Why a sealed enclosure collects water

A gasket keeps rain out. It also keeps moisture in.

Every enclosure is closed in air that already carries water vapour. At 25 °C and 60 % relative humidity, a cubic metre of air holds roughly 13 grams of water. Assemble a cabinet on a humid afternoon and that vapour is now inside, sealed behind the gasket, with nowhere to go.

Cool the cabinet overnight and the internal air reaches its dew point. Water condenses on the coldest surfaces first: metal mounting plates, terminal blocks, the inside face of the door. Nothing crossed the seal. The water was already there.

Warm it again and part of it evaporates, so the cycle repeats with every day and night. The pressure effect is easy to underestimate. Treat the air as ideal and hold the volume constant, and a 40 K rise lifts absolute pressure by roughly 13 %. On a cabinet at sea level that is about 130 mbar across the wall. Real enclosures flex and gaskets creep, so the pressure finds the weakest path: cable entries, gland threads, unsealed knockouts. Each outward and inward breath carries moisture-laden ambient air past seals that were never designed to act as a pump.

The practical consequence is that condensation does not indicate a failed gasket, and no gasket improvement will stop it. The water starts on the inside, and it stays there until something lets the vapour out.

What IP68 actually tests

IP68 is two digits from IEC 60529, and neither one addresses the air inside the box.

The 6 means dust-tight: no dust ingress at all. The 8 means the enclosure survives continuous immersion in water, under conditions of depth and duration agreed between the manufacturer and the user. The test measures what crosses the boundary from outside to inside.

Condensation happens on the inside of that boundary. A cabinet that passes an immersion test can still be wet the next morning, and the two facts do not contradict each other. The rating describes the wall. It says nothing about the humidity of the air sealed behind it during assembly.

This also explains why raising the IP rating of a cabinet does not fix a condensation problem. The failure mode is a closed volume holding water vapour through repeated thermal cycles, while water entering from outside is a separate problem that the seal already handles. Background on how the two digits are defined is in the IEC 60529 overview.

How a breathable vent equalises pressure

A vent plug is a threaded body with an expanded PTFE membrane across the bore. The membrane has pores far smaller than a water droplet and large enough for water vapour and air to pass through.

Three things happen at once when one is fitted:

  • Pressure equalises. As the enclosure heats and cools, air moves through the membrane instead of loading the gasket. The pressure difference stays near zero, so the seal is not worked by every thermal cycle.
  • Vapour escapes. Moisture already inside migrates outward whenever internal vapour pressure exceeds ambient. A vent does not dry a soaked cabinet quickly, but it stops internal humidity from climbing cycle after cycle.
  • Liquid water stays out. The membrane is hydrophobic, so water at the surface cannot push through the pores at normal pressures. Correctly installed against its O-ring, the vent does not reduce the IP rating.

The membrane does not act as a dust filter in the usual sense. Dust reaching the outer face stays there and is shed with the next airflow, which is why a vent that looks dirty is usually still working.

A sealed cabinet without a vent converts every temperature swing into mechanical work on the gasket and the cable entries. Those are the parts that eventually leak, which is why adding a vent extends the working life of the sealing system further than upgrading the seal itself.

Sizing and installing a vent

Three numbers decide the part: thread, airflow, and body material.

Thread should match the tapped hole you already have. M12 × 1.5 covers most outdoor cabinets, inverters and control housings. M5 to M10 suits instrument cases and small luminaires. M20 × 1.5 is common on battery packs and larger cabinets. Full thread and wall-thickness tables are on the vent plug size chart.

Airflow is rated in ml/min at a stated pressure, usually 70 mbar. A large enclosure with a fast thermal cycle needs more flow than a small one that changes temperature over hours. For scale, the CZ6992 brass vented gland is rated 1200 to 2400 ml/min at 70 mbar, which covers most outdoor cabinets up to about one cubic metre. If the cabinet is much larger, or the cycle is fast, go up a size rather than adding a second small vent.

Material follows the environment. PA66 is adequate to about 100 °C and resists most outdoor conditions at low cost. Stainless or nickel-plated brass is the safer choice above that temperature, or where solvents and wash-down chemicals are present. The membrane tolerates a wider range than the housing, so the body is usually the limit.

Installation mistakes cause more failures than the membrane ever does:

  • Fitted at the lowest point of the cabinet, where condensate pools and wets the membrane from the inside.
  • Thread sealed with tape or compound that overlaps the membrane face and blocks the bore.
  • Installed without the O-ring, or with the O-ring compressed on the wrong face.
  • Threaded into a blind boss with no clearance behind the membrane, so airflow is restricted to whatever leaks past the thread.

The correct position is high on a side wall, clear of standing water and clear of the direct path of wash-down spray. On cabinets with several openings, the sequence matters as much as the parts, and there is a separate guide to sealing every opening in an outdoor cabinet.

When a pressure relief valve is the right choice

A vent plug equalises pressure slowly. That suits a cabinet that changes temperature over hours, and it fails a volume that can pressurise faster than a small membrane can pass air.

A spring-loaded relief valve opens at a set pressure, moves a large volume in a short time, and closes again. It is the right part when the cell or module supplier specifies a pressure limit the enclosure must not exceed, or when a liquid-cooled pack operates below zero and the valve seat can ice over. An anti-icing collar keeps the seat free in that case.

For air-cooled cabinets and ordinary outdoor housings, a vent plug is simpler and has no setting that can drift. Where both are used on one enclosure, the vent handles the slow breathing and the relief valve handles the fast event. The breathable cable gland range covers cases where the vent has to sit inside a cable entry rather than in its own tapped hole.

Before ordering, four details let a supplier match the part in one pass:

  • The tapped hole diameter and pitch, taken from the unit itself.
  • Enclosure volume and the temperature range the unit sees in service.
  • Whether the environment includes wash-down, salt spray or solvents.
  • The airflow figure at a stated test pressure, with the test record that ships alongside it.

A vent that is one size too small will look like it is working until the first cold night.