Outdoor Research Interstellar Bivy Review

© Dan Bailey

Hooped bivvys give you a bit more space and comfort than a simple bag, but less so than a tent. In these days of ultralight single person tents, does the hooped bivvy still offer any advantage? I've often pondered this question - and in this case the answer is yes, in some circumstances. Top model in a new range from Outdoor Research, this excellent hooped bivy (alternative spelling) is as light as some bag-only bivvy bags, and it has a unique feature too: The hooped end can be worn on your head, like a large waterproof sombrero, allowing you to sit up and cook etc whilst remaining inside the bag and protected from rain.

I'm not much of a morning person  © Dan Bailey
I'm not much of a morning person
© Dan Bailey

It's designed, say Outdoor Research, for thru-hikers, backpackers and ultralight alpinists. While I'm not sure there's a useful distinction to be made between a backpacker and a thru-hiker in terms of kit requirements (or at all, really) the general theme is obvious - this is a minimalist alternative to a tent that's aimed at all weight conscious mountain travel, whether you're walking or climbing.

Room with a view...  © Dan Bailey
Room with a view...
© Dan Bailey

I think it's ideal for use in the UK, being arguably a bit better than a conventional bivvy bag in wet weather. It also has obvious application in an alpine setting since it is genuinely light enough, while still offering you a good bit more than a simple sack design. I can certainly see myself using the Interstellar on trips like the Cuillin Traverse.

Weight and pack size

Pole and pegs fit in the stuffsack... just  © Dan Bailey
Pole and pegs fit in the stuffsack... just
© Dan Bailey

You're supplied with a bivvy bag and a short collapsible plastic pole. In its stuff sack this weighs just 590g, not a weight that's going to break the bank. By comparison a simple non-hooped bivvy I happen to have here, an old Rab Alpine SL, weighs 416g. The Interstellar is a more spacious bag, and I think that extra 174g is a price worth paying for the additional livability you're getting.

Versus a small single person tent, the Interstellar comfortably beats most mass market models for lightness. At a bearable budget you can expect your one-person tent to weigh around 1kg. Pay a lot more and there are ultralight specials that come in at a similar sort of weight to this bivvy, and in a few cases undercut it. So yes, you can get more space for less weight than the Interstellar. For example, take the new Terra Nova Pulse Ultra 1, billed as the lightest tent in the world at just 490g. Sounds amazing: But this costs around three times as much as the Interstellar, and I'd be willing to bet its fabrics are not half as tough. You'd never take that to the Alps. Overall I think the Interstellar's balance of weight and space is a good one.

Lighter than a tent, slightly more palatial and protective than a standard bivvy  © Dan Bailey
Lighter than a tent, slightly more palatial and protective than a standard bivvy
© Dan Bailey

Pegs add a fraction to that weight, and you have to supply your own.

With cold hands, in the wind, it's a bit of a struggle to cram into its bag (feet first, to squeeze the air out) and I think a marginally larger and more robust stuff sack would have been good, at the cost of only a few grams to the headline weight. The packed size is a negligible 30cm long - easy enough to squeeze into a compact climbing or trekking pack.

Setting up

The pole forms the single upright span. At first I found it a bit hard to force into its sleeve around the head end - and you don't want to push too hard since it's only plastic. But things have softened up after repeated use and it now slides in easily enough. It simply velcros into place - there are no eyelets, as you'd find in most tents. Now peg out each end of the bag and you're done; the whole procedure takes seconds.

You can peg it out at each end  © Dan Bailey
You can peg it out at each end
© Dan Bailey

For added stability in wind you can guy it out at the head end - though no guyline is supplied. While it's not a serious issue I have found that the upright section of the bag can twist a little in use, or lean slightly off the vertical, since the bottom ends of the arch are free to shift about; side pegging points might have stabilised the structure at the head (I'd be tempted to sew my own if I was sure of doing a decent job).

Size and livability

The Interstellar comes in only one size. At 183cm tall I can sleep in it with room to spare, so it's safe to say it's long enough for tall-ish adults. There's plenty of width at the head end too; the experience is not claustrophobic and there's room to wiggle about a bit, and roll over in your sleep. In terms of height, while you won't be sitting upright that hoop does give a bit of headroom and holds the fabric well away from your face so there's less of that enclosed feeling you can get in a non-hooped bivvy. I've found I can read a book inside the bag for instance. With a bit of contorting I could probably change my top layers without emerging. I can also keep a torch, book, bottle and spare clothes at the head end of the bag without losing everything. It's never going to match even a tiny tent for livability, but if I was bag-bound in rainy weather I'd far rather it was in the Interstellar than a non-hooped bivvy.

With a profusion of zips, make sure you've closed them all before the weather comes in  © Dan Bailey
With a profusion of zips, make sure you've closed them all before the weather comes in
© Dan Bailey

Mat in, or mat out of the bivvy bag? It's been an open question for as long as both have existed. I tend to favour mat inside, and in this case doing so holds out the sides of the bag and helps maintain its structure and tautness. However this is still a moot question, because while the Interstellar is roomier than many bivvy bags, its internal space is not limitless. Add a thick inflatable mat inside, especially a wide-footed oblong mat, and bag room at the toe end does begin to get tight. This is something I've noticed when trying to lie on my back, when there's simply not enough slack to comfortably accommodate my feet in upright position. Yes I do have massive feet; but I can't help thinking that more of a box shape at the toe would have been an advantage. Failing that I will just have to remember to use a thinner mat, or a tapered model.

Multiple zippers  © UKC/UKH Gear
Multiple zippers
© UKC/UKH Gear

Insect mesh  © UKC/UKH Gear
Insect mesh
© UKC/UKH Gear

Underneath the waterproof layer is a secondary flap made of insect-proof mesh. Although midge season has yet to get underway this will doubtless be extremely welcome over the summer months, making Scottish bivvies if not pleasant then at least humanly bearable. So far I've not needed it, and have simply tucked it down into the bag where it's completely out of the way. We don't have many bone dry nights in the UK, there's always some dew, but if you do want to sleep entirely open to the stars then you can roll the main door down and secure it with a little retainer, leaving just your legs under cover. I imagine that would be lovely on a warm summer night in the deserts or mountains of the US, where this bag was designed.

It's worth mentioning that the zip-around entry is enormous, so unlike a top-opening bivvy bag there's never a struggle to get in and out.

The sombrero

This is really the unique feature of the Interstellar, the obvious thing that sets it apart from other hooped bivvys. A dimple in the fabric at the hoop end is shaped and sized to perfectly fit onto your head. Assuming you're not pegged down, you can sit upright and the head end of the bivvy effectively becomes a wide brimmed hat, with the pole forming the rim. This looks a bit unusual (no points for style) but it works surprisingly well.

The 'sombrero' lets you sit up yet remain weather proof, while multiple zippers allow for cooking without emerging  © Dan Bailey
The 'sombrero' lets you sit up yet remain weather proof, while multiple zippers allow for cooking without emerging
© Dan Bailey

The result is that if it's raining (or, in the case of the above picture, very cold and windy) you can sit up and perform close tasks such as cooking without having to leave the shelter of your bag. It's a neat idea, and a solution to a genuine issue since such things are normally a bit of a struggle to achieve in a standard bivvy bag. Nice, too, that you can sit around at a camp and still stay out of the weather; in a standard bivvy you're either lying down snug but entombed, or you're out in the cold.

To facilitate the hands-free aspect of this design, OR have had to add a lot - and I mean a LOT - of zippers. The door flap has six zippers, two for each arm and two for the head/main entry, in addition to which the insect mesh has a further six corresponding zippers. That's a total of twelve zippers, each of which is double sided so that you can use them from inside or outside the bag. Add a large zip pull to every possible point and you're left with a potentially confusing array of dangly bits. It's fine when you can see what you're doing, but despite the addition of glow-in-the-dark zip pulls (a nice touch) it can be hard to know which zipper does what when you're half asleep in the middle of the night and fumbling to get out for a pee. Before retiring you also have to be very careful not to have left an inadvertent opening somewhere, or you'll risk a soaking if it rains.

Despite the plethora of zippy bits I think the whole thing is a success, adding a new degree of usability to the traditional hooped bivvy design. I can see it being particularly good on a cramped alpine bivvy ledge. Sounds cheesy but, like OR says, it really does blur the line between apparel and accommodation.

Settling down for an evening of stargazing in the Interstellar  © Dan Bailey
Settling down for an evening of stargazing in the Interstellar
© Dan Bailey


The top side of the bag is Outdoor Research's proprietary waterproof/breathable AscentShell, a 3-layer, 20 denier nylon membrane fabric. I've not yet bivvied out in prolonged wet weather - even a reviewer has to draw the line somewhere - but so far it's kept me dry in a passing shower and light snowfall. It is fully seam taped, and on paper its hydrostatic head of 15,000mm makes it easily waterproof enough for a night out in heavy rain. I can tell you that the Interstellar certainly passes the back garden hosepipe test.

With a Moisture Vapour Transmission Rate (MVTR) of 30,000g/m2/24hr, this is also highly breathable stuff. In practise I've never once noticed any condensation buildup inside the bag - even the morning after, the inside surface of the fabric has been dry to the touch. I think partly this must be because there's a good bit of space inside the bag, and the fabric is held up out of the immediate range of your breath; the hooped design also helps in that you can always keep the door unzipped a fraction somewhere even if it's windy. The one limit to my in-the-field testing to date is that it's all taken place in sub-zero or low single-figure temperatures, and largely dry breezy weather. What will I be saying about the breathability after a warm and humid July night in the rain? I'll have to update this review as the summer progresses.

Pegging point  © UKC/UKH Gear
Pegging point
© UKC/UKH Gear

Waterproof fabric  © UKC/UKH Gear
Waterproof fabric
© UKC/UKH Gear

AscentShell is ever so slightly stretchy, which is good in clothing but I'm not sure really adds much to a bivvy bag. This fabric is also slightly air permeable, (CFM 0.3 if you're counting), which is bound to help keep everything dry and comfortable inside. On a very windy night I did notice some draught on the windward side; I can't say if that was coming through the air permeable fabric, or through the zip. Since it's an 'open' zip rather than a waterproof model, I suspect more of the latter, despite its small external storm flap. Is there a small risk of minor leaks through the zip in wind-driven rain? Perhaps, but I don't think it would be significant. If I'm ever unlucky enough to be caught out in that kind of weather on a bivvy (I'd never go out on purpose on such a forecast) then, again, I'll report back.

The underside is a thicker 40 denier ripstop nylon with a waterproof TPU coating. Like a tent groundsheet this is not breathable, but it doesn't need to be; the main thing is that it's not going to let anything through if you're camped out on a bog. Both fabrics feel good and tough, though due to the cost of the bag I might be inclined to add an under-sheet for use on stony ground.


More spacious than a simple bivvy bag and lighter than most solo tents, this is a first class hooped bivvy. But it's the unique way that you can both sleep in it and practically wear it that sets the Interstellar apart from other such models. It may look like something you'd run the London Marathon in, but for practicality in foul weather this 'sombrero' feature is great, as is the ability to use your hands without getting out of the bag. My only criticism of the Interstellar is that £315 seems a lot to pay for a waterproof bag, even a really nifty and innovative one. However I don't want to make too much of that since it is the best bivvy bag I've used, hands down (or out). You wouldn't want to spend more than a couple of nights in it on the trot, but for alpine climbing or short weight-conscious backpacking trips it would be the absolute business, and I'm sure there are users of either bent who'll readily make the investment.

Outdoor Research say:

Built from the recommendations, feedback, and favorites of Outdoor Research employees, the Interstellar Bivy represents years of idealizing the perfect night spent outside. Seam-taped AscentShell™ technology delivers waterproof breathability, and a single pole can be used to add headspace or left at the trailhead to save weight. A newly designed side entry makes getting in and out of your shelter faster and easier than ever before, while innovative zipper design and built-in sombrero allow you to sit up and perform tasks (like cooking and packing) without stepping outside.

  • Upper fabric : Ascentshell™ 3L 100% nylon 20D ripstop
  • Floor fabric: 100% nylon 40D ripstop with TPU
  • Dimensions: 82" x 26" x 17" (length x top width x top height)
  • Weight : 596g
  • Large Opening
  • One Guy Line Loop
  • Two Stake Loops
  • One External Fly Fastener
  • Derlin® Single-Pole System

For more info see outdoorresearch.com

14 May, 2019

Nice review Dan. Looking at those pics I'm not sure I would actually fit in it....

Thanks Rich. Yes it might be a bit cosy...

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