Does A Bivy Sack Add Warmth?

If your next adventure in nature is likely to be a cold one, you might opt to cut down on the weight of your sleep system with a bivy sack. Bivouac sacks are not only easier to carry and assemble than tents, but they also protect from the elements.

Bivy sacks have a design including a thin waterproof fabric shell. When slipped over your sleeping bag, this shell becomes a protective barrier against drafts, wind, and precipitation. But the question is, does a bivy sack add warmth? 

Bivy sacks can add up to 10 degrees of additional warmth while protecting you from the harsh elements when adequately fitted.

It’s a pretty incredible feat considering the minimal footprint a bivy sack takes in your pack. But how, exactly, does a bivy sack add warmth?

How the Bivy Sack Design Helps with Warmth

The bivy sack, also known by its full name, the bivouac sack, was initially designed for climbers needing emergency weather protection for their sleeping bags without adding excessive weight to their packs.

The two primary functions of a bivy sack include:

  • Keeping your sleeping bag dry
  • Increasing your warming capacity

While bivy sacks have come a long way since their original designs, today, you will find two critical elements to every sack: the bottom and top tiers.

Bottom Tier

The bottom tier, or the bottom half, of a bivy sack, is made from materials similar to tent floors. Commonly durable nylon with urethane coating or something similar, these bottom tiers are crucial in providing waterproof protection from the ground.

Albeit thin, this layer of waterproof protection helps create an added layer of separation from the cold ground, helping to insulate your body within the sack overnight and prevent any moisture from entering.

Additionally, this material tends not to be breathable, ensuring no heat escapes from beneath you. In this way, the bottom tier helps you retain heat, potentially adding a few degrees of warmth.

Top Tier

The top tier of a bivy sack is made in a wide variety of different materials but tends to be a lighter, more breathable fabric.

Typically, this tier will have some type of waterproofing coating. Look for heavier duty top tier materials with waterproofing to ensure you are capturing optimum warmth in your bivy sack overnight.

Head Opening

Bivy sacks come with various head opening designs, depending on the level of protection you seek. Having a head opening means you compromise some of your warming capability and run the risk of allowing moisture into your sack, decreasing body temperature.

Some sacks have drawstring openings that allow you to cinch them up tightly, just around the nose allowing for ventilation.

For the best odds at warming up in a bivy, you will need to keep your head covered and avoid moisture creeping into the sack.

Warmest Bivy Sack Materials

As previously mentioned, several different material options make up bivy sacks. Some are more effective at retaining the most heat possible, even during extreme temperatures.

Silnylon

As the name suggests, silnylon is a combination of silicone and nylon. Silicon brings strength, and the nylon adds waterproofing. Silnylon prevents rips and tears via a ripstop weave, making it an excellent fabric for the bottom tier of your bivy sack.

Silnylon is not breathable and therefore is best for the bottom tier of your bivy sack only. There are some cheaper alternatives out there that use silnylon throughout, but be cautious of this.

Gore-Tex

Gore-Tex is a laminate material made from Teflon, a polytetrafluoroethylene (PTFE). Not only is Teflon waterproof and windproof, but it offers breathability, making it an excellent option for your top tier. 

This material will undoubtedly keep all moisture from entering or exiting the sack, think water vapor from sweat or breath. Hence, it is best recommended for icy weather conditions only.

eVent

Fire departments, police, and the military use material found in some bivy sacks known as eVent. This material uses a dry technology that immediately eliminates moisture and is waterproof, breathable, washable, and fire-resistant.

This specialized material also protects from hazardous substances making eVent an excellent top tier for extreme cold.

Bivy sacks can add warmth, but choosing the right material for your bivy sack is critical when it comes to maintaining the maximum level of heat. 

How Bivy Sacks Minimize Heat Loss

There are three primary mechanisms in which you lose body temperature, especially overnight, in colder climates. Bivy sacks do help to minimize these opportunities and increase warmth inside the sack.

Convection

Heat loss through convection occurs when colder air replaces the warm air immediately surrounding your body.

More than the material of your sleep system, bivy sack, and sleeping bag included, it will be the air around you trapped within it that keeps you warm through the night.

Windproofing in your bivy sack is a crucial component preventing warm air from being moved away from your body and out of your sack.

Conduction

Conduction heat loss occurs via direct contact with cold surfaces. A wet sleeping system means your heat will conduct into them at rapid rates and away from your body.

Waterproofing in your bivy sack will help minimise heat loss through conduction, especially in the bottom tier. Adding even just the bivy sack’s thin layer helps stop the heat from being sucked from you into the ground beneath you.

Evaporation

Sweat keeps the body cool on hot days through evaporation heat loss. Alternatively, if you are wet in cold climates, you will lose significant body heat at rapid rates.

Bivy sacks help reduce moisture levels and evaporation heat loss from coming into contact with your sleeping system. 

Best Practices to Maximize Warmth

Purchasing a practical bivy sack is just the first step of the process!

You can follow several other best practices to help maximize your warmth when it is needed most.

  • Ensure your sleeping kit includes a sleeping bag with enough loft and is small enough to fit comfortably in the bivy sack.
  • Trap plenty of air around your body to insulate you from the cold.
  • Insulate your body from cold surfaces using a low heat conductor like a camping mat.
  • Ensure your sleeping bag is fitted with suitable baffles and drawcords to increase heat retention near your head and face.
  • Keep finger gloves or mittens inside the bivy sack with you so they are not frigid the morning after.
  • Warm your body right before getting inside your bivy sack by engaging in some physical activity.
  • Protect your bivy sack from moisture and condensation as much as possible.
  • Keep your sleep system clean as avoiding doing so may deteriorate insulation properties.
  • Carefully consider your camping grounds as latitude differences of at least three feet can show temperature changes.
  • Eat a warm, hearty meal full of protein.

By following these careful steps, you can be sure to capture as much warmth as possible in your sack to help keep you warm through the night.

Types of Bivy Sacks

Traditional

Bivy sacks typically come in two types, traditional and ultralight. Traditional bivvies are a stand-alone, waterproof sheltering system with poles. They usually weigh between 1-2 pounds and are designed with mosquito netting and hoop to keep the bag a few inches off your head.

While traditional bivvies can be useful for shelter and warmth, they do create some challenges. For instance, during sustained periods of precipitation, reviewers often report them to be claustrophobic and challenging to move around in.

Traditional bivvies are also prone to condensation and breathability issues. Excessive condensation can occur from released vapors due to breathing and may collect on the inner wall of the bivy fabric.

In addition, vapors produced from body heat will not escape without breathable materials like nylon or Gore-Tex.

Ultralight

Ultralight bivvies, or commonly referred to as minimalists, are usually the most reliable and desirable of the two. They weigh considerably less than traditional ones, typically between 4.5 and 7.5 ounces, and are almost always made with water-resistant and breathable materials.

They are useful when it comes to draft protection with a healthy warmth-to-weight ratio. Due to the bivy’s small footprint, you have the freedom to camp almost anywhere, without spending time searching for a suitable building location.

Ultralight bivvies are not stand-alone, however, and do typically require a tarp to shelter from precipitation.

Is a Bivy Sack Right for Me?

When deciding upon using a bivy as your portable sleep system, there are various factors to consider. Some temperatures and climates may not be suitable. Also, the nature of your adventure may impact the comfortability you’ll gain from a bivy and its ability to supply optimal warmth.

You want to ensure the bivy model has ventilation options and is waterproof. It should also have extra zippers to vent out moisture and minimize condensation.

You should always consider the following points before purchasing a bivy sack.

Physical Environment

Consider where you are going and what the weather conditions might be. You might be going somewhere with a lot of moisture and rain, or instead, you may be planning to camp over snow.

Perhaps you’ve set off with the intent of hiking through the woods or possibly mountain climbing to high latitudes.

There are specific environments where you would need a lighter-weight bivy, while in other locations, warmth is a top priority. Areas with a lot of moisture and humidity will likely require a bivy sack with more breathability.

Some environments might also have lots of insects, in which case a mesh attached to your bivy would be ideal.

Weather Conditions

Perilous wind, rain, or snow may be likely at your destination. You’ll need to ensure that the bivy you choose has proper ventilation and breathability.

Humidity from increased levels of body warmth typically condenses on the inner side of the bivy. Because moisture retention severely decreases insulation, breathable materials are essential for warmth.

Many bivy sacks cinch down your face, leaving a small hole to breathe out of as breathing while entirely enclosed will also increase condensation. The tarp is typically used to protect you from rain that might go through this hole.

Additionally, while minimal amounts of precipitation are easy to withstand while inside a bivy sack, being in a location with constant torrential rainfall is less than ideal, and you may be more comfortable and protected under a tent.

Weight of the Bivy

Heavy or bulky bivy sacks are counterproductive to their purpose, especially if your trip requires a high amount of uphill movement.

Mountain climbers would likely need a bivy sack that’s as lightweight and compressible as possible. Backpackers traveling along very far distances will need something light as well, with enough storage for essentials.

For individuals who plan to stop frequently to take in the sights or walk along flat grounds, an Ultralight bivy sack may not be as necessary and could even be replaced altogether by a tent if you so choose.

Your Body Type

Everyone’s body type varies; therefore, yours should be taken into account when taking in generalized information about bivy sacks.

The size of your body will indicate what type of bivy sack you will find the most comfortable. Check the shape of the bivy’s foot-box and hood, as well as its overall size, to ensure you don’t feel trapped in a tight space.

Consider your sleeping style. Do you like to sleep on your back, stomach, or side? In addition, if you are someone with very broad shoulders, you may not be able to move in your sleep comfortably.

Your height and weight will also be a determinant of a bivy’s comfortability and warmth. If you have a relatively petit frame, you might find it more difficult to create enough body heat.

Comparatively, if you are a larger individual, you might become overheated more quickly and create more condensation.

Conclusion

Bivy sacks are a great sleeping system that helps increase body warmth by up to 10 degrees while enduring some of the harsh climates of nature. 

Consider the environment of your destination, the weather conditions, climate, as well as your body type when searching for the right bivy sack for you. Be mindful of the bivy sack’s materials and ability to provide breathability in the event of condensation and the presence of moisture.

Using a bivy sack as a sleeping system is certainly not for the faint of heart, but they are much easier to manage than tents and can offer a rewarding experience with nature as you sleep under the stars.

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