Janine - I am such a HUGE arachnaphobic. EWWWWWW!!!!!! We have the big brown wood spiders here - gosh, I'm getting the willies just typing this. I was in my studio about 7 months ago cleaning beads at my sink when out of the corner of my eye I saw one drop down from over the door and scurry under the table where the student torches are. I FREAKED OUT!!! I ran inside and told my husband what had happened and he went out there to see if he could find it. No luck. I told him that as long as I knew it COULD still be in there I couldn't go in there. I had these horrible visons of it.........EEEEWWWW..............crawling across my foot -----OH MY GOD!!!!!......while I'm torching and me burning down my studio knocking things over trying to get away from it.
Soooooo, he decided the best thing was to set off a fumigation bomb. He put down newpaper, set it off and closed the door. We were leaving town for the weekend, so there was plenty of time for it to do the job and then have the fumes diminish enough that I could go in there. Well, we get back in town on Sunday afternoon and we went out there together. We open the door to the studio and there are about
2 million dead ants everywhere 
. I have no earthly idea where they came from, but they were in piles EVERYWHERE!!! I had to remove glass rods from their tubes, move piles of books, etc. It took me about 5 hours to clean up that mess. I bet I ended up with about a quart of dead ants in the vacuum cleaner. It looked like a mass ant suicide.
I looked everywhere for that damned wood spider and couldn't find it. Then about 3 days later I looked in the can of water where I drop bits of yucky glass and cool my tools, and there he was. Even though he was dead, I still got sort of freaked about it.
Lenora - regarding makeup air, in the summer I have the A/C running constantly, which provides a constant source of makeup air. It is on the back side of the studio, away from both of the vents, which are on the ends. During the winter I open a small vent that I installed right above the A/C unit. I made it from 2 of those dryer vent covers I used the hoods from both of them so that there was a hood on the inside of the wall and a hood on the outside of the wall. I installed it with the flap opening on the inside wall of the studio. Instead of air blowing the vent open to the outside, as would be the case with a clothes dryer, then shutting when the dryer isn't on, I installed it so that the flap is on the inside of the studio and when the vent fan is on, the flap opens and lets in fresh air in from the outside. When the vent system is off, it shuts to keep out air. I put window screening material on the outside under the hood to keep out bugs and other creatures. We do have those in Florida, you know? The hood on the outside keeps rain from getting in. If you need for me to post pics of this let me know.
Becky