Chimney breast venting

[nishnish]

Our chimneys have been removed and cut off inside the loft. The tops of the flues are entirely open to the loft space.
As there is no way for rain to get down the flues, from the top, is the usual vent on each chimney breast needed?
Just lined the external walls (including chimney breast) of a room with battens, 50mm celotex and drywall. I'd rather not cut into it unnecessarily.
Thanks.

 

[Alastairreid]

Your chimney will still require ventilation both high and low if on an external wall.

the vent can be installed externally.

 

[nishnish]

Your chimney will still require ventilation both high and low if on an external wall.

Thanks.
It is on an external wall. Solid brick Victorian. Relatively sheltered as it's about 1.5m from next door and on a south facing wall. Most rain here comes from the West (Irish sea).
That's in case driving rain gets through brickwork into the flue?

the vent can be installed externally.

Think I'll go for the internal. The room is pretty well insulated, so this path up the chimney will provide some ventilation for the room.
It's only 4-5m from the top of the fireplace to the loft floor, where the chimney stacks have been cut back to. So it's not a long flue.

 

[r896neo]

you may be better off going external if you can as it can cause a bit of a sooty stink when windy if the vent is indoors.