Thanks to all.
All the vent holes had already been covered with welded hardware cloth and the doors already secured. So I spent the afternoon trying to make the henhouse more comfortable so the girls will want to sleep there.
I added a new lower roost that I think they'll like and reinforced and secured the roof (which swings up) to get rid of the sag (in 5/8" plywood!) that was the result of a determined raccoon trying to force its way through a 1/8" crack. A week ago I replaced the thin wooden doors with plexiglass ones. I can see through them, they let in light, they slide much better, and they are tough, tough, tough.
The chicken wire had only been in the large covered pen where they spend their days. Sounds like I'll need to replace that. Gail Damerow in Storey's Guide to Raising Chickens recommends chain link (which has large holes) or welded mesh with 1" holes. Is that inadequate if they're in the henhouse at night?