Insulate 60s extension

[Moonhead]

Our bathroom is a single story extension on the back of the house with an uninsulated roof. I'm currently rewiring the downlights and started thinking about insulation (it's freezing in the winter).
I thought for a cheap and easy fix, spray a load of expanding foam up there, through the downlight holes (there's one between each joist. Is this a stupid idea or might it work?

 

[noseall]

Is this a stupid idea or might it work?

Utterly bonkers.

Rip the ceiling down and fully fill the void robustly with PIR foil sandwich insulation. Foil tape every crack crevice and joint. Fix another layer of 50mm PIR foil sandwich insulation across the joists and foil tape all joints. Fix duplex boards to the ceiling.
Then do all the external walls.

 

[rssteve]

And forget cutting any down lighters in.