It honestly sounds like all of your plans are good. The mudroom should be fine, even at night. Your brooder will be big enough to get away from the heat if they need to. Just check them on those cold nights to make sure that they arent piling. After the first two weeks they will start feathering more and you can decrease the heat some by moving the light further away. I also like your plan for moving them at 4-5 weeks, and sounds like you have a good setup. The power tools will freak them out, though. By 8 weeks you should be able to take the heat away and put them outside for goodI am concerned about the temp swings too. The mudroom acts almost like a green house so it can get in the 70s (maybe 80s??) on a sunny day, but plummet below freezing at night. We even keep a space heater behind the washing machine which is out there to keep the pipes from freezing. Should I just plan to move back the lamp before I leave in the morning, and adjust again at night to account for this? The thing is I won't be home all day to check how they are doing and adjust. Boyfriend is home about 4 days a week.
Or do I need to place my brooder somewhere else? Maybe the mudroom is a bad spot.
Again - 20 chicks, 3x5 box, 250 watt bulb.
Also another random question I thought of. I am hoping to eventually move the chicks into their coop in the workshop once they outgrow their box. However, my boyfriend does use power tools out there to the point where he does use hearing protection. Is that going to hurt the chicks ears? They won't be out there until they are at least 4-5 weeks old I think.