It seems like you are thinking of doing a loft conversion without the necessary building regulation approval? And also you probablly need to read up on how condensation forms in lofts and cold spaces, as you will most likely make things much worse by your proposals.
As a guide, once any air hits a cold felt or membrane surface, it will condense, and will need to be removed by ventilation. Not alll breathable felt is breathable enough.
As to the existing problem, I would suggest that the condensation you are experiencing now is not because the insulation is preventing the house from "breathing", but that the insufficient insualtion is allowing more warm air into the loft to condense in the first place. A loft needs to be cold and it needs to be ventilated. And for that reason, the plan to insulate a loft at rafter level is a very bad idea.
On the plus side, the purlin supports are OK
As a guide, once any air hits a cold felt or membrane surface, it will condense, and will need to be removed by ventilation. Not alll breathable felt is breathable enough.
As to the existing problem, I would suggest that the condensation you are experiencing now is not because the insulation is preventing the house from "breathing", but that the insufficient insualtion is allowing more warm air into the loft to condense in the first place. A loft needs to be cold and it needs to be ventilated. And for that reason, the plan to insulate a loft at rafter level is a very bad idea.
On the plus side, the purlin supports are OK