A sealed fixture is only as good as the hole somebody drilled in it. That sounds obvious written down, and it still accounts for most of the water ingress we see reported on industrial vapour tight lighting fixtures during the first two years.
The housing was IP66. The gasket was silicone. The installer ran a conduit into the end cap with a standard connector and no sealant, and the fixture became a very expensive funnel.
Product selection gets most of the attention. Installation decides most of the outcome.
Cable and conduit entry is where seals fail
Every sealed fixture has to be fed from somewhere, and that entry point is the weakest part of the assembly by design.
Use cable glands rated to the same IP level as the fixture, and match the gland to the actual cable diameter. A gland sized for 12mm cable clamped onto 7mm cable does not seal — the compression ring never closes fully. It looks tight. It leaks.
For conduit entries, liquid-tight flexible conduit with a sealing connector is the standard approach, and the connector needs a gasket or thread sealant on the fixture side. Rigid conduit into a sealed housing wants a sealing fitting rather than a plain locknut and bushing.
The detail people miss: run the final drop so the cable makes a low point before it enters the fixture. Water tracks along cable sheath. A drip loop costs nothing and stops it dead.
Unused knockouts and spare entries
Any factory knockout you open and do not use must be closed with an IP-rated plug, not tape and not a blank washer. This is a routine inspection failure and an easy one to avoid.
Equally, resist the urge to drill your own entry to suit the conduit run. Manufacturers put entries where the internal geometry supports a seal.
Gasket compression: tight enough, not tighter
Over-tightening the latches on a vapour tight housing deforms the gasket permanently. Once the silicone takes a set, it stops recovering after thermal cycling and the seal opens up at the corners.
The right amount of compression is the point where the gasket visibly flattens and the lens sits flush along its whole length. Work the clips in sequence — opposite corners first, then the middles — the same way you would torque a flange. Doing them left to right pulls the lens out of square and leaves a gap at one end.
Condensation is not ingress, and it needs a different fix
A fixture can be perfectly sealed and still show water inside. Warm humid air gets trapped in the housing during assembly, the fixture cools overnight, and the moisture condenses on the inside of the lens. Installers see it and assume the seal failed.
Better industrial fixtures include a breather membrane — a small vent that passes water vapour but blocks liquid water and dust. If your application swings through big temperature changes, a wash bay that gets steam cleaned, a cold room, an unheated building in a humid climate, specify a fixture with one. Fitting an industrial vapour tight fixture light with a pressure-equalising breather vent is far easier than diagnosing a phantom leak six months later.
Listings the inspector will actually look for
Three things get checked on industrial jobs, and only one of them is the IP number.
UL or ETL wet location listing. The IP rating is a European test standard. North American inspectors want the North American listing. Reputable fixtures carry both. If a datasheet quotes only an IP figure, ask.
NSF certification where food is handled. NSF/ANSI 2 covers splash zones in food processing and commercial kitchens. It is a construction standard — smooth surfaces, no dirt traps, cleanable — and it is separate from ingress protection.
Hazardous location listing where the atmosphere is classified. Vapour tight is not explosion proof. If the area drawing shows Class I or Class II, the fixture needs the matching UL listing, and no IP66 rating substitutes for it. Grain handling, flour milling, woodworking and solvent areas fall into this more often than people expect.
Mounting: allow for the building moving
Suspended fixtures on chain or aircraft cable need enough slack that roof deflection does not load the housing. Surface-mounted units on a metal deck should use the manufacturer’s clips rather than screws driven through the housing, which is a hole through the seal by another name.
Frequently Asked Questions
How do I stop condensation inside a vapour tight fixture?
Specify a fixture with a breather membrane, which equalises pressure without letting water or dust in. Where fixtures are already installed, retrofit vent plugs are available for some models. Sealing the fixture harder makes condensation worse, not better.
Can industrial vapour tight lighting fixtures be pressure washed directly?
IP66 fixtures are tested against high-pressure jets and handle routine washdown. Keep the nozzle at least three feet away, avoid aiming directly at gaskets and cable entries, and do not use hot water on a cold fixture — thermal shock stresses the lens and the seal.
What IP rating do I need for a food processing plant?
IP66 as a minimum in splash and washdown zones, together with NSF/ANSI 2 certification for the construction standard. IP69K is worth specifying where high-temperature, high-pressure cleaning is used on a daily cycle.
Do vapour tight fixtures work in explosive atmospheres?
No, unless separately listed for hazardous locations. Ingress protection stops water and dust getting in; it does nothing to prevent the fixture igniting a flammable atmosphere outside it. Check the area classification drawing before specifying.
