BIAB newbie with a mash volume question

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.

For BIAB on the kitchen stove your pot is fine. That's the same size batches and pot I use. You can fit up to ten pounds, but efficiency may suffer. Fill it with water one inch short of the top. After you later check the volume and add the difference. I like to do the addional water in a second pot as a dunk sparge, but as you said, that's not true BIAB.

recently I did a blog post on this:
http://woodlandbrew.blogspot.com/2012/09/biab.html

Thanks. Also, when you first boil wort in AG recipes, do you have the huge hot break foam-over you see with extract? None of the videos I've watched have shown that, but they may just have skipped over it. I'm hoping it doesn't foam much so I can boil closer to the top of the pot.

It depends on which beer you are making but yes, AG can boil over if you aren't careful. I've found higher gravity AG batches tend to boil over easier. Just have your pot holders ready

andycr said:

Thanks. Also, when you first boil wort in AG recipes, do you have the huge hot break foam-over you see with extract? None of the videos I've watched have shown that, but they may just have skipped over it. I'm hoping it doesn't foam much so I can boil closer to the top of the pot.

Click to expand...

A small plastic spray bottle filled w/water can keep the boil over from happening. I have a digital thermometer w/an alarm you can set for any temp. I set it to say 208* and when the alarm goes off I grab my spray bottle and get ready.

As the wort begins to boil and rise toward the boil over point I turn down the heat a little and begin spraying water on the wort. Since doing this I've never had a boil over.