Look, I respect the ambition here. Turning a neglected shed into a chicken coop is the kind of project that separates the casual backyard enthusiast from the truly committed. But let's talk about that bug bomb question, because it's the part of this plan that makes me want to gently grab you by the shoulders and say, "Friend, no."
You've got a solid list of chores, cutting a pop-hole, patching leaks, uncovering a ventilation window, ripping out old insulation, and excavating decades of rodent real estate. That's real work, and it's the right work. But a bug bomb is a blunt instrument for a problem that needs surgical precision. Those foggers coat every surface in a layer of insecticide that lingers long after the fog clears. You're planning to air the place out for a month, which is smart, but wood is porous. Nooks and crannies hold residue. Your chicks will be breathing that, walking on it, pecking at it. Even if the label says it's safe once dry, the word "safe" in pest-control marketing is doing a lot of heavy lifting. I'd rather you tackle the bugs mechanically: shovel out the dirt layer that's hosting the ant and spider hotels, vacuum every corner after the insulation is gone, and seal cracks with caulk or hardware cloth. A good cleaning is boring, but boring is safe. Boring doesn't give your pullets a chemical welcome mat.
What I'd tell you if you asked me directly is this: skip the fogger and double down on removal. That top layer of dirt is the real problem, it's a buffet and a nursery for everything you don't want. Haul it out, then treat the remaining floor with food-grade diatomaceous earth if you must. It's mechanical, not chemical; it dries out exoskeletons without poisoning the air. And that ventilation window you're uncovering? That's your best friend. Chickens need airflow far more than they need a sterile environment. A coop that's too tight traps moisture and ammonia, which causes respiratory issues that a few spiders never will. You've got the right instinct to air it out for a month, just let fresh air do the work instead of a chemical cloud.
The rest of your plan is solid. Cutting a door sized for a chicken (not a human) is a detail too many people overlook. Patching leaks keeps bedding dry, which keeps birds healthy. And building a run? Essential. Just remember that a coop is a shelter, not a vault. Your goal isn't a germ-free lab; it's a dry, ventilated, predator-proof space where your birds can be birds. The spiders will find their way back eventually, and that's fine. A few bugs in the coop are a snack, not a crisis. So finish the demolition, skip the fogger, and let that month of airing out be the final step. Your chicks will thank you, probably by not dying of pesticide exposure, which is honestly the best thanks a chicken can give.