Comfort is not ‘one size fits all’, which is why the Boost 650 empowers you to customize its shape to find your perfect balance of space and warmth. As a spacious semi-rectangular sleeping bag the Boost provides all the room you need to side sleep, shift about, spread out and relax. The semi-rectangular shape gives you considerably more internal space than a mummy bag—you’ll feel unrestricted and free to sleep in whatever position you want all night long. But more space demands more body heat to warm it up and cold isn’t comfortable. That’s where WarmZip comes in. Zipping up the WarmZip center zipper transforms the shape of the Boost 650 from a semi-rectangular bag to a thermally efficient mummy bag. It pulls the 650-fill down around your body and reduces the amount of space your body needs to heat. WarmZip gives you the power to add up to 10F/6C to the Boost’s temperature rating. Standard Therm-a-Rest features like a puffy draft collar, a draft tube along the entry zipper line and a cinchable hood round out the Boost’s bliss-inducing comfort.

The Boost 650 is designed to be comfortable when you’re falling asleep, when you’re asleep and when you’re wide awake and hanging out in your tent. The arm holes give you easy access to the WarmZip without having to use the side zipper. They also allow you to access your belongings, read a book, play cards or high-five your friends without leaving the cozy warmth of your sleeping bag. Getting too warm? Use the arm holes to ventilate or comfortably sleep with just your arms out. You can also ventilate around your torso or down by your feet with the two-way side zipper. Every zipper line on the Boost has a durable YKK anti-snag pull to eliminate annoying snags and damaging tears.

All that comfort must be bulky and heavy, right? Think again. The Boost 650 was built to backpack—Therm-a-Rest dialed in the materials and design to make it light, packable and high-performing in variable backcountry conditions. Zoned Insulation places the 650-fill hydrophobic down in key warmth-retention areas (on top, around your core and shoulders) and reduces it where it’s less efficient (on the bottom), allowing the Boost to create more warmth with less down. The shell and liner are made of 100%-recycled nylon that was handpicked for its light weight and super-soft feel. The PFAS-free DWR coating and the hydrophobic down keep the Boost warm and comfy even in wet weather.