Emergency Shelter Materials: How to Judge Tarps, Pads, and Cordage

Most guides to emergency shelter materials hand you a shopping list. It quits being useful the moment you stand in an aisle looking at 6 tarps that all say waterproof at 4 different prices. So this is a specifications guide: what each material is physically good at, which number on the tag means something, and which is marketing.

Two companion pieces cover the rest. Our guide to choosing a site, insulating from the ground, and staying dry covers where to put a shelter, and our guide to essential knots for emergency shelter construction covers the ties. This one is what you carry.

What do emergency shelter materials actually have to do?

Three jobs: shed water, block moving air, and stop heat draining into whatever you lie on. Rank your kit against those 3 and the gaps show fast.

Most kits handle water and wind, then quit. The ground is a heat sink that never fills up. National Park Service hypothermia guidance says to get the person into dry clothes and make sure there is a barrier between the ground and themselves. The CDC notes hypothermia can set in at cool temperatures above 40 degrees F when someone is chilled by rain or sweat, and that a body temperature below 95 degrees F needs help immediately.

Water is the accelerant. An NIH review of cold physiology puts convective heat transfer during water immersion at perhaps 70 times what it is in air of the same temperature. Wet ground is the slow version.

How do you read tarp specs without getting fooled?

Look at 4 things: fiber and coating, fabric weight, hydrostatic head, and tie-out construction. The rest is color.

Denier is the mass of the yarn, not the strength of the fabric. A 70D silnylon tarp beats a 20D one in the same weave, but denier across different fibers tells you nothing. Mil measures thickness in thousandths of an inch, and it is how polyethylene sheeting is sold.

Hydrostatic head is the one honest waterproofness number. It comes from a standardized test, ISO 811, Determination of resistance to water penetration, Hydrostatic pressure test: a specimen is held against rising water pressure until water comes through, reported as a column of water in millimeters. The part most guides skip: no published standard defines what number is adequate for a camp tarp. The quoted 1,500 mm and 3,000 mm thresholds are industry convention. Use the figure to compare 2 tarps, and remember that a pressure point, a pack corner pushing into the fly, beats the rating locally.

• Silnylon. Silicone-coated nylon, light and strong for the weight. Nylon absorbs water and stretches when wet, which is why a ridgeline drum tight at dusk sags by 2 a.m. Use tensioners so you can retighten in the dark.

• Silpoly. Silicone-coated polyester, slightly heavier per unit of strength, but polyester barely absorbs water and holds its pitch in the rain. The quieter choice overnight.

• Polyethylene. The hardware store tarp. Waterproof, cheap, abrasion tough, and also heavy, loud in wind, and brittle after UV. Great staged, poor carried.

• Canvas. Cotton duck, waxed or treated. Heavy and slow to dry, but breathable, far more fire tolerant than synthetics, and repairable with a needle.

Tie-outs decide whether the tarp survives the night. A grommet is a ring punched through single-layer fabric, so it puts the whole load on a few threads. A reinforced tie-out sews a webbing loop into the seam so the load spreads. With only grommets, back them up: fold a pebble into the fabric and cinch a loop of cord behind it, which makes a load point anywhere on the tarp.

Which cheap emergency shelter materials actually work?

Several, and all worth staging.

• Contractor-grade trash bags. The 3 mil class, not kitchen bags. One makes a poncho with a head hole or a vapor barrier over a sleeping bag. Two with the bottoms cut out and taped together make a 6-foot sleeve.

• Painter's plastic sheeting. Sold by mil, and thin film tears at every stress point. EPA's lead renovation guidance is instructive: the agency declined to name a thickness and told contractors to pick one or more layers of plastic sheeting of a sufficient thickness to prevent puncture based on the job, while noting that some materials recommend 6 mil. Treat 6 mil as the floor for anything you kneel on or stake out.

• Housewrap. Spun polyolefin. Water resistant, vapor permeable, wide, tough, and often free as jobsite offcuts. Excellent ground sheet or wind wall.

• Roofing felt. Asphalt-saturated paper. Heavy and it smells, but waterproof and good over a frame or under a pallet bed.

• A reflective windshield sunshade. Already in the car, torso sized, and stiff enough to stand as a wind baffle.

Do emergency blankets and bivvies really work?

They do one thing well and three poorly. A metalized polyester (mylar) blanket is a radiant barrier: it reflects radiant heat and has almost no insulating value of its own. Virginia Cooperative Extension is blunt about the condition attached: a radiant heat barrier must have an air space next to a reflective side to be effective, and without that gap it "acts as a conductor and just passes the heat along." The same bulletin notes there is no current method for assigning an R-value to radiant barriers.

1. Reflective needs a gap. Draped loosely, with air between the film and your clothing, it returns radiant heat. Shrink wrapped against you, it is just a windbreak.

2. It fails as a ground layer. Lying on it crushes the gap and you get bare conduction into cold dirt. Reflective goes above you, foam below you.

3. It does not breathe. Expect condensation inside, and wet insulation underperforms. The Department of Energy notes R-values depend on temperature, aging, and moisture. Vent the foot end.

A bivvy sack is the upgrade, because its shape holds the gap and adds a wind and rain shell. One more limit: that bulletin warns dust and dirt decrease a radiant barrier's effectiveness, so a blanket crumpled in a pocket for 5 years is not performing like a new one.

Why is ground insulation the emergency shelter material people underestimate?

Because the ground never stops taking heat, and it does not care how good your tarp is.

The number is R-value. The Department of Energy defines it as a material's resistance to conductive heat flow. Two things get missed. It adds up, so a thin foam pad plus leaves beats either alone. And DOE says plainly that compressed insulation will not provide its full rated R-value. Every pound of you compresses the pad under your hips and shoulders, exactly where the heat leaves.

• Closed cell foam. Fixed R-value, cannot fail, doubles as a splint, cheap, bulky. The right pad for an emergency kit, because a hole in it does not matter.

• Inflatable pads. Far more R-value for the weight and packed size, and one puncture takes them to roughly zero. Carry a patch kit, and never make one your only ground layer in the cold.

The free substitute is dry debris, and the mistake is always depth. Leaves and duff insulate by trapping air, and your weight squeezes most of it out. Pile loose material roughly knee deep before you lie down, and expect a few inches of real loft. If your elbow finds dirt when you press down, it is not deep enough.

What cordage, stakes, and repair gear belong in a shelter kit?

Cordage first, and buy it by its rating rather than its reputation. Manufacturers building Type III paracord to the military specification state 550 pounds minimum breaking strength for new cord in a straight pull. I could not verify that against the specification document itself, so treat it as a claim. It is a breaking strength, not a working load, and every knot is a weak point.

• Tarred bankline. Twisted nylon, grippy, holds a knot without slipping, resists rot, costs little. Number 36 is the usual shelter size.

• Reflective guyline. Thin, low stretch cord with a tracer woven in. Not a gimmick: a guyline at shin height in the dark is a trip hazard, and a headlamp finds it instantly.

• Ridgeline force. A line pulled tight between 2 trees multiplies the load at its anchors far beyond the weight hanging on it, because the angle at each end is shallow. Anchor to a trunk or a buried log, not a sapling.

Stakes depend on what is under you. In firm soil, a 7-inch aluminum Y or V stake driven at an angle leaning away from the pull. In soft ground, sand, or snow, surface area beats length: a wide snow stake, a stuff sack of sand or snow buried as a deadman, or a stick laid horizontally in a trench.

Repair gear gets skipped. Pressure sensitive adhesives lose tack as they get cold, so keep tape warm in a pocket, warm the fabric too, and wipe it dry. Cold, wet, dirty fabric holds no brand of tape. Carry a heavy needle and waxed thread for canvas, webbing, and torn tie-outs, plus 4 tarp clips: spring or ball grips that make a tie point on any edge with no grommet, which is what turns plastic sheeting into a pitchable tarp.

When do natural materials beat carried gear?

When you need volume, because volume is heavy.

• Debris. Thatch over a frame works only when thick. Treat under roughly 2 feet of piled, settled leaves and duff as a windbreak, not insulation.

• Conifer boughs. Shingled from the bottom up they shed water well, and underneath you they make a springy mattress that holds loft better than loose leaves.

• Snow. The National Snow and Ice Data Center describes snow as a good insulator, similar to the insulation in the ceiling of a house, because most of a snow layer's volume is air. Snow near the ground in a deep pack is warmer, and the top 30 to 45 centimeters (12 to 18 inches) take the brunt of the cold air above, which is the physics behind a snow trench. Snow is still not a ground pad, so put foam or boughs between you and it.

What should each layer of your emergency shelter kit contain?

The National Park Service puts emergency shelter (tent, space blanket, tarp, or bivy) on its Ten Essentials list. Here is that idea in 3 layers, each item chosen by the spec that matters.

Pocket layer, a few ounces. An emergency bivvy rather than a flat blanket, because the shape holds the air gap. 25 feet of number 36 bankline or 2 mm reflective guyline. 4 feet of repair tape wrapped around a card. 2 tarp clips.

Day-pack layer, 2 to 3 pounds. An 8 by 10 foot silpoly tarp with reinforced tie-outs and a published hydrostatic head. A torso length closed cell foam pad chosen on stated R-value. 50 feet of cordage and 4 guylines with tensioners. 6 stakes matched to your terrain plus 1 wide snow or sand stake. Heavy needle, waxed thread, spare cord.

Vehicle or home-staged layer, where weight stops mattering. A 10 by 12 foot or larger polyethylene tarp, 10 mil or heavier, grommets every 18 inches. A roll of 6 mil sheeting and a roll of housewrap. 2 contractor bags and a full length foam pad or wool blanket per person. 100 feet of rope, bankline, 12 stakes, a mallet.

Staying warm and being found are 2 halves of one job, and our guide to ground-to-air symbols, mirrors, and whistles that get you found covers the other.

Frequently asked questions

What is the best material for an emergency shelter?

For a carried kit, a silicone-coated polyester tarp plus a closed cell foam pad. The tarp handles water and wind, the pad handles conduction into the ground, and neither has a failure mode that leaves you with nothing. Staged at home, a heavy polyethylene tarp and 6 mil sheeting do the same for less.

How thick does plastic sheeting need to be for a shelter?

Treat 6 mil as the practical floor for anything you stake out or kneel on. EPA declined to set a thickness in its lead renovation guidance, telling contractors to choose "one or more layers of plastic sheeting of a sufficient thickness to prevent puncture." Thinner film works only where nothing rubs it.

Do emergency mylar blankets actually keep you warm?

Only as a radiant barrier, and only with an air gap. Virginia Cooperative Extension states a radiant heat barrier must have an air space next to the reflective side, and without one it acts as a conductor and passes heat along. There is no accepted method for assigning it an R-value, so mylar is not insulation. Use it over you, not under you.

What hydrostatic head rating do I need for a tarp?

No published standard defines what is adequate, which is worth knowing before you pay for a bigger number. ISO 811 defines the test (rising water pressure until penetration, reported in millimeters of water column), but the quoted 1,500 mm and 3,000 mm thresholds are industry convention. Use the figure only to compare tarps.

Is paracord strong enough for a shelter ridgeline?

For a tarp, yes. For anything holding body weight, no. The stated 550 pounds for Type III cord is a manufacturer claim about new cord in a straight pull, not a working load. Knots cut into it, and much of what is sold as 550 paracord is not built to spec.

The numbers, in one place

You never know, but you can always be ready.

Sources: CDC, Hypothermia; National Park Service, Hypothermia (Rocky Mountain National Park); National Park Service, The Ten Essentials; NIH NCBI Bookshelf, Physiology of Cold Exposure, in Nutritional Needs in Cold and in High-Altitude Environments; NOAA National Weather Service, Wind Chill Chart; US Department of Energy, Insulation; Virginia Cooperative Extension, What About Radiant Barriers? (2908-9021); National Snow and Ice Data Center, Science of Snow; ISO 811, Textile fabrics, Determination of resistance to water penetration, Hydrostatic pressure test; US EPA, What mil plastic is considered impermeable?