I am considering putting a ceiling in my conservatory to make it more like a sun room and to improve insulation in the winter.
In theory it appears really simple. I would use 4x2 for the joists and plasterboard it, placing loads on insulation in the roof space. Also I would consider sheeting the inside of the conservatory roof incase it appeared unsightly from the outside.
Location: essex, then chongqing, china and now essex again
i wouldn't do that until i had tried other things if i were you.
having said that, no need for 4x2 - 2x2 should do the job...just need to make sure the roof structure can cope with the additional load and how to attach to the roof plate without losing too much ceiling height. is it timber or aluminium framed?
have you tried those solar reflective films? i presume you have a glass roof rather than polycarb (if you had polycarb i expect you would have tried solar relective inserts by now)