only screw the batons to some of the rafters, i.e. not the protruding one?

And what do you mean by 'packing pieces'?

Thanks for your help! :)

[stoneyboy]

Hi diy_luke,
Unless you have an exceptionally bad roof you will need to add a batten to every rafter, you can buy packs of plastic packing pieces (various thicknesses) do screw through these to keep them in place.
Start with the lowest rafter, screw on a batten with no packers and work away from it using battens and packers to give you a flat surface.
Regards S

[DIY_Luke]

Thanks for the guidance S :)

[DIY_Luke]

Hi S,

As I have now found time to get back to this project, I wanted ask for some additional advice.

The joists are around 35mm thick. I'm assuming I should be okay then to use battens that are 30mm wide? This seems obvious, but for the fact i'll be leaving a slight gap. My thinking is that it will be minor anyway and can be covered over by the foil tape when I insulate the roof, but welcome your own thoughts.

Thanks,
Luke