>> There is a expanded metal grill that might prohibit/reduce bee access.
>> I think they might discourage future swarms if they are available to you.
>
>6 mesh hardware cloth should do the trick and reduce sparks, without
>restricting draft at all.
>
>BTW 5 mesh hardware cloth is often used as an exclude as bees can pass
>thru, leaving drones and queens behind.
>
>Allen
 
My experience here in the San Francisco Bay Area is that once the original
swarm has arrived, even if the swarm doesn't come, the scouts will.
Even though the queen can't pass (usually) the 1/4" mesh hardware cloth,
scouts can. Unless the chimney has been cleaned very well, immediately
after winter, the flew will be dusty with soot. If 1/4" mesh is used,
the scout will get through the square; then as it bangs around trying
to get back through the screen, exaustion and soot will cause it to
drop down and shortly arrive  in the fire box. Of corse, any large
screen will do nothing. I use 1/8" galvy screen; making a "hat" that
fits snugly on to the top of the fire clay smoke stack. Most people
don't use their fire places here in this mild climate. so the news
that if the fire place is used, the hat must be removed and hung up
in the garage till next spring, is met with little resistance. If they
forget to remove, there will be 3-6 fires before enough creasote
precipitates on the grid to shut down the draft. If the screen is placed
in responce to scouts, the odds are very good that another call will
be comming soon to remove the swarm, now hanging of the side of the
chimney.
Our consumer hardware stores don't sell hardware cloth with  size
indications such as 5 or 6. How does that translate into fractions.
Alas poor metric, I knew him well.
 
David Crawford
Pinole, Ca.
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P.s. In another post, Lars writes about the use of spark aresters, using
expanded metal screen. I have earned many $s removing the swarms that
decide these caps are a wonderful place to live.