A sealed enclosure keeps liquid water out under its specified exposure conditions. Its rating does not establish vapour tightness or remove water already trapped inside when you close the door. So yes, condensation can form on a PCB inside an enclosure that genuinely meets IP66 or IP67, especially when equipment produces little heat.
That is not necessarily a defect in the rating. Ingress protection and moisture control are separate design tasks. Understanding what the seal does and does not do tells you which fix actually works: desiccant, conformal coating, a heater or a breather vent. Use the IP rating guide for the ingress-test fundamentals.
What the rating actually promises
An IP code from IEC 60529 describes protection against solid objects and liquid water ingress under defined tests. The second digit grades water exposure: 4 covers splashing, 6 covers powerful water jets, and 7 covers temporary immersion. IP67 alone does not establish protection against powerful jets. A NEMA 4X-type rating includes hose-down protection and corrosion resistance, among other requirements.
Neither rating establishes that the enclosure will remain free of internal condensation. Water vapour behaves differently from liquid water in three ways that matter here.
- It is already inside. The air sealed in the box at assembly carries whatever humidity the assembly shop had that day.
- It can enter later. Vapour can permeate some gasket materials, while leaks and pressure changes can exchange humid air. The rate depends on the construction and conditions.
- It condenses on cold surfaces. When a wall or board drops below the internal dew point, vapour can form a liquid film there.
Even a vapour-tight box can develop condensation from moisture trapped during assembly. A leak is not a dependable drying method; it can admit more humid air or liquid water. The task is to control both the moisture source and the temperature of vulnerable surfaces.
Why a low-heat-load device is the worst case
Equipment heat can reduce condensation risk by keeping internal surfaces above the dew point. A drive or panel PC running continuously may provide useful warming, but cold walls can still collect moisture. Heating also does not remove water from a sealed enclosure. Establish the equipment layout and dimensions with the enclosure sizing guide before assessing heat loss or cooling needs.
A weight indicator, a signal junction box or a logger that wakes briefly may produce little useful warming. Its internal temperatures follow outdoor changes, with delays caused by the components' thermal mass. On a clear night, an exposed metal wall can radiate heat to the sky and cool below the surrounding air temperature. Condensation forms wherever a surface falls below the internal dew point; that surface is not necessarily the PCB.
Low-power devices also provide little heat to help re-evaporate a film. In a shaded location, moisture can persist. If it evaporates without leaving the enclosure, it remains available to condense during the next cooling cycle.
Where the moisture goes in a real panel
Typical moisture sources and entry paths to check are:
- Air trapped at assembly at high humidity, then cooled below dew point on the first cold night.
- Vapour permeating gasket materials or entering through imperfect seals over time.
- Daily temperature cycling drawing humid air through leaks as internal pressure changes.
- Damp components, cable spaces and polymers releasing previously absorbed moisture as conditions change.
Do not assume visible water is condensation without checking the door gasket, cable entries and conduit paths. Water concentrated beneath an entry or following washdown may indicate liquid ingress instead. Both mechanisms can occur in the same enclosure.
If you have an ambient humidity sensor soldered to the board inside the box, note what it really measures: internal humidity, not outdoor humidity. Condensation can also affect sensor accuracy or recovery, depending on its design. How a humidity sensor in a sealed enclosure behaves covers that measurement problem directly. If the reading must reflect ambient conditions, use an appropriately protected external sensing element. A breather vent alone does not guarantee representative ambient readings.
What each fix actually does
There are four approaches, and they solve different problems.
Dry the air at assembly and afterwards: desiccant. A sachet of silica gel adsorbs moisture trapped during assembly and moisture entering later. It is silent and needs replacement or regeneration on a defined schedule. For a low-heat-load device, it can be a useful first measure when the moisture load is limited. Size it from the desiccant supplier's capacity data, expected humidity, leakage and service interval rather than enclosed volume alone. Secure the sachet away from terminals and moving parts.
Help the board survive a wet event: conformal coating. A suitable acrylic, silicone or urethane coating can reduce moisture-related leakage and corrosion on a clean PCB. Protection depends on material selection, coverage, curing and compatibility with the components. Connectors, switches and sensing elements may require masking or separate protection. Coating does not stop condensation or make it harmless, and it is not a substitute for controlling liquid ingress.
Keep vulnerable surfaces above dew point: a heater. An anti-condensation heater can maintain a temperature margin when equipment heat is insufficient. It needs an appropriate power supply, mounting clearance and control arrangement, such as a suitable hygrostat or temperature controller. Size it for enclosure heat loss, minimum ambient temperature and the required temperature rise. Do not select wattage from volume alone. Consider the exposure conditions described in the outdoor enclosure guide, then verify the coldest internal surfaces remain above dew point. Heating lowers relative humidity but does not remove moisture from a sealed box.
Let the box breathe: a breather vent. A membrane vent can equalise pressure and permit water-vapour transfer while resisting liquid entry under its specified conditions. It can help moisture leave when outside conditions favour drying, but it can also admit vapour when outside air is humid. A correctly selected vent does not automatically sacrifice jet or immersion protection. The fitting, mounting arrangement and completed assembly must support the required rating. A drain serves a different purpose: removing accumulated liquid. Specify either fitting using the cutout and gland guide, rather than adding an unqualified opening.
Why the rating is still worth having
A common conclusion from all this is to skip the sealed enclosure, since moisture can remain inside anyway. That is the wrong trade.
The rating addresses the external exposure you specify: dust, rain, splash, hose-down or immersion, depending on the actual rating. These are not interchangeable requirements. Condensation can cause immediate electrical leakage as well as longer-term corrosion. You are not choosing between ingress protection and moisture management; you need both where conditions demand them.
There is also a material dimension. Compared with 304, 316L generally provides better resistance to chloride-related pitting, making it worth considering near coasts and in some washdown environments. Neither grade is immune to corrosion. Check chemical identity, concentration, temperature and contact time rather than treating every sanitizer alike. Use the 304 and 316L comparison to separate material selection from ingress protection.
What to specify
For a low-heat-load device outdoors or in a washdown area, a workable starting specification is:
- A suitable enclosure for the actual exposure. Our stainless wall-mount enclosures are manufacturer-rated NEMA 4X-type / IP66, not UL listed. Select 304 or 316L after checking the environment and chemical compatibility. A 10 x 8 x 6 in outside enclosure is one available size; verify the board, wiring and moisture-control hardware fit before selecting it.
- Conformal coating where appropriate for the PCB, with cleanliness, coverage, masking and inspection requirements defined.
- Silica gel desiccant sized to the moisture load and service interval, bagged so granules cannot reach the board.
- A membrane breather vent only after checking its vapour-transfer performance, exposure rating and installation requirements for the completed assembly.
- A heater with suitable controls where power is available and temperature measurements justify it. Include the equipment's powered-off condition in the assessment.
If the sensor reading must track ambient air, move the sensing element outside the enclosure wall using suitable environmental protection. For a custom enclosure, put vent and gland positions on the drawing at order time rather than drilling the wall later. Installation details are covered in the guide to cutouts, glands and HMI windows. Use the stainless enclosure size finder to compare grade, outside size and nominal inside dimensions.
The seal can be doing its job even when condensation forms inside. Check for liquid ingress, design the inside for moisture, and specify the outside for the actual water exposure.
Questions
Does an IP67 enclosure stop condensation on a PCB?
No. IP67 describes dust protection and temporary liquid-water immersion under specified test conditions, not freedom from condensation. Moisture trapped during assembly or entering later can condense when a surface falls below the internal dew point. That surface may be the enclosure wall, a component or the PCB. Better sealing does not remove existing moisture.
Is a humidity sensor mounted on a PCB inside a sealed enclosure still usable?
It can measure internal humidity if its operating conditions remain within the sensor's specification. It does not directly measure outdoor humidity, and condensation may affect its accuracy or recovery. For ambient measurements, use an appropriately protected external sensing element. A breather vent may reduce isolation from outdoor conditions, but it does not guarantee an accurate ambient measurement.
If condensation gets in anyway, why bother with an IP-rated enclosure?
The rating addresses external dust and liquid-water exposure under defined tests. Select the code for the actual exposure; temporary immersion and powerful water jets are different requirements. Condensation needs separate moisture management. Both liquid ingress and condensation can cause immediate electrical faults or longer-term corrosion, so neither should be ignored.
How much desiccant should I put in a small sealed enclosure?
There is no reliable universal grams-per-litre rule. Select the quantity from the desiccant supplier's capacity data at the intended temperature and humidity. Include moisture trapped during assembly, absorbed moisture in components, expected leakage, door openings and the service interval. Secure the sachet away from electrical parts and verify its replacement or regeneration schedule in service.
What temperature difference causes condensation inside a box?
Condensation occurs when a surface falls below the dew point of the internal air. At 86 F [30 C] and 60% relative humidity, the dew point is approximately 70 F [21 C]. A surface cooling below that temperature can collect moisture. The cooling time depends on radiation, wind, mounting, thermal mass and equipment heat.
