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Outdoor Research Interstellar Bivy Review

Wearable Bivy Sack

The Outdoor Research Interstellar Bivy is a 4-season bivy that features an upper made with AscentShell, a proprietary waterproof breathable fabric made by OR that has some built-in stretch. While the floor of the bivy is nylon coated TPU for more conventional waterproofness and abrasion resistance, the addition of the AscentShell upper and Derlin single-pole system provide ample yet stable headspace inside a waterproof/ breathable bivy system that weighs a mere 23 oz when fully packed for the trail.

Outdoor Research Interstellar Bivy

Interior Room
Ease of Access

Highly Breathable and Waterproof

The Interstellar Bivy is weather-proof and roomy enough to replace a one-person tent in the right circumstance. Highly breathable and lightweight, it's a great option when weight and packability are must-have shelter attributes.

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Specs at a Glance

  • Opening: Side Entry and Clamshell
  • Head Pole: Yes
  • No-Seeum-Mesh: Yes
  • Seasons: 3 and 4 (winter)
  • Fabric: AscentShell 3L – seam taped 100% nylon 20D ripstop upper, w/ 100% nylon 40D ripstop with TPU floor
  • Dimensions: 82” x 26” x 17” (length x top width x top height)
  • Trail weight: 19 oz


A bivy is a lightweight, low volume sleeping shelter for one person. Made up of varying yet simple components, a bivy is designed to provide protection from the elements like a tent would do, but without the weight, bulk or complexity of one. When I think of a bivy I think simple, fast, lightweight, and small enough to fit inside a low volume backpack. Bivies come in all types and differ in functionality. Some offer bug protection, emergency survival, military expediency, waterproofness, breathability, 3- season, 4-season, structured and unstructured options or some such combination or all of the above. The Interstellar 4-season bivy seems to have checked all the boxes and improved upon the bivy concept.

The Interstellar Bivy is a waterproof/ breathable bivy system that weighs a mere 23 oz when fully packed for the trail. shelter system that provides a structured headspace by using a single-pole for stand-alone structure or tying a guy line from the top of the bivy to something above it. The fabric is comprised of a waterproof/ breathable upper made from OR’s AscentShell fabric, and a nylon TPU-coated floor for robust waterproofing and abrasion resistance, which eliminates the need for a ground tarp. The bivy zipper system allows for side entry and the user can sit up while inside it, put arms out and complete camp tasks all the while covered by a bivy “sombrero” that is created by the single-pole and an indentation on the top of the bivy for one’s head.

Bivy features
The Delrin rod improves livability at the head end of the bivy by keeping the mesh or cover off your face, without the need for a tarp overhead.


The AscentShell upper is made of a 20D ripstop, 3-layer fabric that is waterproof while remaining breathable (air-permeable) and a bit stretchy. According to OR’s website, this proprietary fabric is created “by an electrospinning process where charged polymers are affixed to a grounded surface. Those polymers gather into a breathable membrane that, under a microscope, you can see forming a web that allows sweat vapors out but stops water from coming in.” The fabric is soft to the touch and feels a bit stretchy, and these are attributes that OR claims result from the electrospinning process.

Now, whether or not you understand the finer points of electrospinning, the bottom line for any technical fabric is its performance in the field. I must say I’ve been impressed by how well it breathes while keeping me warm, comfortable, and free to toss and turn (as I usually do) throughout the night.

The floor of the Interstellar Bivy is 40D nylon ripstop that is coated with TPU. TPU is a protective thermoplastic polyurethane membrane, which is bonded to the nylon floor fabric creating a durable, abrasion-resistant, and waterproof fabric that is also UV resistant. This is a robust fabric coating designed to stand up to any type of weather, intense UV exposure, and multiple ground types. It is also strong enough to eliminate the need for a ground tarp, even on surfaces that can cause punctures or tears.

Interior Room

The single-pole keeps the top of the bivy well away from your head and provides enough head and shoulder space for all types of sleepers, even tossers, and turners like me. Not only that, but it is remarkably stable and doesn’t fall in your face while sleeping and moving around inside. While zipped inside, I had plenty of room for my head and upper torso, and that sleeping on either side, on my stomach or on my back was easy. The pole, which is threaded through a sleeve on the top head end of the bivy, is effective while free-standing, secured to the ground with one tent stake, or suspended from an overhead branch with a guy line.

Side entry makes it much easier to get in and out of the bivy than a clamshell opening

Breathability and Ventilation

The upper (zipper-able) portions of the AscentShell and bug mesh allow the top third of the bivy to open for easy entry and adjustable ventilation. The AscentShell upper can be partially unzipped or completely unzipped and rolled up neatly and secured with an external fastener, thus increasing the ventilation and ensuring bug protection.

At 6’0’’ and 190 lbs, I have a relatively long yet lean frame, and this bivy does not feel constricting at all. I’ve used a Therm-a-Rest NeoAir mattress and a 32 degree Mountain Hardware Ratio down sleeping bag inside in freezing weather without experiencing any internal condensation. That’s pretty impressive when you consider internal and external temperature differential that would normally result in internal condensation, regardless of shelter type. The breathability of the AscentShell fabric is apparent when rolling up the bivy to put it back in its stuff sack. It is easy to roll up from the head/mesh end to the foot tightly and quickly as air easily escapes from the AscentShell fabric.

Both the zippered AscentShell upper and no-see-um mesh sections of the bivy can be completely zipped up to seal off the shelter from the elements. The bug screen and AscentShell zippers are labeled “head” and “arms”, and the most effective way to seal the bivy is to zip all head zippers up from the sides to the top of the bivy above one’s forehead. The arm zippers are best left zipped down along the bivy’s sides. These zippers allow for side entry to the bivy, which is far more convenient than the old school clamshell openings you find on most four-season bivy sacks and which require you to slide in feet first.

But with so many zippers it is important to employ some focused zipper management to the process of setting it up – willy nilly zippering will lead to frustration, especially in the dark! I suggest you add some distinct zipper pulls to them so you can tell them apart by touch in the dark.

The OR Interstellar Bivy has lots of interior headroom and it’s easy to stargaze too,

Wearable Bivy

What really makes this bivy interesting is that it was designed to allow the user to sit up, put both arms out, zip it open in front of the face and wear the top part of the bivy like a sombrero. The top of the bivy even has an indentation for the head, which truly makes it feel like a sombrero. The ability to sit up and do camp tasks, while nice and cozy in your sleeping bag on your mattress, is useful when facing inclement weather in camp.

The OR Interstellar Bivy weighs 19 oz alone and the single-pole, stuff sack, and two aluminum ground stakes (not included) add an additional and optional 4 ounces extra for a total packed weight of 23 oz. The bivy can be used effectively with or without the poles, stakes, and stuff sack; a feature the gram shavers will no doubt appreciate. Personally, my preference is to use the entire 23 oz package rather than ditching the pole and stakes for the guy-line-only approach. I really like the relative roominess of the head and shoulder space created by the pole, and the ability to stake out the bivy at the head and the foot. I felt like this keeps me rooted in my chosen sleeping spot, and better able to toss and turn until morning.

Comparable Bivy Sacks

Comparable 3-4 season bivy sacks with head pole, no-see-um mesh, and side entry.

Make / ModelWeightWPB Fabric
Big Agnes Three Wire Bivy30 ozProprietary (37.5)
Black Diamond Spolight Bivy17.6 ozNanoshield
Black Diamond Bipod Bivy27 ozToddTex
Outdoor Research Stargazer Bivy19.2 ozAscentShell
Outdoor Research Interstellar Bivy23 ozAscentShell


The Outdoor Research Interstellar Bivy is a 4-season waterproof/ breathable bivy that is light and effective enough to replace a 1 person tent in a backpacking kit. The breathability of the AscentShell fabric and the resulting lack of internal condensation is impressive. Designed to function well in varied terrain and weather, this bivy is an all-in-one option that provides ample head and torso space for comfortable sleeping and functions as a wearable shelter while performing seated camp tasks in inclement weather. Weighing less and priced lower than most one-person tents available today, it is a good go-it-alone option for trekking the world over.

Disclosure: Outdoor Research donated a bivy for this review.

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  1. I would be interested to hear from one or two people who have used this in a good soaking all-night rain.

    • I’ve been using it in the AT during my thru hike and it’s a great bivy sack. We’ve had terrible weather this year. Keeps me dry in rain with hardly any internal condensation, The big netting and side entrance are also great especially when I can get a spot inside a shelter. I’ve owned other bivy sack and they simply don’t compare.

    • Without a tarp you will have to keep it zipped up in the rain, and Ascentshell’s breathability however great, is significantly compromised when there is a thin film of water running over it. I usually leave three 2-3inch openings using the zippers in the head area to help with ventilation, but my sleeping bag is soaked in the feet and head areas by morning usually, though admittedly a little less than other bivys. Ascentshell is great, but isn’t a miracle fabric when saturated, and have the same limitations as other ‘breathable’ fabrics. So there will be internal condensation sans tarp in prolonged rain.

      Now when it doesn’t rain… it will breathe like a dream! I don’t think I’ve ever experienced condensation when it didn’t rain, even with the zippers closed completely in humid weather.

      • Sounds like you need to reapply the DWR coating.

      • I shouldn’t have used the word ‘saturated’ when I was leaning towards inundated or overwhelmed. Iit is a bit misleading, I admit.

        I actually had the same hunch and inspected the bivy after my first night, worried that the culprit might be the dwr, but the fabric itself wasn’t saturated at all. It had rained all night and was still raining in the morning, and the water was beading off the dwr as well as any new coating I’ve seen. There was no pooling, no telltale sign of darkened fabric. The interstellar seems to handle short showers well, but I’ve always been a little wet after all nighters in it.

  2. Nice review Erik, thanks for the info. I may buy a bivy this year and have researched a few. In addition to the OR bivy’s, I also had the Mountain Laurel bivy on my short list. Do you have any experience with the Mountain Laurel bivy that comes in at 8 ounces for a three season bivy? Thanks for your input.

    • MLD sells several bivy sacks.
      Based on the weight, I assume you mean the MLD Superlight. For one, it’s not waterproof and not meant as a standalone shelter. You need to use it under a tarp.

    • Thanks Len. No I do not have any experience with that bivy. What I really like about the OR bivy is that it doesn’t require a tarp and it is lighter than many one person tents at that price point. It’s a good solo sleeping option from my perspective.

  3. Thanks for the additional information guys. Yes I was thinking of the superlight model. I wasn’t aware of the need for a tarp for that bivy and I did want a stand alone type of shelter. Appreciate the help with this.

  4. Thanks for this review. Have you used it in winter? I’m researching bivies for emergency kit on winter dayhikes.

  5. Doing some more looking, it seems the Stargazer is also 4 season and just lacks all the extra zippers?

  6. Hello Erik,
    I’m 6ft3 tall
    I see your feet are touching one end while your head is touching the other !!
    the spec states 82 inches = 6ft10 … Is that going to be enough for me ??
    especially when sat up with a sleeping pad !!!

  7. No water column, no mph, Open research doesn’t provide the most fundamental spec for this very expensive product. I had the Helium bivy, they announced 20000 mph and water column. Worst result ever, my tarp wrapped around me performs better. I can’t trust their product anymore, I just remind when my sleeping bag was wet by 30°.

  8. I like the design but for extra comfort try adding two pole pockets near the shoulder blades. Add the small criss-cross poles and you have back support. No different than a separate hikers sling seat, but built into the bivy.

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