Home / Blog / Materials & Construction

Purge Valve or No Valve on a Dry Bag

What a purge valve is actually for

A dry bag closure is a roll of fabric pressed against itself, and it works because three folds create three long contact lines that water has to cross sideways. Air trapped inside that roll is the enemy: it makes the bag buoyant in a way you did not plan, it pushes the roll open when the bag is squeezed into a hatch, and it wastes volume that could hold gear. A purge valve is a one-way path through the shell that lets you push that air out after packing, or pull it back in with a breath.

The second job is buoyancy control. A sealed 20L bag with a decent air pocket supports roughly 15 to 19 kg of positive buoyancy in freshwater, because a litre of displaced water is a kilogram. Ducks, hunting guides and photographers use that deliberately: a bag with a valve can be topped up with two or three breaths before it goes in the water so it floats high if it goes overboard. Without a valve you get whatever air happened to be trapped, which varies with how carefully you packed.

The three valve designs you will meet

Screw valves are the oldest and most serviceable design: a threaded body welded into the panel with a captive cap on a tether. They seal with a compression face rather than a membrane, tolerate grit better than anything else, and can be opened with gloves on. Push-button valves use a spring-loaded stem you depress to vent; they are fast and one-handed but the stem is the first thing to jam with sand. Flap or duckbill valves are the simplest of all, a moulded lip that opens under slight pressure, and they are the easiest to weld and the easiest to defeat with a single grain of grit.

Those vent rates matter more than they look. Compressing a 40L bag with a hand pump takes about 8 seconds through a duckbill and about 2 seconds through a push-button, and the difference is the difference between something the crew does every morning and something they stop doing by day three. A valve nobody uses is dead weight plus a leak path, so choose the fastest design the grit environment allows.

Compression: the honest gain in litres

Compression is the headline benefit and it is worth being precise about it. Loose clothing packed by someone in a hurry traps 25 to 35% of its volume as air. Sitting on the bag and pushing the air out through a valve typically recovers 1.2 litres on a 10L bag, 2.5 litres on a 20L, and 4 to 6 litres on a 60L full of insulation. That is why valves earn their keep on big soft loads and are nearly pointless on a 5L grab bag holding a phone and a sandwich.

There is a second, less obvious gain: a compressed bag behaves better in a hatch. A 40L bag with five litres of air inside resists deformation and wedges in an oval opening; the same bag with the air removed squashes through a gap 30 mm smaller than its nominal diameter. If your bag has to pass through a hatch rather than sit on deck, a valve earns its cost for that reason alone.

Buoyancy and the valve: the real relationship

Buoyancy is a function of trapped air, not of fabric, and a valve turns it from an accident into a setting. Add three breaths, roughly 1.5 litres, to a sealed 30L bag and you gain about 1.5 kg of lift. That will not support a person and should never be treated as a flotation aid, but it does keep a bag that goes overboard on the surface where you can see it, which is the entire point for anyone working from a boat or a wet bank.

The trap is symmetry. A bag with air in it floats, and a floating bag sitting on a deck line will try to leave when a wave crosses it. Guides who work moving water often do the opposite: they pack the bag, vent every bit of air out, and accept that it will sink if lost, because a bag that stays put is more use than one that has to be chased. Decide which behaviour you want before you decide whether to fit a valve; more detail on the physics sits in our air trap guide.

Every valve is a hole in the barrier

A welded TPU shell is continuous; the moment you weld in a valve you create a boundary between two materials with different coefficients of thermal expansion, different stiffness and different surface energy. Water that cannot cross a welded seam now has a circumferential joint to work on. In practice a well-welded valve is not the weakest point on a good bag, but it is the second weakest, and it is the part most likely to be damaged by being sat on, dropped on a rock or dragged across a beach.

Rank the risk by how the bag is used. A bag that lives in a raft frame and gets hosed out every evening will almost never trouble a valve. A bag that is stuffed into an over-packed hatch, compressed by 8 kg of other gear, then sits in 30 °C sun is a different story: heat softens the film, pressure pushes against the valve housing, and a marginal weld starts to creep. This is the failure profile that makes valve placement a design decision rather than a decorative one.

Welding a valve into a TPU panel

Radio-frequency welding is the standard method for thermoplastic polyurethane, because the polar molecules in the film heat under a high-frequency field and melt at the interface rather than being heated from outside. A valve body is normally moulded from a compatible TPU or from a polypropylene with a welded flange, and the flange is welded to the panel across an annulus of 8 to 12 mm. That gives a weld area of roughly 6 to 10 square centimetres, which in peel terms is far stronger than the film itself when the parameters are right.

Three parameters decide whether the weld is right: electrode pressure, dwell time and power. Too little energy and the flange lifts at the edge; too much and the film thins and the housing sinks into the panel, which looks fine and leaks under load. Placement matters just as much: keep the valve at least 60 mm from any seam so the weld heat does not re-melt an existing join, and put it on a flat panel rather than near a corner where the bag flexes. Our overview of radio-frequency welding for dry bags covers the machine settings in more depth.

Which valve faults actually happen most

Field returns and warranty claims cluster around a short list, and the ranking is fairly stable across brands. A cap left open or cross-threaded is by far the most common complaint and it is a user problem, not a product defect. A stuck-open duckbill from a single grain of sand is second. A cracked housing from a hard impact is third. An actual weld failure around the flange is rare on a well-built bag and common on a cheap one, which is the distinction worth paying for.

Notice how much of that list is maintenance rather than design. Rinse the valve in fresh water at the same time you rinse the closure, work the cap through its full travel while it is wet, and check the seating face for salt crystals once a month in summer. A valve treated that way outlasts the bag around it; a valve ignored for a season will be the reason the bag gets retired early.

Sand, salt and cold: where valves struggle

Fine sand is the natural enemy of any moving seal. A quartz grain around 0.2 mm across is large enough to hold a duckbill lip open by a fraction of a millimetre, which is all it takes for water to creep in over an hour of paddling. Salt works more slowly: crystals form on the seating face as the bag dries, and the next closure grinds them into the sealing surface. Cold is the third problem, because a stiffened seal at 0 °C does not conform the way it does at 25 °C.

The practical answer is to match design to environment. Screw valves with a flat seating face tolerate all three far better than flaps, which is why they dominate on saltwater boats despite weighing twice as much. If your programme is desert or dune based, consider leaving the valve off entirely and relying on the technique described below. If it is cold-water paddling, specify a housing moulded from a grade that stays flexible below freezing, the same consideration that governs cold crack temperature in TPU film.

When a valve is worth specifying

Four conditions point clearly towards fitting one. The bag is 20L or larger. The contents are soft and compressible, meaning insulation, clothing or a sleeping bag rather than camera bodies. The bag has to pass through a hatch or a locker opening where a squashed profile helps. And the user actually wants to control buoyancy, which covers duck hunting, photography from a boat, and any situation where a lost bag should float rather than sink.

• Volume above 20 litres with soft contents that trap air.
• A hatch, locker or frame opening that the bag must be squeezed through.
• Deliberate buoyancy control for bags that may end up in the water.
• Compression straps or a vacuum-style system that needs somewhere for air to escape.
• Multi-day expeditions where 4 to 6 litres of recovered space changes the size you buy.

Retail ranges benefit in a way that is easy to overlook: a valve is a visible feature on a shelf. Two otherwise identical bags hang side by side and the one with a valve reads as more technical, which justifies a price step of 10 to 15% when the part costs a fraction of that. That is a merchandising argument rather than an engineering one, but it is real, and it is why valves appear on bags where they deliver little practical benefit.

When leaving the valve off is better

The case against is straightforward. Every valve adds 3 to 16 g, a weld operation, a leak path and a maintenance task. On a 5L or 10L bag the compression gain is under 1.5 litres, which nobody notices, and the buoyancy effect is negligible on a bag that small. On bags used in fine sand or silt, a flap valve that jams open turns a waterproof bag into a wet bag on the first day, and the user will blame the bag rather than the grit.

There is also a cost and lead-time dimension for anyone specifying a range. A valve is a bought-in component with its own tooling, its own minimum quantity and its own approval file. On a 500-piece run that is manageable; on a 200-piece colourway it can be the item that holds up the order. Where the practical benefit is marginal, spending the same money on a heavier film or a better mouth stiffener buys more real-world performance. Our roll-top closure engineering guide sets out where that money goes further.

Alternatives that get most of the benefit

1. Pack in layers and press each layer down before adding the next, which removes 60 to 70% of trapped air.
2. Close the bag halfway, kneel on it to expel air, then finish the folds: the classic body-weight method.
3. Use external compression straps on a duffel or backpack format to squeeze the load from outside.
4. Roll soft items rather than folding them, which removes air inside each garment before it goes in.
5. Choose a size that matches the rolled volume rather than the brim volume, so no compression is needed.

The body-weight method deserves a specific note because it is what most experienced paddlers actually do. Kneeling on a half-closed 40L bag expels 4 to 6 litres of air in about five seconds, which matches what a valve delivers, and it costs nothing, adds no weight and introduces nothing to fail. The one thing it cannot do is let air back in afterwards, so it is useless for buoyancy control and for bags that must be re-fluffed at camp.

Compression straps are the better answer on duffel and backpack formats, where two 25 mm straps with cam buckles remove 3 to 5 litres from a soft load and double as lash points. They cost about 40 g and a few cents more than a valve and they cannot leak. For a bag that lives on a boat rather than inside a hatch, that trade is usually the right one; see the compression bag capacity analysis for measured figures.

Testing a valve before you trust it

Two checks cover everything. The first is a pressure test: close the bag with three folds, sit on it gently and listen at the valve for a hiss, then repeat with the valve open and confirm air actually escapes. A valve that hisses under light pressure with the cap closed has a seating problem. The second is the bath test: submerge the closed bag with about 5 kg of weight on it for five minutes, dry the outside, and press a paper towel against the valve housing from inside.

Run both checks at the start of every season and after any impact, and re-run them after the bag has been stored for months. The full procedure for the air test, including how much pressure to apply and how to interpret a slow leak, is in our air inflation leak test guide. If a valve fails either check, retire the bag rather than sealing over the housing: a glued-over valve is a rigid spot in a flexible panel and it will crack the film around it within a season.

Writing the valve into a specification

• Name the valve type by mechanism, not by part number: screw, push-button or duckbill.
• State the housing material and require a cold-flex test below the lowest service temperature.
• Specify the weld annulus width and require a peel test on the flange at production sampling.
• Fix placement rules: flat panel, minimum 60 mm from any seam, away from the base contact area.
• Require a tethered cap so the cap cannot be lost, and a torque note on the hang tag.
• Ask for a grit test if the bag is aimed at beach, desert or silt environments.
• Reference ISO or ASTM test methods for any strength or ageing claim on the housing.

Programme buyers should also decide early whether the valve is a functional part or a feature. If it is functional, it goes on the drawing with tolerances and a test method. If it is a feature, it goes on with a cost target and an appearance standard, and nobody should be surprised when it does nothing measurable. Our production team has built waterproof bags since 2014 in an SGS-verified 4,950 m2 facility with seven lines, and the fastest programmes are the ones that make that call before sampling rather than after.

Whichever way you decide, the closure remains the primary seal and the valve is secondary. Roll three times, press the air out, clip the buckle, and the bag is waterproof with or without a valve fitted. Everything in this guide is a refinement on top of that, and the closure sealing guide is the place to start if the basics are not yet second nature to the people using the bags.