Just like hundreds of good-quality jackets, mittens, and sleeping bags, Hydrapak have chosen Primaloft insulation to sit between the Wooly Mammoth’s walls. The fuzzy sock fits over a 20oz BPA- and phthalate-free polypropylene inner bottle and the two layers slide into a polypropylene outer bottle. The package is topped by a wide-mouth screw-on lid with a nicely-sized soft nipple. Dishwasher friendly, the Wooly Mammoth is too tall for my top rack and has suffered the indignities of the bottom rack without any issue. The polypro bottle seems resistant to taking on flavor from energy drinks, too.