Key takeaways
- A sealed enclosure is not a dry enclosure. IP66 stops water getting in; it does nothing about the moisture already in the trapped air.
- Condensation happens when a surface drops below the dew point of the air inside the box — usually the roof or the upper side wall, on a clear still night.
- Size the heater from P = k × A × ΔT, with ΔT of 3–5 K above the lowest expected ambient. Oversizing wastes energy and eats your summer headroom.
- Fit both a breather and a drain — the breather stops the cabinet inhaling humid air through the gasket, the drain lets liquid water out at the true low point.
- Feed the heater from a supply that stays live when the control supply is isolated. A heater switched off with the process is off exactly when the panel cools.
- Cable entries cause more wet cabinets than shell defects do — roughly three to one in our field log.
Enclosure condensation is the failure mode that survives every IP rating you can buy. A telecom OEM installed 60 outdoor cabinets along a coastal highway last summer — all IP66, all with compression gaskets, all type-tested. Within four months, 14 of them had water beading on the underside of the roof and corrosion starting on the terminal blocks. Nothing had leaked. The cabinets had manufactured their own water: warm humid air sealed inside at 3 p.m., cold roof radiating to a clear night sky at 2 a.m., and a roof panel sitting four degrees below the dew point of the air beneath it.
This guide is the moisture-management package we now quote with every outdoor enclosure. It covers the dew-point arithmetic that tells you whether you have a problem at all, the six control measures and what each one actually buys you, how to size an anti-condensation heater without cooking your electronics, where the breather and the drain go, and the eight failure modes that show up in our 12-year service log. For the sealing side that keeps the macro-environment out in the first place, pair this with our gasket sealing to IP65 guide.
Why a Sealed Enclosure Still Gets Wet
Warm air holds more water vapour than cold air. The dew point is the temperature at which the air can no longer hold what it has, and the surplus condenses onto whichever surface is coldest. Seal a cabinet at 30 °C and 80% RH and the air inside holds roughly 24 g of water per cubic metre. Cool the roof to 22 °C overnight — which a clear sky will happily do, because the panel radiates heat away faster than it absorbs it — and the air next to it can only hold about 19 g. The missing five grams appear as liquid water on the underside of the roof, and gravity does the rest.
Two things make this worse in a metal box. First, steel is a good conductor: the roof and the side walls reach ambient almost immediately, far faster than the air volume inside. Second, the daily cycle pumps air in and out. Every afternoon the internal air expands and pushes out through the path of least resistance; every night it contracts and draws fresh humid air back in through the gasket, the gland threads and any unused knockout. A perfectly sealed IP66 cabinet is, over a year, a slow pump.
| Ambient air | Dew point (approx.) | Practical meaning |
|---|---|---|
| 40 °C / 60% RH | 30.5 °C | Tropical afternoon — any air-conditioned surface inside is at risk |
| 30 °C / 80% RH | 26.2 °C | Coastal summer — a 4 K overnight drop is enough |
| 20 °C / 70% RH | 14.4 °C | Temperate spring/autumn — the classic shoulder-season failure |
| 10 °C / 90% RH | 8.4 °C | Temperate winter, near-saturated — 2 K of margin |
| 5 °C / 85% RH | 2.6 °C | Cold damp — a clear night will almost certainly cross it |
Read that table as a design warning: you do not need a large temperature swing to make water. You need a surface that is a couple of degrees colder than the air — and a sheet metal roof on a clear night is exactly that.
Six Control Measures — and What Each One Actually Buys
There is no single device that solves this. Every cabinet we ship for a humid or cycling climate uses a combination, and the combination is chosen from the table below.
| Measure | Mechanism | Best when | Typical cost | Limits |
|---|---|---|---|---|
| Anti-condensation heater + hygrostat | Holds internal surfaces above the dew point | Cold swings, night-to-morning cycling, standby periods | $35–180 per cabinet | Adds heat load in summer; must be controlled, not run continuously |
| Pressure equalisation breather (ePTFE membrane) | Lets pressure equalise so the box stops inhaling through the gasket | Sealed enclosures with daily temperature cycling | $8–40 | Must match the enclosure IP rating; not a substitute for a drain |
| Breather drain plug | Lets liquid water escape at the true low point | Any outdoor cabinet where water can collect | $5–30 | Does nothing to prevent condensation; only removes it |
| Conformal coating on PCBs | Makes the electronics tolerant of an occasional wet surface | High-value boards in humid zones, telecom, municipal | $6–40 per board | Insurance, not prevention — per IPC-CC-830 / MIL-I-46058C |
| Desiccant + VCI packaging | Removes moisture during shipping and storage | Export transit, long warehouse holds | $2–15 | Saturated within weeks; useless once commissioned |
| Double wall / insulated roof | Raises the internal surface temperature and cuts radiative loss | Strong solar gain plus cold nights; desert and high-altitude sites | +15–35% on the shell | Cost, weight and depth; needs its own condensation strategy |
The combination that works for most outdoor cabinets is heater + hygrostat + breather + drain. Drop any one of the four and the other three carry a load they were not sized for.
Sizing the Heater Without Cooking the Electronics
Start from the steady-state heat loss through the shell:
P = k × A × ΔT — where k is the surface heat transfer coefficient of the wall, A is the effective radiating area, and ΔT is the rise above ambient you want to hold.
| Wall construction | k (W/m²·K) | Notes |
|---|---|---|
| Single-wall painted steel | 5.0 – 6.0 | The default for most industrial cabinets |
| Single-wall aluminium | 6.0 – 7.0 | Conducts faster; loses heat faster |
| Stainless steel, single wall | 5.0 – 6.0 | Similar to painted steel |
| Insulated double wall | 2.0 – 3.0 | Roughly halves the heater requirement |
Worked example. A floor-standing cabinet 1.2 × 0.8 × 1.8 m has about 7.5 m² of effective radiating area (excluding the face against the wall or plinth). With k = 5.5 and ΔT = 5 K: P = 5.5 × 7.5 × 5 = 206 W. Select a 250 W heater, which gives headroom without pushing into the next price band.
Three rules that our commissioning team insists on:
- Use 3–5 K, not 15 K. Three to five kelvin above the lowest expected ambient is enough to keep surfaces off the dew point in most temperate climates. Sizing for a large ΔT wastes energy and eats the summer headroom you need for the internal heat load.
- Check the other end. Add the dissipation of the PLC, the power supply and any drives to the solar-gain-driven ambient, and compare against the lowest maximum operating ambient of any device in the panel. That one number — usually the switch-mode supply or the PLC CPU — sets whether you need cooling instead.
- Mount it low, with the convection path clear. Warm air rises; a heater mounted low circulates through the whole cabinet. Keep the clearance to PVC-insulated cables required by the heater datasheet — element surfaces can exceed 100 °C.
Where the Breather and the Drain Go
These two accessories are frequently specified as one, and they are not interchangeable.
- Breather (pressure equalisation vent) — mount high on a side wall, above the expected internal water line, on a flat surface with the membrane facing down or sideways so it cannot be directly rained on or splashed. Its own IP rating must match or exceed the enclosure rating, or you have downgraded the whole assembly. Typical threads are M12, M20 and M32 with a locknut.
- Breather drain plug — mount at the true lowest internal point, on the same face as the gland plate. "True" matters: if the gland plate is bowed or the base has a stiffening rib, the lowest point may not be where the drawing suggests. The plug must let water out in seconds, not hours.
- Never put cable entries on the top face. Use a bottom gland plate, form a drip loop below every entry, and blank every unused knockout with a rated plug.
Our standard drawing note for outdoor cabinets reads: "1 × M20 ePTFE breather, IP66 minimum, mounted upper side wall, membrane shielded; 1 × breather drain plug at true low point on gland-plate face; no top entries; all spare knockouts plugged to the enclosure rating."
Eight Field Failure Modes from Our Service Log
| Symptom | Root cause | Prevention |
|---|---|---|
| Heater burns out within 12–18 months | Running continuously with no thermostat or hygrostat | Always control it. Controlled elements last 5–8 years versus 12–18 months uncontrolled |
| Water still forms with the heater running | Undersized for the surface area, or blocked convection path | Recalculate from A and k; keep the area above the heater clear of cable bundles |
| Heater trips the breaker on cold start | Inrush current of a cold element | Size the breaker at 150% of the heater rating |
| Condensation only during shutdown | Heater fed from the control supply that is isolated with the process | Feed from a supply that stays live; it is needed most when the panel is cold |
| Corrosion at the bottom corners | Unsealed spare knockouts, or conduit entering downhill | Blank all spares; run conduit so it cannot drain into the cabinet |
| Wet streak on the hinge side only | Hinge-side gasket compression relaxes while the latch pulls the strike side tight | Check witness marks all round; add a second latch or adjust the strike |
| Water pooling instead of draining | Drain not at the true low point, or gland plate bowed | Pour 0.5 L through a temporary opening; it must exit within a minute |
| Cabinet wet again one season after repair | Gland nuts backed off by thermal cycling and vibration | Re-torque glands and latches at the first scheduled visit; log it |
Five Steps to a Condensation Specification
- Define the climate. Minimum and maximum ambient, maximum RH, and whether the site is coastal, tropical, temperate-cycling or high-altitude. Without this, nothing downstream is checkable.
- Calculate the dew point at the worst-case RH and compare it with the coldest expected internal surface temperature. If the margin is under 3 K, you need active control.
- Choose the combination from the six-measure table. Default to heater + hygrostat + breather + drain for anything outdoor in a cycling climate.
- Size the heater from P = k × A × ΔT at 3–5 K, then verify the upper end against the internal heat load and the lowest device rating.
- Specify verification. A drain-flow test at FAT, plus a temperature/humidity data logger left inside for one week that includes a clear still night and a sunny afternoon.
What the Sheet Metal Shop Can Do for You
Four geometry decisions made at the flat-pattern stage remove most of the problem before a single accessory is bought:
- Slope the roof and the base. A 3–5° fall to the drain point prevents the shallow pools that a drain plug cannot reach.
- Avoid water traps in the base. Stiffening ribs, boxed feet and welded channels that cross the floor create dams. Detail them with drain notches.
- Weld a drain boss, do not just drill a hole. A formed or welded boss gives the plug a flat sealing face and thread engagement; a bare hole in 1.5 mm sheet gives neither.
- Insulate or shade the roof. Our sunshade hood design guide covers the passive option; a ventilated shade gap kills most of the solar gain and blocks the night-sky radiative loss that drives the roof below dew point in the first place.
Send us the site climate data, the cabinet dimensions and the internal heat load, and we will return a one-page Moisture Control Pack within 24 hours — dew-point check, heater wattage, breather and drain positions, the four drawing notes, and a fixed-price line item. Use the contact form or email gongtianbao@xhlmarketing.com. If you are also working through the spacing implications of a humid internal environment, see our clearance and creepage guide — pollution degree 3 is what a wet cabinet becomes.
Frequently Asked Questions
Does an IP66 enclosure stop condensation?
No. IP66 means dust-tight and protected against powerful water jets from outside. The air sealed inside still contains moisture, and when the shell cools below the dew point of that trapped air, water condenses on the coldest internal surface. Sealed enclosures in climates with large day-night swings usually still need a heater, a breather, or both.
What size anti-condensation heater do I need?
Use P = k × A × ΔT with ΔT of 3–5 K. For single-wall painted steel, k is about 5.5 W/m²·K. A cabinet with 7.5 m² of effective area at ΔT = 5 K needs about 206 W, so a 250 W heater is the usual selection. Always check that the added heat does not push the cabinet past the lowest maximum ambient rating of any device inside.
Should I use a drain plug or a breather vent?
Both. A pressure equalisation breather with an ePTFE membrane stops the cabinet inhaling humid air through gaskets and glands as it cools; a breather drain plug at the true lowest point lets liquid water escape. Mount the breather high on a side wall and the drain at the low point on the gland-plate face.
What relative humidity should the hygrostat be set to?
55–65% RH is the standard band. Below about 50% the heater cycles more than necessary; above about 70% a fast overnight drop can still deposit water before the heater responds.
Can condensation come from the cable entry rather than the enclosure?
Yes, and it is one of the most common causes. Conduit run downhill carries water in, unsealed spare knockouts admit humid air, and gland nuts backed off by thermal cycling break the seal. In our field log, cable-entry problems outnumber genuine shell defects by roughly three to one.