Just don't let it agitate in the top loading washer; that central agitator will stress the batting too much. You need to stop the washer to allow the batting to soak and absorb water, then advance to the spin cycle before turning the washer back on.
If your dryer is small, be careful not to stuff such a large batting into it that the batting cannot turn around. I stuffed my dryer with a king size mattress cover and put it on high. The cover came out with brown spots where it had touched the dryer's air holes and couldn't move away from them!
It would take forever to dry a batting in my dryer on air only. For max shrinkage, I would use the regular cycle. Just check every 10 or 20 minutes to see if the batting needs to be re-arranged in the dryer.
Edit: I should add that this method does not work on all battings! It works for Warm and Natural because it is needlepunched through scrim. Vintage cotton battings (such as Mountain Mist Blue Ribbon cotton) will fall apart in the soaking water. Hobbs 80/20 is held together with heat bonding, so for that one you would probably want to use cold or warm water only and either line dry or machine dry on air or low; it wouldn't be able to handle the high dryer heat that Warm and Natural can.
Last edited by Prism99; 07-12-2014 at 09:28 AM.