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 splashback 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, which 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 as 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 insulation, 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 drawstring 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″ Therm-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 ripstop 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 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 splashback 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.
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.
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This looks very good … with the exception that I would have to use a head net to keep out the bugs. I am not a fan of that idea!
That claims about being twice as breathable as Gore-Tex always seem like marketing spin to me. Not that I'm saying Gore-Tex is definitely better, but looking at the older BPL articles on breathability of waterproof fabrics, and the new ones about the war on Gore-Tex, and going by my personal experience, most waterproof breathable materials are pretty similar in functionality. And what matters most is that they work at all. It sounds like you've found one that works pretty well.
I'd be interested to hear how that tyvek cover you had a few weeks ago worked for long-term use, too. I've found fabrics like driducks and tyvek are the most breathable and inexpensive, if not quite as feather light as the Montbell.
I have been using a space blanket under my sleeping pads when winter camping in a floor less tent or tarp. I am intrigued about using a bivy/overbag. I am interested in one that is long and big enough to hold a sleeping pad. Since I am usually under a shelter breathability is more important than waterproofness. This bivy sounds near ideal, except that it lacks a zipper.
Does anyone have a recommendation/ suggestion of a bivy for me to try?
Jim – I drove myself crazy trying to find a lightweight bivy sack last winter with a zipper. If weight is important you pretty much have to go to Mountain Laurel Designs and pay to have one made (last years pricing – $330). If weight is not as important, I'd recommend you look at the ones from Integral Designs.
I end up buying this tarp from ID – https://sectionhiker.com/integral-designs-event-mi… – the zipper proved a lot less important than the price and it was only about 5 or 6 ounces heavier than a custom made bivy sack with a zipper. The long length 88" really helps for getting out at night to pee.
Great timing. I was helplessly confused about which bivy sack to buy but needed to make a choice in the next week for a trip I'm taking. Thanks much!
I've been reading your posts on bivies. You've reviewed several, this your newest. Are you using this to replace your MLD superlight and your ID Micro? I'm trying to decide what to get in conjunction with a Monk tarp for at least 3 season use. Thanks. Great site!
This is my oldest bivy actually. Nope, the superlight is my main bag for May through September, the Montbell for winter day hikes and for November, December and for March and April if I'm not in the mounains, and the ID from December through the beginning of April. They all do slightly different things and are sized for slightly different bags. The ID fits my -25 winter bag for instance. The Montbell is good for shoulder season is the superlight is good when I want to go UL and there are bugs.
Wow, thanks for the super fast response! If you could only have a jack of all trades bivy, perhaps minus winter, which would you choose? MLD or Montbell? There seems like there is overlap and I can't afford that many bivies. The Montbell with a headnet seems like it might be a great choice for the cost in CA mountain scenarios nearly year round? Thanks again for your insights.
That is one option,although the MB might be too warm in summer. You might have a look at the other bags that Ron Bell has at MLD that might work across seasons. My superlight is 2 years old now and his materials and designs are changing constantly.
Also check out this bivy – I bet the guy who made it will sell you one. You can ask at least. https://www.williswall.com/willis-wall-blog/2011/7…
Much better than a very cold night I spent at Yellowstone in an “emergency” bivy which I thought was a cheap alternative. not really an alternative at all since it was not breathable and cause a significant condensation problem. For a while I tried out OR bivy’s but they’re pricey and I enjoy being able to share bodyheat with my partner. How about a 2 man bivy… or is that just called a tent?
The bivies from Titanium Goat are nice and also affordable. I have the Ptarmigan with an optional side zip (not on the site, just ask for it). Now, I’d probably get the Raven Omni, since it is roomier, the zip is standard, and it is only an ounce heavier. They also make a bug bivy.
Does this bivy has enough room for a 4 seasons sleeping bag ?
Yes. It’s pretty huge.
Is this the wide and long version or the normal ?
Haven’t you noticed a peeling of the inside coating in this bivy bag ? here are some pictures of what i’m talking about.
You’ve probably just worn it out. That does happen.
Hi, it is brand new, just tried it at home…
I made some test at home with water on it and this little shading of the white interior face does not affect the bivy water column as i first thought.