I've had a Bunch of the chemical/Grabber packs,& A Few Merino wool Scarfs from Italian Military,aprox.5' long x 12" wide,Wrapped a Scarf around my torso,secured with safety pins,& Inserted a Hand warmer pack,at each Kidney,With a Silk-Longsleeved Shirt& My Goretex-Flecktarn Jacket,I was Warm for Longer than 10hrs,with a Nominal Temp. of 20 F.,I even had to wrap the packs with a Merino sock,at 1 point,As they got Downright,Too Hot! Those things work,Very Well,Are Compact,& If purchased in Bulk,Such as Costco,come out to about $1 ea.!Goose Down,Such as Falcon 5000 Recommends is By far,The Best Insulator,But when it Gets Wet,All bets are off!