Originally Posted by WilliamWS
Just curious: If you're using a cooler as a mash tun then why BIAB, why not just drain and batch sparge?
I don't do BIAB but my understanding is the only real advantage is that it is a one vessel system, it allows you to do all AG without a separate mash tun.
When I BIAB indoors, I usually do it via Partial Mash with 2 kettles, no cooler. Some people use a cooler and a kettle. Others just use one of these vessels. But it doesn't have to be a one vessel system. It saves time to heat up your main volume of water to the appropriate temp. while you're mashing the other volume. That way your mashed wort and main volume of water are premeasured and at the appropriate temps exactly when you need them to be... So you're good to go with the next step instead of waiting for the main volume of water to heat up after the mash.
Your mash & mash volume = 5 lbs. grain mashed in 7.5 liters of water
Your total boil volume = 20 liters of water
So, an additional 12.5 liters of water will be required for the full boil... which you don't want to be cold. So you can heat it up in a separate vessel.
Also, some people who brew BIAB (whether Partial Mash or All Grain) will batch sparge; others will no sparge; others will dunk the grain bag several times in the 170 F-ish main kettle volume of plain water to sort of combine the idea of a sparge and mashout together as one. In summation, there are many more than one ways to skin a cat.