Walk-In Cooler Sweating on the Outside: Dew Point and Fixes

Water beading on the outside of a walk-in means some part of the box's exterior has become colder than the surrounding air can tolerate without dropping its moisture. Sometimes that is just a very humid day. More often, when it keeps happening in the same place, it's a sign that insulation, a heater or a seal has failed, and the wet surface is the only visible symptom.

The dew point, in plain terms

Air holds water vapor, and warmer air can hold more. The dew point is the temperature at which a given batch of air can no longer hold all its moisture. When air touches a surface colder than its dew point, water condenses on that surface, the same way a cold drink sweats on a summer patio.

A walk-in is a box full of cold air wrapped in insulation. A healthy panel keeps its outer skin close to room temperature, well above the dew point, so it stays dry. Federal efficiency rules require walk-in walls, ceilings and doors to be insulated to at least R-25 for coolers and R-32 for freezers, with freezer floors at R-28, which is enough to keep the outer skin warm under normal conditions.

Sweating happens in two situations:

1. The room air is unusually humid, so its dew point climbs close to the temperature of the panel skin.
2. Part of the exterior is colder than it should be, because insulation, heat or a seal has failed there.

What's usually behind it

High humidity and the room

A walk-in in an air-conditioned store is rarely the problem. One in a hot, humid back room, a kitchen near the dish machine, an unconditioned warehouse or a garage is exposed to air with a much higher dew point. In summer, a sound box may sweat lightly at the door frame and seams. If the sweating comes and goes with the weather and appears evenly, humidity is the likely driver.

Poor ventilation around the box makes it worse. The site's page on ventilation requirements covers clearances, and indoor vs. outdoor walk-ins explains how location affects the box.

Wet or damaged insulation

When a panel sweats in the same place in all weather, the insulation behind that spot is probably no longer doing its job. The most common reason is water. Moisture that works into a panel through a damaged skin, an open seam or years of condensation can soak the foam. Wet insulation conducts heat far better than dry, so the outer skin gets cold, sweats, and lets in more moisture.

Other signs of a wet or damaged panel include soft spots when you press the skin, bulging or delaminating skins, rust stains, ice on the inside surface at the same location, and mold at the base of the wall. Panels can also be crushed by forklift or cart impacts, which compresses the foam and leaves a cold dent. See panels and insulation, and the site's comparison of polystyrene vs. polyurethane insulation for why foam type matters.

Failed heaters: door frames and glass

Freezer door frames carry heater wire (U.S. Cooler specifies it on all four sides of freezer doors) to keep the frame warm enough that it doesn't sweat outside or freeze to the gasket inside. When a section fails, the frame sweats outside and frosts inside at the same spot. See frost around the walk-in door and door frame heaters.

Glass merchandising doors have anti-sweat heaters in the frame and glass. Federal rules cap heaters that run without controls at 3.0 W per square foot of door opening on coolers and 7.1 W on freezers; above that, controls must adjust the heat for outside humidity or condensation. A failed heater, a failed control, or a control set too low lets the glass fog and the frame sweat. Fog between the panes is different: it means the sealed glass unit has failed and needs replacing. See glass display doors.

Seams, leaks and thermal bridges

Cold air is heavy and leaks out low through open seams, chilling the outside of the joint below it. A line of sweat along a vertical seam, or a wet stripe at the base of a wall, points to a joint that has opened. Cam locks that were never fully tightened, or were loosened by floor movement, are common culprits.

Thermal bridges are metal paths that carry cold from inside to outside: fasteners, structural members, trim and some older panel edges. Federal rules exempt structural members from the R-value requirement, so some cold conduction there is expected. Beads of water on screw heads and trim in humid weather may be this, not a failure.

The ceiling above the box

The top of a walk-in is often the forgotten surface. If the box sits under a lower building ceiling, or the space above it is boxed in, warm, humid air collects there with no air movement. The box top stays cold, condensation forms, and water can drip down the sides, stain ceiling tiles or rot framing. Clearing the space, improving air movement or sealing the gap properly is usually a facilities or installer job.

Checks staff can do safely

1. Map where it sweats and photograph it. Note whether it's the same places every day.
2. Compare humid and dry days. Weather-dependent sweating on many surfaces points to humidity; constant sweating in one spot points to the box.
3. Press panel skins gently near wet spots for softness or bulges. Don't puncture anything.
4. Check the inside of the same spot for frost, ice or damp insulation smell.
5. Look at seams for gaps and at cam-lock covers for missing caps.
6. Check glass doors for fog on the glass versus between the panes.
7. Wipe surfaces dry and keep the floor dry around the box.

Safety: Water running down panels onto the floor is a slip hazard. Keep it mopped and marked. Don't open door heater wiring, glass door frames or electrical boxes to look for the fault.

What a technician or installer will check

• Door frame and glass heater circuits, and any anti-sweat control settings.
• Panel condition, including moisture in the insulation, often with a thermal camera.
• Seam and cam-lock tightness, and sealing at penetrations.
• Room conditions and ventilation around and above the box.

When it points to replacement

Heater faults, open seams and room humidity are repairable. A panel that is saturated usually isn't; once foam has absorbed water, it rarely dries out in service, and replacing the panel is the lasting fix. Panel warranties vary widely by maker, from one year to ten or more, so check yours before paying for repairs; see warranty claims. If several panels are wet, or the box is also failing elsewhere, JayComp can quote replacement panels or a new box from lines such as Leer.