small shells fired from a large cannon that are hallowed out and hold some sort of posion inside killing the crew in that module.the posion would also kill any boarders on that ship or building as well.
EDIT-also the shell would cause little hull damage since the shell is halow and tnin to fit the gas or posion.
I think what you're aiming for here is something more along the lines of a poison gas weapon, not a bio weapon. I mean, if you want to be technical, bio weapons have been around since people figured out dead bodies were associated with disease, but I can't think of a single industrial society that threw corpses at the enemy from a catapult. And the nations in this game -certainly- do not possess the capacity to engineer a plague virulent enough to have an effect within seconds.
On the other hand, poison gas is EASILY within their reach and would definitely have the effect you described. So, this would be a weapon that kills crews but leaves ships and buildings relatively intact, right?
Well, TheClockworks beat me to it, and he/she said basically what I was going to say.
So, myeah, what you're thinking of are probably chemical weapons, in which case, they would be an interesting addition. However, crew members are an important resource, and a weapon that hurts crew specifically would have to be balanced very carefully.
Lukasmah but with the new addition of moving crew from ship to ship it might work without to much messing with balancing.
Commander, Engineering Corps
Be kind of cool if the gas will eventually settle into the lower modules. So you could design ships that are specifically built to counter poison gases.
My only concern of a system that fills a room and kill crew within that room is that you can't directly control where crew go inside the ship. So if it hits an ammo room it basically takes that room out of commission as anyone who enters will die - but because the crew need ammo they'll keep sending people into the room.
The way fires work sorta similarly, but don't 'only' kill crew. But it spreads through halls and makes a mess of the ship and her crew.
Air Lord, Engineering Corps
You could treat the gas like how fire works in terms of mechanics (like you said) but it does 0 module damage and cannot be put out. It could spread like fire but the duration goes down 1/3 rounded up per spread room.
First room hit, poison for 15(?) seconds in that room,
Spreads to two more rooms, each poisoned for 5 seconds
Two more For 2 seconds,
To counter the damage without needing to overhaul crew movement orders (or lack of) ships could become equipped with a module for holding gas masks, any crew in that ship get an updated gas mask sprite and takes much less damage from the gas (enough to barely survive 15 seconds).
Also, I'm not dead!
-edit- I had another idea to build on this, what if the gas duration is REALLY long, but drastically reduced for Windows, hull breaches and fast speeds, possibly with the option of having allied ships purposefully target a gassed ship with a breaching weapon (once) by command to help out the situation? This may be overpowered, though.