I would, as a principle, want to ensure a good airflow through any space
that might accumulate dampness. Therefore an airbrick at the top and the
bottom would allow air to move freely and take moisture with it.
Must be vented top and bottom. Proper ventilation is
much more important than capping.
If the chimney is on an outside wall, I would suggest
venting to the outside rather than in to the room, as a
an open flue in a room will create its own updraft and
hence lose some heat/energy from the room.
I've seen it done with a slate, mortared in place (on top), seemed to work &
v unobtrusive but then you still have to vent.
The other option would be to fabricate a square version of these:
http://www.fluesystems.com/cowls/index_disused.htm , just use
something to stand it off the pot a little, leaving a gap for airflow.
Also, these guys:
http://www.flues.co.uk/product_cowls_pots_and_terminals.html have a
huge range of cowls, may be worth a call or email to see if they can help
with a square version or maybe a large round hat could be adapted.
On Andy's point about venting the bottom of the chimney to the outside,
I'm not sure that will create enough airflow, my thinking is that the vent to
the inside (and warm air) is needed to create the updraught that will keep
the chimney dry. That said you are stopping rain entry so how much
airflow is needed?
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