I'm a complete all grain newbie, and want to get into BIAB. I do 2.5 gallon batches in a 4 gallon pot. Can I mash with 2.5 gallons of water, remove the bag, then top up to the 3.2 gallons required to collect 2.5 gallons at the end of the boil? Is it a cardinal sin of all grain to top up, or will it not matter since I'm mashing with the full volume, and only topping up to cover evaporation losses?
If that's not kosher, will heating the top up water to 170 and sparging the bag as it hangs in the strainer work? (Though I don't think sparging is part of traditional BIAB (?) it may save me here...)
I'm pretty sure 3.2 gallons plus the grain bill won't fit in my pot, so I'm evaluating whether to get a bigger pot.