Moisture Build-up During Route Execution

Every frozen delivery operator knows about heat infiltration — warm air rushing into cold loadboxes. There are formulas for it, equipment sized against it, entire articles written about it.

But heat is only half the problem.

The warm air rushing through your door carries something else: water vapour. And while your refrigeration system fights the heat, nobody is fighting the moisture. It freezes on your evaporator coils, chokes your airflow, condenses on your ceiling, soaks your cardboard boxes, rusts your door hardware, and degrades your insulation — silently, relentlessly, every single route.

At The Frozen Food Courier, we’ve driven to the moon and back twice multi-dropping frozen delivery across Gauteng and Western Cape. We’ve measured this. Calculated it. Lived with it. And we can tell you exactly how much water your loadbox is swallowing every day — because nobody else has bothered to do the maths.

Part 1: What You Need to Know

The Water You Can’t See

At the end of a 15-stop summer route, open your loadbox and look at the ceiling. You’ll see condensation droplets — sometimes running in streams. Look at your door frame hardware: rust, not from age, but from daily moisture cycling that no amount of maintenance prevents.

This isn’t equipment failure. This is physics.

Every time a driver opens the door, warm humid air floods into -18°C cargo space. The moisture in that air has nowhere to go but onto the coldest surfaces — evaporator coils first, then walls, ceiling, and product packaging. Over 15-30 stops, litres of water accumulate inside your loadbox.

The industry’s workaround? “Sleep the truck open” overnight — leave doors open so moisture can escape. A manual dehumidification process for a problem nobody engineered a solution for. That tells you everything about how seriously equipment suppliers take multi-stop operations.

Why We Ask for Good Quality Cardboard Boxes

If you’ve shipped with us, you’ve heard us talk about packaging quality. Here’s why it matters more than you think.

Inside a multi-stop loadbox, temperature doesn’t sit at a steady -18°C. It cycles — dropping to -18°C while running, rising to -10°C or -5°C during door opening recovery, then pulling back down. Four to eight freeze-thaw cycles per route is normal.

Each cycle means moisture condensing on your box exteriors, then freezing, then thawing again. Cheap single-wall cardboard absorbs this moisture like a sponge:

It softens within 2-3 hours of multi-stop delivery. Stacking strength collapses — bottom boxes crush under weight they’d normally support. Labels become illegible from moisture wicking. Boxes tear during handling because wet corrugated board has no structural integrity.

The product inside stays frozen at -18°C. But if the box around it disintegrates, you’ve got a delivery problem that looks like a temperature problem but is actually a packaging problem.

Our recommendation: Wax-coated or moisture-resistant corrugated boxes. Double-wall construction for heavy items. A sealed polybag liner inside the box as a moisture barrier. And proper taping — water wicks into every open flap.

We control our refrigeration, our routes, our handling. We cannot control the packaging you give us. Good boxes are your insurance policy against physics.

What It Means for Your Deliveries

Temperature fluctuations during multi-stop delivery are not equipment failure — they are the unavoidable physics of fighting moisture and heat simultaneously. Your product stays safely frozen. But outer packaging takes a beating from condensation cycling, and labels can suffer.

Here’s the comparison that matters: a long-haul truck opens its doors 2-3 times per trip. A courier opens doors 30-60 times per route. The moisture load difference is enormous. Yet both operations use essentially the same equipment, designed for the same assumptions. This is the gap that nobody specs for.

Part 2: The Engineering Reality

The Physics — Moisture Infiltration Per Door Opening

Heat infiltration through door openings is well documented in our Technical Formulas Reference. But thermal load and moisture load are separate problems requiring separate analysis.

The mass of water vapour introduced per door opening:

m_water = ρ_air × V_cargo × η_exchange × (ω_ambient − ω_cargo)

Where:

m_water = mass of water vapour introduced per door opening (grams)
ρ_air = ambient air density (kg/m³) — 0.95 at Johannesburg altitude, 1.2 at Cape Town sea level
V_cargo = cargo volume (m³) — typically 4-6 m³ for a 1-tonner
η_exchange = air exchange fraction per opening (0.3-0.6)
ω_ambient = humidity ratio of ambient air (kg water / kg dry air), from psychrometric relations at ambient temperature and relative humidity
ω_cargo = humidity ratio of cargo air — effectively zero at -18°C, because air at that temperature holds negligible moisture

The critical insight: this formula isolates the moisture component. The heat infiltration formula on our Technical Formulas Reference captures the thermal energy. Both happen simultaneously with every door opening, but require different engineering responses.

Worked Example — Gauteng Summer Route

Given:
  Ambient: 30°C, 60% relative humidity → ω ≈ 0.016 kg/kg
  Cargo volume: 5 m³
  Air exchange per opening: 40% (η = 0.4)
  Air density at Johannesburg: 0.95 kg/m³
  Cargo humidity ratio: ≈ 0 at -18°C

Per door opening:
  m_water = 0.95 × 5 × 0.4 × 0.016
  m_water = 30.4 grams

Route total (30 openings):
  30.4g × 30 = 912g ≈ 0.9 litres per route

Weekly (5 routes): 4.5 litres
Monthly: ~18 litres

Nearly a litre of water entering your loadbox every single route. Not from leaks. Not from defrost drainage failure. From air.

Cape Town — The Counterintuitive Result

You’d expect Johannesburg’s hot summers to be worse. They’re not — for moisture.

Given:
  Ambient: 28°C, 75% relative humidity → ω ≈ 0.018 kg/kg
  Higher air density at sea level: 1.2 kg/m³
  Same cargo volume and exchange rate

Per door opening:
  m_water = 1.2 × 5 × 0.4 × 0.018
  m_water = 43.2 grams

Route total (30 openings):
  43.2g × 30 = 1,296g ≈ 1.3 litres per route

Monthly: ~26 litres

Cape Town faces 44% higher moisture loads than Johannesburg despite lower ambient temperatures. Higher humidity combined with higher air density at sea level puts more water into every door opening.

Equipment suppliers who size systems based on ambient temperature alone — ignoring humidity and air density — get this completely wrong. Johannesburg needs more cooling capacity (heat load at altitude). Cape Town needs more moisture management (humidity load at sea level). Different problems, different solutions, same “standard” equipment sold to both.

Where the Water Goes — The Icing Cascade

Understanding the sequence matters because it explains why multi-stop frozen operations experience compounding performance degradation across a route:

1. Door opens — warm humid air floods cargo space.
2. Door closes — TRU starts pulling temperature back down.
3. Moisture condenses on evaporator coils — the coldest surface (-25°C to -30°C) attracts moisture first.
4. Ice forms on coils — reducing fin spacing, blocking airflow passages.
5. Airflow drops — evaporator heat transfer efficiency falls.
6. Next door opening — more humid air enters, but now recovery is slower.
7. Compounding effect — each stop performs worse than the last.
8. Timer defrost triggers — if the 6-8 hour interval happens to fall during the route (often it doesn’t).
9. Defrost heaters activate — adding 2-3 kW of heat directly into cargo space.
10. Ice melts — water drains if drainage works, or re-evaporates into cargo air.
11. Re-evaporated moisture condenses — on ceiling, walls, and product packaging. This is where your ceiling condensation comes from.
12. Post-defrost recovery — TRU runs at maximum capacity to pull temperature back down.
13. Cycle repeats — until route ends or ice overwhelms the system.

This is why the last stops on a route experience worse conditions than the first. Not because the equipment is failing — because ice accumulation is progressively strangling the evaporator.

Ice Thickness — The Numbers

0.9 litres water per route × 70% deposited on coils = 630g ice
Typical 1-tonner evaporator fin area: ~0.5 m²
Ice layer thickness: ~1.4mm per route

At 3mm: airflow drops 30-40%
At 5mm: evaporator effectively blocked

Multi-stop route can reach 3-5mm within 3-4 hours in summer

A 6-hour timer defrost interval means the evaporator may be blocked for hours before defrost fires. And when it does fire, it dumps 2-3 kW of heat into your cargo space whether the route is finished or not. Timer-based systems respond to clocks, not physics. Our Technical Formulas Reference quantifies the energy waste from this approach — 60% of timer-initiated defrost cycles are unnecessary under steady-state conditions. Under multi-stop conditions, the timing is wrong in the opposite direction: too infrequent during heavy door opening periods, too frequent during light periods.

Structural Damage — What Moisture Does Over Months and Years

The daily moisture cycling described above doesn’t just affect route performance. It systematically destroys your loadbox.

• Door seals: Rubber gaskets degrade from constant freeze-thaw cycling. Degraded seals admit more air, creating a positive feedback loop — more moisture entry, faster seal degradation. Replacement every 12-18 months instead of the expected 3-5 years.
• Panel joints: Freeze-thaw expansion works moisture into panel joints. Insulation absorbs water. Wet insulation loses thermal resistance — a panel with saturated insulation can lose 40-60% of its R-value. Higher thermal load means more compressor runtime, more fuel, worse temperature control. The insulation damage is invisible until panels are cut open.
• Floor corrosion: Water pools at low points. Over months, floor panels corrode from beneath. Structural integrity is compromised before visual signs appear externally.
• Electrical systems: TRU wiring, temperature sensors, door switches, and control boards all corrode. The result is intermittent faults that drive technicians and operators mad — systems that work sometimes but not others, because corroded connections make and break contact randomly.
• Hygiene risk: Standing moisture combined with organic residue from food contact creates conditions for bacterial and fungal growth. This is a direct R638 compliance concern. Regulatory inspectors look for visible moisture, condensation damage, and evidence of inadequate cleaning — all symptoms of unmanaged moisture infiltration.
• Cost quantification:
  Annual moisture-related maintenance: R8,000-R20,000 per vehicle. Over a 10-year vehicle life: R80,000-R200,000 in damage that could be mitigated through proper moisture management.

Solutions — What Actually Works

Moisture management for multi-stop frozen delivery exists on a spectrum from operational discipline to engineering solutions. No single intervention eliminates the problem — physics guarantees moisture entry with every door opening. The question is how effectively you manage it.

Tier 1: Operational Discipline (No Hardware Cost)

Sleep trucks open overnight — manual dehumidification. It works, but it’s a workaround, not a solution. Route planning to minimise door open time. Pre-sorting cargo for quick stop access reduces average door opening duration from 90 seconds to 45 seconds — halving moisture entry per stop. Driver training on door discipline. End-of-day inspection and wipe-down of condensation accumulation.

These are table stakes. Every operator should do them. They don’t solve the problem; they reduce it.

Tier 2: Strip Curtains — The “Butcher’s Curtain”

Freezer-grade PVC strips, 200mm wide with 50% overlap, reduce air exchange 60-80% per door opening. Cost: R1,500-R3,000. Local suppliers include Maxiflex and Strip Curtain Solutions (Western Cape).

On paper, this is the obvious answer. In practice, it’s a borrowed solution from the wrong use case.

Strip curtains work brilliantly for cold room doorways with forklift traffic — predictable entry angles, powered vehicles pushing through, maintained by facility staff. Courier operations are different. Drivers handling multiple boxes per stop whip curtains aside, fight through them with arms full of product, and inevitably tie them back, cut them short, or tear them off within weeks.

The solution designed for warehouse dock doors fails in the chaotic reality of 30-stop hand-loading courier operations. It’s not that strip curtains don’t work — it’s that courier duty cycles destroy them faster than they deliver value.

Tier 3: Demand-Based Defrost (Engineering Fix)

Replace the timer-based defrost with sensor-triggered defrost — an NTC thermistor on the evaporator coil or differential pressure sensing across the coil face. Defrost fires only when ice has actually accumulated, not when a clock says so.

This doesn’t reduce moisture entering the loadbox. But it optimises the response — defrosting when needed rather than on arbitrary schedules. For multi-stop operations where ice accumulation is rapid and variable, demand-based defrost is essential rather than optional.

Cost: approximately R12,000 for retrofit. The Technical Formulas Reference details the energy waste from timer-based systems — 60% of timer-initiated cycles are unnecessary. Demand-based systems eliminate this waste while responding faster to actual ice accumulation during heavy door-opening periods.

Tier 4: Vehicle Air Curtains (Premium Solution)

An invisible barrier of recirculated air across the door opening — no physical curtain for drivers to fight. Solutions such as BlueSeal create a high-velocity air stream that separates cargo air from ambient air when the door opens, activated automatically by a door switch.

Testing on comparable vehicles (Mercedes Vito class, approximately 4.8 m³ cargo) shows air curtains outperform PVC strip curtains on both temperature stability and hygiene, because there’s no physical barrier to become contaminated, damaged, or bypassed.

These systems run on 12V vehicle power and are most effective when designed into new-build specifications rather than retrofitted, though retrofit installation is possible.

Tier 5: New-Build Specification (Comprehensive)

Design moisture management into the loadbox from day one: air curtain integrated with demand-based defrost, proper drainage channels, corrosion-resistant hardware throughout, anti-condensation coatings on interior surfaces, sealed panel joints with vapour barriers, and insulation systems that maintain R-value when exposed to incidental moisture.

This is the subject of our next technical article — the ultimate loadbox specification for multi-stop frozen courier operations. Because the real failure isn’t that operators don’t manage moisture. It’s that bodybuilders don’t design for it.

The Question Nobody Asks

The transport refrigeration industry sells you a loadbox designed for long-haul operations — 2-3 door openings per trip — and a timer defrost calibrated for steady-state conditions. Then watches you use it for 30-stop courier delivery and blames “operator error” when coils ice up, doors rust, and panels degrade.

Nobody asks: “How many times per day will you open the door?”

Because answering honestly means specifying different equipment, different materials, different controls. And that costs more than the standard package.

It means acknowledging that multi-stop frozen delivery is a fundamentally different engineering problem from long-haul frozen transport — not just a shorter version of the same thing.

At The Frozen Food Courier, we ask that question first. We know the answer is 30-60 openings per day. We know that means 0.9-1.3 litres of water entering our loadboxes every route. We know where that water goes, what it does to our equipment, and what it takes to manage it. We don’t pretend that standard equipment handles non-standard duty cycles. We engineer for the physics we actually face — because the Second Law of Thermodynamics doesn’t care what the sales brochure promised.