A bivy sack or shelter is actually a bit of a conundrum for most users – it is both a tent and a sleeping bag, but in its real sense, it’s neither of these things. At its most fundamental, a bivy is a private shelter for climbers, skiers or backcountry hikers who are more concerned with their get-out objectives than they are with enjoying their time in tents.
There are a whole lot of bivy bags and sleeping bags on the market. Just remember that when you decide on getting one, there will always be a trade-off between weight and features.
A sleeping bag can be stuffed into a small package, but it will not be as comfortable or offer enough protection as well as a heavier or bulkier bivvy bag when you’re caught in a severe storm. So, when you’re in the market for a sleeping bag or bivy bag, first decide whether you require a full shelter or just a rain fly – if you already have a sleeping bag.
To this end, you’ll get a lot of bivy bags that offer a range of technical features that correspond to their weight and intended use.
Bivvy Bag vs Sleeping Bag – Key Differences
A bivy is quite bigger than a sleeping bag – also making use of tent poles to create a minimalist shelter. In other words, it’s an outer shell meant for your bag, built with waterproof and breathable fabrics that can repel nasty and extreme weather. It’s the most suitable choice of shelter for alpine climbs, solo through hikes, big wall expeditions and other adventures where taking a full tent isn’t really feasible.
A bivy (AKA bivouac sack) can act as a waterproof cover for a sleeping bag that may be used instead of a tent by experienced and minimalist hikers. A bivy bag can also be used by day hikers as an emergency shelter or backup which may be used in the event that they can’t make it back to their starting point when it gets dark either because they got lost or the weather deteriorated.
When selecting a bivy shelter or bivy sack, ensure that you choose one that is highly rated as being waterproof with pre-taped seams. This is imperative if you plan to use the bivy sack as a standalone shelter in the snow, rain, or inside a snow cave.
Having a breathable waterproof top is also important for venting water vapour and helping to minimise the internal condensation that will make the sleeping bag wet.
It is also important not to overlook the importance of zippers or vents which can also be very effective in reducing the level of internal humidity you might experience.
One of the key differences between bivy sacks and sleeping bags covered with waterproof/breathable laminates are the seams. A lot of sleeping bag manufacturers don’t seal or tape all of the seams in their bags, which is something actually required for complete waterproofing. Water could pass through all the tiny needle holes in the baffling of a down bag.
On the other hand, most of the bivy bags that are made with waterproof/breathable fabrics have taped seams. They can also be easily sealed using seam sealer. This cannot be done with a sleeping bag.
Experience has shown that when you cover the exterior of a sleeping bag using a waterproof breathable shell, it’ll tend to store more perspiration inside the insulation of the sleeping bag than one that has a lighter shell fabric. This perspiration gets trapped in synthetic or down insulation, thereby reducing the bag’s ability to store heat.
This then means when you sleep inside a bag that has a waterproof cover, there’s a chance you might feel colder. In extremely cold climates, there’s a chance of the perspiration freezing inside the bag and then melting when you get back into the bag.
Contrary to what you’d normally expect, breathable and waterproof fabrics are actually less breathable than a lot of the non-waterproof shell fabrics that are used on the exterior of sleeping bags these days. You can get a similar waterproof benefit by spraying some waterproof coating on the outside part of these lighter weight and more breathable fabrics.
This way, the fabric will repel water droplets that fall on the outside of the bag – making them bead and roll off like it would on a raincoat/jacket.
A lot of sleeping bag manufacturers already do this at the factory level. However, if the waterproof coating of your sleeping bag wears off, you can reapply it at will using Nikwax TX Direct or a similar product.
Ability to Absorb Moisture
One of the biggest disadvantages of sleeping bags – particularly down sleeping bags – is that they absorb a lot of moisture, which takes a long time to dry. Drying a wet down bag outside in wet conditions is virtually impossible. However, it isn’t actually difficult to keep your down bag dry and it doesn’t have to be a chore.
All you have to do is pack your bag in a waterproof stuff sack and ensure you sleep in a bivy, tent or tarp whenever it rains.
Adhering to these simple rules should be enough to ensure that your bag remains dry. You should also remember to air down sleeping bags to remove moisture that has been picked up from humid air or from the user’s body.
This problem that down sleeping bags have isn’t really evident in the range of bivy bags.
Waterproof/breathable sleeping bags are usually heavier and more expensive than bivy bags with normal lightweight shell fabric. They are also quite specific in the destinations where they can be used when compared to regular sleeping bags. It would be advisable to get a regular sleeping bag, then accompany it with a lightweight, breathable and waterproof bivy sack on trips or hikes where or when you need it.
In my own opinion, I prefer to invest in a number of different outdoor items that I can decide to mix and match as required for the different trips I’m embarking on.
For example, if you get a bivy sack, you can decide to combine it with several sleeping bags or sleeping quilts – depending on the requirements and needs you have for a particular destination.