chimney removed to the roof level

[zemster]

We just removed the chimney to the roof level but I found that cold air goes to the living room through the fireplace.
Must the stack be sealed on the loft and in the living room?
I also found a lots of condensation on the membrane in the attic. Is the open stack causes all this probelm?

 

[kp00110]

I believe per building regs once a chimney has been lowered to within the loft void it must be capped to stop anyone inadvertently using the fireplace still and filling the loft with smoke/fire (in case someone forgot the chimney was no longer venting outside??) Similarly unless you have breathable roof membrane or suitable loft ventilation it makes sense that condensation would also do the same i.e. warm air from your rooms with the open fireplaces