Foil blanket, bivvy, bothy bag or tarp? What actually protects you when a trip goes wrong

“Shelter” gets treated as one category, but the products sold under that name are doing different jobs — some reflect heat, some block wind, some insulate, and some do all three badly and one well. Understanding which mechanism you’re buying — and which one you need for the conditions you’re in — matters more than any brand name on the packet.

The physics behind the protection

The body loses heat four ways: conduction (direct contact with something cold — wet ground, wet clothing), convection (moving air stripping heat off your skin — this is what “wind chill” actually is), radiation (infrared heat leaving your body into the surrounding air), and evaporation (sweat or wet clothing changing state, which costs a large amount of energy per gram). In still, dry, mild conditions, radiation and convection dominate. Add wind, and convective loss increases sharply — the reason a 5°C day with a stiff breeze feels far colder than a still 5°C day is that moving air continuously replaces the thin layer of air your body has warmed, forcing you to keep re-heating it. Add wet clothing or wet ground contact, and conductive and evaporative losses take over entirely, which is why a soaked, wind-exposed casualty cools far faster than a dry, sheltered one at the same air temperature.

Every product below is, underneath the marketing, a different combination of answers to three questions: does it block wind (stops convection), does it stop you touching something cold or wet (stops conduction), and does it trap or reflect your own heat (manages radiation)? None of them do all three perfectly. The Wilderness Medical Society’s hypothermia guidelines frame field management the same way — insulation, wrapped in a vapour barrier, with wind and wet excluded — and that framework is the cleanest lens for judging any shelter product on this list.

Flat foil blankets: real physics, poor execution

The classic “space blanket” — a flat sheet of aluminised PET film, typically 12–15 microns thick — works on a genuine mechanism. Testing on the aluminised coating has found it reflects a large majority of incident infrared radiation; one study measuring rescue-blanket optical properties found a single layer blocked roughly 82% of thermal radiation, which is also why search-and-rescue thermal-imaging teams specifically instruct conscious casualties to remove one if they hear a drone approaching. A separate randomised trial comparing foil rescue blankets against conventional wool blankets during ambulance patient transfer found comparable performance at preventing further heat loss. The radiant-reflection mechanism is real, and in a controlled, low-wind setting — wrapped snugly around a patient in a vehicle, or layered over insulation as a vapour barrier — it does meaningfully reduce further heat loss.

Where it falls apart is as a standalone wilderness shelter, and the reasons are mechanical rather than thermal. The film is thin enough to tear on scrub, rock, or a fingernail, and once torn it stops functioning as a sealed layer. It has no structure of its own — nothing to hold it against a moving, shivering body in wind — so in practice it flaps, gaps open at the edges, and the wind you most need to block finds its way straight in underneath. It provides no insulation whatsoever from conductive ground loss; sitting or lying on one over cold, wet ground does nothing for the heat you’re losing downward. And not all blankets are equal: independent emissivity testing of several branded and generic space blankets found real differences in actual infrared reflectivity between products that look identical, with cheaper generic brands reflecting noticeably less than better-made ones — the $2 keychain version and a quality branded blanket are not interchangeable, despite looking the same.

The sealed foil bivy bag: the same material, a far smarter shape

This is the upgrade most people skip past without realising it exists, and it deserves to be far better known than the flat blanket it sits next to on the same shelf. Take the identical aluminised film, and instead of leaving it as a flat sheet, seal it into a bag you climb into. A SOL Emergency Bivvy, for example, weighs around 108g against roughly 71g for the equivalent flat blanket — about 37g heavier, barely noticeable in a pack — but because it’s a sealed, closed bag rather than a sheet you have to hold shut, wind can’t get underneath and peel it away, and there’s no gap to lose heat through at the edges. Manufacturers claim it reflects around 90% of radiant body heat, consistent with the reflectivity figures above, and because it fully encloses you, it also does what the flat blanket can only do if someone wraps it carefully: functions as a genuine vapour barrier around your insulating layers, exactly as the WMS guidelines describe.

A step up again is the heavier, multi-layer survival bag — the Blizzard Survival Bag is the standard example, adopted widely by UK mountain rescue teams. At 385g, it’s meaningfully heavier than a simple mylar bivy, but it isn’t just reflecting your heat back at you — its three-layer construction is rated at roughly 7.5 togs, in the ballpark of a proper sleeping bag, and it’s really waterproof and windproof rather than just wind-resistant. For a day-hiking or overnight kit where 385g is a rounding error, it’s a legitimately different, more capable class of device than either the flat blanket or the simple mylar bag beside it.

Breathable ultralight bivvies: the condensation fix

Every plain aluminised bivy — flat blanket or sealed bag — shares one flaw: the film that makes it a good radiant reflector is also completely vapour-impermeable, so your own sweat has nowhere to go and condenses on the inside. For a short emergency use, that’s a manageable trade-off. For anything longer, it gets unpleasant fast. Products like the SOL Escape (and the heavier Escape Pro) swap the plain aluminised film for a waterproof-breathable laminate — Sympatex is the common choice — with the reflective coating on the inside. Independent testing has found reduced condensation build-up compared to plain mylar under equivalent conditions, at the cost of roughly double the weight of a simple sealed bivy and a meaningfully higher price.

The important caveat, confirmed in the same testing: ground conduction dominates once you’re lying down, so an insulated sleeping pad underneath is close to essential — reflectivity alone doesn’t solve for cold ground, no matter which layer you buy. Treat this category as the reusable, more comfortable option for someone who wants a genuine backup sleep system rather than a one-shot emergency device, not as a guarantee of warmth on its own.

Reusable waterproof-breathable bivvies: the multi-night step up

A different category again from the aluminised products above: a bivy sack built from ordinary waterproof-breathable fabric — no reflective coating at all — designed to be pitched over a sleeping bag night after night rather than used once and discarded. The Alpkit Hunka (around 330g, 10,000mm hydrostatic head, 10,000g/m²/24hr breathability rating) is the commonly cited budget-durable option; a Gore-Tex equivalent like Rab’s Ridge Raider sits at a similar weight with a step up in fabric performance and price. These split into non-hooped versions, which lie flat against your sleeping bag and are lighter and more packable, and hooped versions with a pole at the head end that lifts the fabric off your face and gives condensation somewhere to go — useful on longer trips where you’re sleeping in the bivy on consecutive nights rather than using it as a one-off emergency layer.

This category isn’t really competing with the emergency bivvies above — it’s solving a different problem. It won’t fit in an emergency kit the way a 108g mylar bivy does, and it isn’t there to save your life on a day hike gone wrong. It’s the right choice when a lightweight sleep system, rather than an emergency backup, is what the trip calls for — fastpacking, alpine bivvying, or anyone who wants a bothy-night option on multiple trips rather than a single-use device sitting in a pack for years.

Bothy bags and group shelters: shared-warmth insurance, not a tent

A bothy bag (also sold as a “group shelter”) is a windproof, waterproof nylon or polyester sack with no poles or stakes — you pull it over the whole group, sit on the inside edge to hold it down, and let trapped, shared body heat warm the interior. The Rab Group Shelter 2 weighs around 320g; the Lifesystems Survival Shelter 2-person version is similar at 345g, with taped seams and a 5,000mm hydrostatic head rating. Both pack down to roughly the size of a large water bottle.

The mechanism is straightforward and the wind- and rain-blocking performance is strong — this is a sealed fabric shelter, not a reflective film — but there’s no dedicated randomised trial measuring how much warmer a group actually gets inside one; the evidence here is mechanistic and experiential rather than a clinical trial, and that distinction deserves stating plainly. What experienced users consistently report is that the difference is large and immediate: several people’s combined body heat in a sealed, windproof space raises the internal temperature noticeably within minutes, which is exactly why mountain and hill-walking guides treat a bothy bag as standard kit for any extended stop in poor weather, not just genuine emergencies. The trade-off is condensation, which builds quickly in a sealed bag full of breathing, sweating people — bothy bags are designed for stops and short-term emergency shelter, not as an overnight sleep system, and get unpleasant fast if used as one.

Tarp and trekking pole: the flexible, skills-dependent option

A tarp pitched off a trekking pole (or two) is the most adaptable shelter on this list, and the one that most rewards — and punishes — the skill of the person using it. Since most hikers are already carrying poles, it adds minimal extra weight for genuine rain and wind protection over a much larger footprint than any bivy, and it works whether you’re solo or sheltering a small group. It also provides zero insulation on its own — you still need ground and body insulation underneath — and it isn’t a sealed shelter, so wind direction and pitch quality matter enormously. A well-pitched A-frame or lean-to in the right orientation to the wind is effective; a poorly pitched one flaps, lets rain in sideways, and can be worse than no shelter at all if it collapses on you in the night.

The honest limitation is that this option is entirely dependent on practice. Unlike a bothy bag or sealed bivy, which work more or less the same way regardless of who’s using them, a tarp shelter is only as good as the person pitching it — guylines, stake points (or improvised anchors above the treeline where stakes won’t hold), and orientation to the wind all have to be judged correctly, under pressure, in poor conditions. It’s an excellent complementary or backup option for someone who already knows how to pitch one; it’s a poor first line of defence for someone who’s never practised it before they needed it.

Tube tents (the Life Tent / PrepMate style): sealed material, unsealed design

Products like the Go Time Gear Life Tent are a length of puncture-resistant mylar sewn into a tube, with taped seams along its length and a reflective interior claimed to bounce back roughly 90% of radiant body heat — comparable reflectivity to the sealed bivy bags above, at a similarly low weight (around 230g). Where it differs structurally is that a tube is, by definition, open at both ends. To pitch one at all, you need two fixed anchor points — trees or posts — to string it between, which immediately rules it out above the treeline or on an open ridge unless you’re carrying poles specifically to substitute. And because both ends stay open unless you actively block them with a pack, rocks, or snow, wind can funnel straight through the tube exactly the way it can’t through a sealed bivy bag or a bothy bag — the fabric itself is sealed and weatherproof, but the overall design leaves a real gap in wind protection that the sealed-seam marketing doesn’t make obvious.

That makes it a reasonable low-cost, low-weight backup specifically in forested terrain where two anchor points are easy to find and you’re willing to actively block the ends — and a poor choice as a primary shelter above the treeline, in open terrain, or anywhere the wind is coming from a direction you can’t control once it’s pitched.

Snow shelters: quinzees and snow caves

A different category entirely — no gear beyond a shovel, and a different mechanism: several hundred millimetres of packed snow is an excellent insulator, and a well-built quinzee or snow cave is reported by experienced winter campers as warmer and dramatically quieter than any tent, tarp, or bivy in the same storm, with internal temperatures that stay close to freezing even when it’s far colder outside. The trade-off is time and effort: building one properly takes roughly two to three hours of continuous, physically demanding digging, and for safety reasons it needs at least two people — a solo dig risks collapse with nobody around to help if it goes wrong.

That time cost is the reason this belongs in a different mental category from everything else on this page. It’s an excellent option for a planned overnight in snow-camping terrain where you have daylight, energy, and manpower to spare. It’s a poor fit for the scenario the rest of this article is really about — an unplanned night out, where cold, fatigue, dwindling daylight, and a smaller or less able group are already working against you, and two to three hours of heavy digging may simply not be a realistic option. Know it as a genuine tool for planned snow travel; don’t count on it as your answer to an actual emergency unless conditions and group size make it realistic.

Bringing it together

Foil blankets are underrated for what they do well and overrated as a shelter: useful as a vapour-barrier layer over insulation in a controlled setting, poorly suited to standing alone against wind, rock, and cold ground in the field.

The sealed foil bivy bag is the single most overlooked upgrade on this list: for roughly 30–40g more than a flat blanket, it stays sealed against wind, needs no one to hold it shut, and does the vapour-barrier job properly — there’s little reason left to carry the flat version instead.

Breathable and reusable bivvies solve comfort, not the core problem: they manage condensation and durability far better than plain mylar, but ground insulation still has to come from somewhere else, and they’re answering a different question — a sleep system, not an emergency device.

Group and improvised shelters (bothy bags, tarps, tube tents, snow shelters) all trade a specific strength for a specific weakness: shared warmth versus needing a group, adaptability versus skill dependence, low cost versus an unsealed design, excellent insulation versus a prohibitive time cost — none of them is a universal answer, and matching the option to your actual terrain, group size, and conditions matters more than picking a single “best” one.

The common failure mode across all of it is the same one that shows up everywhere in gear folklore: a product that works well in one specific context gets marketed — or assumed — to work everywhere. A flat blanket that performs adequately wrapped around a patient in a warm ambulance gets carried as if it will perform the same way flapping in an exposed, windy col. Match the mechanism to the conditions you’re actually likely to face, and the right choice for your kit gets a lot more obvious.

Building a shelter and kit list for a specific trip?

What belongs in your kit depends on group size, terrain, season, and how far you are from help — a mountaineering group above the treeline needs a very different answer to this than a solo day hiker on a maintained trail. If you’d like your kit reviewed against the real conditions of a specific trip, it’s worth talking through properly before you go.