I got hooked on using a bivy bag about two years ago, mainly as an adjunct to a tarp based shelter system, and originally to eliminate splash back onto my sleeping bag in the rain. Since then, I’ve expanded my use of bivy sacks year round and bring them on nearly every overnight trip I take and on many winter day hikes as an emergency shelter.
In addition to rain protection, bivies provide:
- 5 to 10 degrees of extra warmth
- additional wind protection
- bug protection when coupled with a head net
- keep me on top of my sleeping pad at night
However bivy bags can be very expensive to buy, and it often costs upwards of $200 or more to purchase ones that are made of high-tech breathable materials like Gore-tex or eVent. Instead of spending that kind of money on my first bivy sack, I bought the 6.3 ounce Montbell Breeze UL Sleeping Bag Cover ($140), is made of a proprietary breathable laminate that Montbell calls Dry-Tech. Montbell claims that Dry-Tech is more breathable than Gore-tex XCR, which is itself more than twice as breathable than previous Gore-tex products.
Breathability is important because you sweat at night and there’s bound to be condensation in your bivy sack if the air under your shelter is cooler than the air on the outside surface of your sleeping bag (see this post about tent condensation for the reasons why.) While most sleeping bag shell fabrics will vent this moisture, you don’t want it trapped by an additional layer like a bivy sack because it can soak your bag’s exterior shell and potentially compromise your sleeping bag’s fill, particularly over the span of a multi-day trip.
Bivy Sack or Sleeping Bag Cover?
What is the difference between a bivy sack and a sleeping bag cover? I’m not sure I know (it’s a marketing construct to a large extent), but if I had to hazard a guess, it’s because many sleeping bag covers don’t have a mummy hood and all bivy sacks do.
If that is the difference, then the Montbell Breeze is a full-on bivy sack because it has a draw string hood like many of the other bivy sacks made by Rab, Integral Designs, Outdoor Research, or Black Diamond. Keep that in mind if you’re looking for an affordable 3 or 4 season bivy sack.
In addition to a mummy hood, the Breeze is cavernous in size, and at 88.6″ long x 33″ wide, it is big enough that I can put a 3 season down sleeping bag and a 72″ Therma=-a-Rest Neoair sleeping pad inside it when I’m sleeping. The bag itself has a two layer construction with fully taped seams and a thin 12 denier rip-stop nylon exterior shell. There is no zipper to facilitate nighttime pee breaks unfortunately, but it’s easy to slip out of the Breeze hood and get back in again without too much bother.
One of the nicest features of this bivy sack is its pack-ability, because it compresses down to the size of your fist (or a Montbell down vest) in its own stuff sack, making it easy to bring on any kind of day hike or expedition.
Waterproofness and Breathability
In terms of performance, I’ve owned the Montbell Breeze UL Sleeping Bag Cover going on three years now, and I have slept outdoors in it many times under a tarp in dry and very wet conditions. It’s proven to be very waterproof to external splash back and wet ground and I have a lot of confidence in it as part of a bad weather system. I mainly carry it as a lightweight emergency shelter on early spring and late fall day hikes or overnights, when I don’t need a head net and my heavy-duty and much heavier winter bivy sack is overkill.
In terms of breathability, the Breeze works as well as my other more-expensive bivy sacks made of eVent or Momentum. I do occasionally wake up with dampness on the surface of my sleeping bag’s foot box in the morning. This doesn’t happen often, but it evaporates quickly when I put the sleeping bag into the sunlight while I make breakfast. You can spend as much as money as you want on breathable fabrics, but I’ve concluded that waking up with damp sleeping bag foot box periodically is just a fact of life. Condensation happens to all of us, even in a much more expensive Gore-tex of eVent bivy sack.
At $140, the Montbell Breeze is a very good value for money if you’re looking for a very compact and lightweight bivy sack or sleeping bag cover for use in combination with a tarp for ultralight camping and backpacking. Its large size, mummy hood, waterproofness, and breathability make it a good piece of gear for shoulder season day hikes or overnights where you want to move fast and light, but want to carry a little more thermal or weather protection for your sleeping bag or quilt.
Disclosure: SectionHiker owns this product and bought it with their own funds.
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