What is Dew Point? And Why Does it Matter for Your Coldroom?

Understanding Dew Point and Your Coldroom

Understanding dew point is essential for anyone managing or maintaining a coldroom or walk-in freezer. It explains why condensation forms, why ice builds up on door frames, and why a seemingly small gap in a door seal can cause a surprisingly big problem.

What is dew point?

Dew point is the temperature at which air becomes fully saturated with water vapour and condensation begins to form. Think of air like a sponge - it can hold a certain amount of moisture depending on its temperature. Warm air holds more moisture than cold air. When air cools down to its dew point, it can no longer hold that moisture and it is released as liquid water on whatever surface it contacts.

Dew point is determined by two things: air temperature and relative humidity. Relative humidity is simply how much moisture the air is holding relative to how much it could hold at that temperature. When relative humidity reaches 100%, the air temperature and the dew point are the same - any further drop in temperature causes condensation to form. A higher dew point means more moisture in the air and a greater risk of condensation forming on cold surfaces.

To lower the dew point, something must change. Either the air temperature rises, which allows the air to hold more moisture without reaching saturation, or the relative humidity falls, meaning there is less moisture in the air to begin with. Both bring the dew point down. Condensation can also be prevented by raising the temperature of the cold surface itself - if the surface stays above the dew point, condensation cannot form on it, even if the dew point of the surrounding air remains unchanged.

The frost point - what happens in a freezer

In a walk-in freezer the situation goes one step further. When air cools below 0°C at its dew point, the moisture does not form as liquid water. It freezes directly onto the surface. This is called the frost point. It is why you see frost forming on freezer door frames and panels rather than water droplets. If the frost point is reached repeatedly and moisture builds up faster than it can dissipate, ice accumulates.

How does this apply to your coldroom?

Any cold air leaking from your coldroom, whether through badly fitting door seals, thermal bridging, or poor insulation, causes the door, frame, or panels to become cold on the outside surface. When warm, humid air from a working kitchen or warehouse comes into contact with those cold surfaces, it reaches its dew point and condensation forms. In a walk-in freezer that condensation freezes and ice begins to build up.

This is why the environment around your coldroom matters as much as the room itself. A coldroom in a busy commercial kitchen with high humidity is far more susceptible to condensation than one in a dry warehouse, even if both rooms are identical.

A practical example

Consider a walk-in freezer set to -20°C opening into a commercial kitchen running at 30°C with 70% relative humidity, typical during a busy service. The dew point of that kitchen air is around 24°C. When the door is closed, the warm side of the door is exposed to the kitchen. Where insulation is poor or cold-bridging occurs, typically around the door frame and at the bottom of the door, the surface temperature on the warm side drops below that 24°C dew point. Condensation forms on those cold spots and freezes on contact. When the door opens, the cold side is exposed directly to the warm kitchen air and the same thing happens immediately and on a larger scale.

In this environment, a working door frame heater and a good door seal are not optional extras. They are essential.

The solution is a combination of good door seals, a working door frame heater to keep the frame surface above the dew point, and minimising the time the door is left open.

What can you do about it?

Start with your door seals. A damaged or poorly fitting seal allows warm moist air in and cold air out continuously, every time the door is closed. Check them regularly and replace them if they are hardening, splitting, or no longer sitting flush against the frame.

Make sure your door frame heater is working. Its job is to keep the frame surface above the dew point so that condensation cannot form there. A failed heater in a freezer room will result in ice build-up on the frame quickly, especially in a humid environment.

Minimise door open time. Every second the door is open introduces warm humid air into the space around the room. A strip curtain or thermal curtain reduces the volume of warm air entering each time the door opens and makes a significant difference in busy operations.

Finally, consider the humidity of the area surrounding your coldroom. High ambient humidity raises the dew point and increases the risk significantly. If your coldroom sits in a busy kitchen or a humid loading bay, the conditions outside the room are working against you before the door even opens.

Check your own dew point

You can calculate the dew point for your specific environment using this free online calculator at dpcalc.org. Enter the air temperature and relative humidity of the area around your coldroom and it will give you the dew point. If that figure is above the surface temperature of your door or frame, condensation will form.