Had the same issue myself... ended up rigging a few cheap box fans on timers under the tarp edges. Just enough airflow to keep humidity down without blowing the budget. Worked surprisingly well.
Interesting idea with the fans... hadn't thought of that one before. I've usually found that humidity isn't the only culprit when it comes to lumber warping—sometimes it's just the quality of the wood itself. Even with proper airflow, cheaper lumber can twist or bow unpredictably. One thing that's helped me is carefully selecting boards at the store, looking down their length to spot any obvious curves or twists before buying. Takes a bit more time upfront, but saves headaches later.
Also, bracing is key. Temporary diagonal braces can really help keep walls straight during framing, especially if you're working solo or with limited help. Learned that lesson the hard way after a particularly frustrating afternoon spent wrestling with a stubborn wall section... never again, haha.
Good points on lumber selection and bracing—definitely learned the hard way myself. Another thing I've noticed is that uneven floor surfaces can throw off wall alignment too. Anyone else had issues with floors causing framing headaches? Curious how you tackled it...
Ran into this exact issue renovating an older place—floors were all over the place. Ended up shimming the bottom plates to level things out before framing walls. Took some extra time but saved headaches later on... worth it imo.
Ran into something similar myself a couple years back. Shimming definitely helps, but I also found it useful to spend extra time sorting through lumber at the yard. Even though it's tedious, picking out the straightest pieces you can find makes a noticeable difference later on. Also, don't underestimate how much humidity and temperature swings can warp lumber after you've brought it home—especially if you're storing it for a while before framing. Learned that one the hard way...
Another thing I noticed is that older homes often have subtle dips or rises in the subfloor that aren't immediately obvious. Using a laser level or even just a long straightedge to map out those problem areas beforehand can save you from surprises mid-build. It's a bit more prep work upfront, but honestly, it's worth it to avoid frustration down the line.