In reply to a crap climber:
Sounds exactly like my loft - wavy tiles with felt underneath. You can just feel the heat radiating into the loft on sunny days from the southern side of the house. I'm going to go down the radiant bubblewrap route I think as I've done kingspan in the roof of my old house garage and it was a pain.
Look up 'cold roofs and warm roofs' - I think the idea is that you need to avoid condensation on the outside of your bubblewrap that'll rot the rafters. That means sealing that void completely to stop moist air getting in at all, or having ventilation to allow any cold damp air to escape. So, simply bubblewrapping to the rafters isn't likely to be airtight enough thus it looks like the only way forward is to leave a gap at top and bottom which is simply going to allow the air heated up in the void to rise into the top of the roof and warm up the loft.
EDIT - I've not read the links yet, but apparently it's not very good? I can do a test next week as I have some kicking around I intend to fit. Googling for 'how to fit reflective bubblewrap' comes up with plenty of links to how-to pages and general articles about it.
Post edited at 23:34