Gear Closet: Big Agnes Thunderhead SL 30 Zipperless Sleeping Bag Review

Big Agnes Thunderhead SL 30: We’ve seen a lot of innovation in the sleeping bag market over the past couple of years. For instance, the introduction of hydrophobic down was downright revolutionary. In contrast, the introduction of body mapping and gender-specific design has made it easier for both men and women to get a warmer, more comfortable bag.

The outdoor industry isn’t finished revamping the humble sleeping bag just yet, as there are still ways to make this vital piece of gear smaller and lighter while improving performance as well. Case in point, the new Big Agnes Thunderhead SL 30, which is designed to appeal to ounce-counting backpackers who don’t want to scrimp on comfort.

The biggest selling point on the Thunderhead SL is that it has done away with zippers altogether. Most bags use at least one – and sometimes two – zippers to seal themselves up and keep the person snuggled up inside the warmer.

Zippers also add weight and tend to malfunction and create excess noise that wakes other campers inside the tent. Big Agnes overcame those issues by creating a clip and loop system that effectively seals up the bag nicely while shaving ounces from its overall weight. This system is also easy to use and is virtually silent too.

So just how much does this sleeping bag weigh? The Thunderhead SL comes in at 2 pounds, 2 ounces (964 grams) for the regular version, and 2 pounds, 5 ounces (1.04 kg) for the long model. That isn’t the lightest sleeping bag I’ve ever come across, but it is certainly in the ballpark and is definitely a respectable number. This bag is particularly rated for 30ºF (-1ºC) and stuffs down to a minimal size.

Without a zipper, some campers will probably be concerned that cool air can still make its way inside the bag. But Big Agnes has designed the Thunderhead with a cocoon-like construction that keeps warmth in and cool air out, making it just as comfortable as any normal sleeping bag that I’ve used with a zipper.  In fact, other than using the hook and loop system, I didn’t notice much difference in overall performance, although it was nice that the bag was quieter when getting in and out.

Other nice features of the Thunderhead SL include a vaulted foot box to give your feet a little extra room, which can sometimes be at a premium in a mummy-style bag. The bag also has interior loops for attaching a liner, which is great for adding extra warmth, and an exterior half-sleeve for connecting to a sleeping pad.

This prevents the Thunderhead from sliding around too much and allows the person inside to move about more naturally without fear of rolling off their pad. Thanks to plenty of interior space, it is even possible to sleep on your side if you choose.

Big Agnes has given the Thunderhead 650-fill power of insulation by way of water-resistant DownTek down. This makes the bag a much better option when used in wet and cold environments, and quite frankly, I’m not sure why you’d buy a sleeping bag (or down jacket) that isn’t using hydrophobic down these days.

The price difference is negligible, but the difference in performance is enormous. If you take your backcountry or adventure travel excursions seriously, you’ll definitely want this material in your important down products.

If you’re in the market for a new sleeping bag and are looking for something lightweight, easy to pack, and takes up very little room in your backpack, the Thunderhead SL 30 should be on your shortlist. Its zipperless design shaves ounces off of traditional models but still provides the warmth and comfort you need on cold nights in the tents. The hook and loop closure system perform nicely and still manages to keep warmth in, while the bag’s interior is spacious and very comfortable.

Priced at $249.95 for the regular and $269.95 for the long, the Thunderhead SL 30 is about at the middle of the road in terms of price. Yes, you’ll find 30º bags that cost less, but very few of them will offer the same weight-to-warmth ratio, nor will they deliver in terms of overall performance. The few extra dollars that you pay for this model will pay off in the field, making it a great choice for backpackers, through-hikers, and adventure travelers for sure.

Kraig Becker