Adding insulation to an uninsulated 1995 conc tile roof, what product and how much is needed?

[GarethM]

Pretty much, bitumen isn't breathable.

It's like shed felt minus the gravel.

 

Vapour barrier is usually white and has tiny holes, like a miniature string vest.

Edited March 11 by GarethM

[difflock]

But, as I mentioned/queried, the eaves to ridge ventilation should make a breathable membrane unecessary, surely?

Hazarding a guess, pure belt an braces?

cheers

[Welshfred]

Breathable membrane would sit on top of the joists with a drape to allow any moisture that gets on top to run down into the gutter. Your felt is doing this job but has no breathability, except at the laps. If you have air gap under the felt, between the joists, there may be enough airflow to vent the space but it must be a continuous gap and the airflow must enter and leave from somewhere (lap leakage?). A vapour control layer (membrane) is fitted below the joists to slow moisture flow up into the joist space to a rate slower than the moisture can leave through the upper membrane, keeping the space dry.

Consider too whether boxing the felt lap air leakage out of the main roof space might reduce ventilation too far if no soffits...??

My knowledge doesn't come from retrofitting old houses. There's so many panel insulation products out there, some with integral VCL I believe, talking to a pro will make your life easier as they know what's available and suitable. But more research required I reckon...