Cold store door condensation: why the frame drips when the panel is fine
The door of a -20 °C freezer is a thick panel with a warm surface, yet water still forms and frost builds up on the frame and edges. Taking the dew point of the dock air as the criterion, we put numbers on the panel, the frame thermal bridge, heater power, and the fog when the door opens.
Situation
Say that on a summer day water drips around the door of a -20 °C freezer on the loading dock and thick frost builds up on the door frame. The dock air is 30 °C and 70 % RH, and the door is a polyurethane panel. If the broad face of the door is fine and only the frame and edges are wet, the problem is not the panel's insulation but the parts through which heat passes easily (thermal bridges). Three things to find: the dew point of the dock air, the surface temperatures of the panel face and of the frame, and why fog forms when the door opens. The conditions are illustrative; a real decision uses measured dock temperature and humidity and the product data for the panel and frame.
Given conditions
| Item | Value |
|---|---|
| Air inside the store | -20 °C, 90 % RH (freezer, example value) |
| Dock air | Dry-bulb 30 °C, 70 % RH (summer afternoon, example value). For comparison 25 °C / 60 % and a chilled anteroom at 10 °C / 70 % are also calculated |
| Door panel | Polyurethane 50 to 200 mm, conductivity k = 0.025 + 0.0001 × (mean temperature − 10) W/m·K (design reference value; real values come from the product test certificate, and the sub-zero side is a linear extrapolation) |
| Jacket emissivity ε | Painted steel 0.9, bright galvanized steel 0.26 |
| Door frame | A metal frame with a polyurethane thermal break of 10 · 20 · 30 · 50 mm. The metal's own resistance is ignored; painted, ε 0.9 |
| Air movement outside | Still air (natural convection only). Wind would warm the surface, so this is the safe side |
| Margin | Surface − dew point ≥ 1 K is safe |
| Pressure | 101.325 kPa (sea level) |
The dock-side surface receives heat by convection from the air (hcv) and by radiation (hr), and that heat flows through the panel into the store. The surface settles at the temperature where the two balance. The thicker the panel, the less heat leaves and the closer the surface sits to the dock air.
Reading it on the chart
- Plot the dock air point A (30 °C, 70 %).
- Follow the line of constant humidity ratio left from A to the saturation line; the dry-bulb at that point DP is the dew point. Air touching a surface cools at constant humidity ratio.
- Draw a vertical at the panel surface temperature. To the right of DP (by at least 1 K) is safe.
- The vertical for the frame surface sits just left of DP (condensation), and the thinner the thermal break, the further left it goes toward the store air point K (-20 °C). The straight line from A to K is the path when the two airs mix at the open door; where it leaves the saturation line to the left, fog forms.
Step-by-step calculation
1) Dock dew point. At 30 °C / 70 % the humidity ratio is W = 18.80 g/kg, the vapour pressure 2.972 kPa and the dew point tdp = 23.93 °C. The criterion is a surface of 24.93 °C or more.
2) A bare metal frame. Metal has almost no thermal resistance, so the surface equals the store air temperature, −20.0 °C, which is 43.9 K below the dew point. Water forms and, below 0 °C, freezes as frost.
3) 100 mm panel (painted). Both k and the surface coefficient depend on the surface temperature, so it is solved iteratively. The converged values are mean temperature Tm = (-20 + 28.37)/2 = 4.18 °C, k = 0.0244 W/m·K, Rins = t/k = 0.1 / 0.0244 = 4.095 m²K/W. At the surface hcv = 1.60, hr = 5.64, total hse = 7.25 W/m²K and Rse = 0.138 m²K/W. Solving (Ta − Ts)/Rse = (Ts − Tf)/Rins gives Ts = (Ta·Rins + Tf·Rse)/(Rins + Rse) = (30 × 4.095 + (-20) × 0.138)/(4.095 + 0.138) = 28.37 °C. That is 4.44 K above the dew point, so it is safe, with a heat gain of 11.8 W/m².
4) A 20 mm thermal break in the frame. Applying the same equations to the break thickness gives a surface of 23.28 °C, 0.6 K below the dew point, so it condenses. With a 10 mm break it is much worse at 18.82 °C, and with 30 mm the surface is 25.22 °C, 1.29 K of margin, which is safe. The frame uses the same polyurethane as the panel, so the thickness needed is the minimum thickness in step 5. The difficulty is that a frame face is narrow and metallic, and a break that thick is often hard to fit (assumed 20 mm in this example).
5) Minimum thickness. The thinnest panel with surface − dew point ≥ 1 K is 28.0 mm with a painted jacket and 57.5 mm with a bright jacket, which round up to the standard 30 mm and 65 mm. A bright jacket (ε 0.26) receives little radiant heat, so its surface runs colder and the panel must be thicker.
6) Heater power. To hold the surface of a frame with a 20 mm break at the dew point + 1 K = 24.93 °C, the heater must supply the heat leaving the surface to the store, 54.5 W/m², minus the heat the dock air gives the surface, 38.9 W/m² (hse = 7.68), which is 15.5 W/m². With a frame face width of 0.1 m that is 1.6 W per metre of frame.
Interpreting the result
By panel thickness (dock 30 °C / 70 %, dew point 23.93 °C):
| Panel thickness | Painted surface | Margin | Verdict | Heat gain (W/m²) | Bright surface | Verdict |
|---|---|---|---|---|---|---|
| 50 mm | 26.94 °C | 3.02 K | safe | 22.9 | 24.33 °C | at risk (margin too small) |
| 75 mm | 27.88 °C | 3.95 K | safe | 15.6 | 25.90 °C | safe |
| 100 mm | 28.37 °C | 4.44 K | safe | 11.8 | 26.77 °C | safe |
| 150 mm | 28.88 °C | 4.95 K | safe | 8.0 | 27.70 °C | safe |
| 200 mm | 29.14 °C | 5.22 K | safe | 6.0 | 28.20 °C | safe |
With a bright jacket a 50 mm panel has a surface of 24.3 °C, only 0.40 K of margin. The painted 50 mm panel has 3.02 K and is safe. As with duct insulation, a shiny jacket reduces heat gain but is worse for surface temperature.
The frame changes with the thermal break thickness (same dock, painted):
| Thermal break | Surface | Margin | Verdict | Heater needed (W/m²) | Per metre at 0.1 m face (W/m) |
|---|---|---|---|---|---|
| 10 mm | 18.82 °C | −5.1 K | condensation | 70.0 | 7.0 |
| 20 mm | 23.28 °C | −0.6 K | condensation | 15.5 | 1.6 |
| 30 mm | 25.22 °C | 1.3 K | safe | 0.0 | 0.0 |
| 50 mm | 26.94 °C | 3.0 K | safe | 0.0 | 0.0 |
If the break is thinner than the minimum thickness for the dew point (28.0 mm), a heater makes up the difference: 70.0 W/m² for 10 mm and 15.5 W/m² for 20 mm, and none is needed from 30 mm. That is why frames whose break cannot be made thicker carry heaters. The power is for a high dock dew point and falls sharply when the weather is milder or the door opens into an anteroom.
The needed panel thickness and heater power by environment (panel painted or bright; heater for a 20 mm break):
| Environment | Dew point | Min panel thickness (painted) | Min panel thickness (bright) | Heater (W/m²) |
|---|---|---|---|---|
| Summer dock 30 °C / 70 % | 23.9 °C | 28.0 mm | 57.5 mm | 15.5 |
| Milder dock 25 °C / 60 % | 16.7 °C | 16.3 mm | 32.1 mm | 0.0 |
| Chilled anteroom 10 °C / 70 % | 4.8 °C | 21.7 mm | 42.6 mm | 2.4 |
Going through a chilled anteroom (10 °C) lowers the dew point to 4.8 °C, so the heater for a 20 mm break falls to 2.4 W/m² (15.5 W/m² directly from the dock) and the minimum panel thickness drops too. An anteroom also cuts the moisture that enters when the door opens, so it addresses both condensation and frost.
Fog when the door opens. When dock air at 30 °C / 70 % mixes with store air (-20 °C / 90 %, W = 0.57 g/kg), compare the humidity ratio of the mixed point with the saturation humidity ratio at its temperature:
| Share of dock air | Mixed temperature | Mixed W (g/kg) | Saturation W at that temperature (g/kg) | W / saturation W |
|---|---|---|---|---|
| 1 % | −19.5 °C | 0.75 | 0.67 | 1.13 |
| 5 % | −17.4 °C | 1.48 | 0.81 | 1.83 |
| 10 % | −14.8 °C | 2.39 | 1.03 | 2.32 |
Even 1 % dock air puts the mixed point beyond the saturation line (ratio 1.13), so moisture comes out as fog and frost. At 10 % dock air the mixed point is 2.32 times its saturation humidity ratio. The ratio is an index of supersaturation; the actual amount deposited is smaller because the heat of condensation warms the air. This is why the evaporator of a freezer whose door opens often frosts up quickly.
Condensation outside the calculation: threshold, gasket, frost build-up
The calculation above is a steady-state one for a flat panel and frame face. In practice the commoner causes are the threshold and floor junction (thermal bridges), a gasket that is folded or gapped, and humid air leaking through the gap between door and frame. Humid air that gets into a gap turns to frost on the cold side and builds into ice, and the ice pushes the door and widens the gap.
If the panel itself is wet, a split in the joint seal has probably let moisture into the panel. Wet insulation conducts more heat, making the surface colder and the condensation worse, a vicious circle. The calculation assumes dry, continuous insulation.
For the sub-zero side the site calculator extends its linear conductivity equation (referenced to 10 °C) below zero. With a mean temperature around -5 °C the error is not large, but a design should use the low-temperature conductivity from the product test certificate. The still-air and 1 m height assumptions are on the safe side; on a tall door natural convection weakens near the top.
Common mistakes
- Assuming a thick panel makes the whole door safe. A 100 mm panel has 4.4 K of margin, but a frame with a 20 mm thermal break is at 23.3 °C and condenses.
- Thinking a bright jacket helps. It cuts heat gain but makes the surface colder: at 50 mm, 26.9 °C painted against 24.3 °C bright.
- Sizing the heater for one condition. At a dock dew point of 23.9 °C versus 4.8 °C through an anteroom, the power differs by a factor of 6.6. Control it with a dew point sensor.
- Running the heater but ignoring the gasket gap. Humid air leaking through the gap is not stopped by the heater.
- Treating door-opening fog as separate from condensation. Even 1 % dock air is supersaturated. Door-open time, anterooms and air curtains set how much frost forms.
Try this calculation on the site
Insulation surface condensation: open the 100 mm painted panelOpen a 20 mm frame breakOpen the frame check on the chart
The dew point tool links fill in dock air 30 °C / 70 % and the flat surface of "Insulation surface condensation", with -20 °C on the cold side, polyurethane and a painted jacket, and show the surface temperature, margin, verdict, minimum thickness, thickness table and calculation steps. Change the thickness, jacket and dock conditions to see where the verdict flips. The chart link plots the dock air point with the frame surface temperature as a condensation check. The same kind of calculation for ducts is Duct insulation surface condensation, for wall surfaces see Winter exterior wall condensation check, and for plotting see How to read a psychrometric chart.
References
- ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: humidity ratio, saturation vapour pressure (Hyland-Wexler), dew point and enthalpy. The psychrometric numbers here were computed with this site's engine, which implements those equations at 101.325 kPa.
- ISO 12241: calculation rules for heat transfer through insulation on equipment and pipework. The site's insulation calculation follows this form (insulation resistance plus a surface coefficient split into radiation and natural convection). For flat surfaces the natural convection coefficient is the simplified air correlation from Holman, Heat Transfer.
- ASHRAE Handbook Refrigeration: general principles for refrigerated facilities on preventing frost and condensation at door frames (door frame heaters), vestibules, and moisture entering when doors open. This article shows those principles by psychrometric and heat balance calculation; the heater power here is a simple heat-balance value, not a manufacturer's standard rating.
- The polyurethane conductivity (k10 0.025 W/m·K) and the jacket emissivities (0.9 · 0.26) are typical design reference values from the site's calculator. The store -20 °C / 90 %, dock 30 °C / 70 %, thermal break thicknesses and face width are example values.