The Short Circuit in Every Loadbox

Think of your refrigerated loadbox as an electrical circuit.

The insulation panels are high-value resistors — 75mm of polyurethane foam with a thermal conductivity of 0.024 W/m·K. Heat has to fight through that foam molecule by molecule. It’s a slow, difficult path. That’s why you paid for it.

Now look at what connects those panels together. The door threshold. The corner posts. The hinges. The lock rods. The floor-to-wall joints. Every one of these is a metal component that pierces or bypasses your expensive insulation.

And every one is a short circuit.

The physics problem: Mild steel conducts heat at 50 W/m·K. Your polyurethane foam conducts at 0.024 W/m·K. That’s a 2,083:1 ratio. A mild steel door threshold bypassing your insulation is like wiring a 1Ω resistor in parallel with a 2,083Ω resistor. The current — the heat — takes the easy path. Every time.

Your 75mm PUR panel is a 2,083Ω resistor. Your mild steel door frame is a 1Ω bypass wire. Guess where the heat goes.

This is what thermal bridges are: conductive pathways through the building envelope that bypass insulation entirely. They’re not a minor inefficiency. In poorly designed loadbox construction, thermal bridges contribute more heat infiltration than the insulated panels themselves. You paid for R-2.88 insulation and then drilled thermal highways through it.

Where Thermal Bridges Hide: Every Connection Point Is a Heat Leak

Walk around your refrigerated vehicle. At every location where two surfaces meet, where hardware penetrates the envelope, where structural members connect panels — there’s a thermal bridge. Most operators never think about these. Most bodybuilders don’t either.

We’ve mapped every thermal bridge location in a typical South African loadbox and ranked them by severity:

Location	Typical SA Material	Heat Transfer Rate	Severity
Door threshold / sill	Galvanised mild steel	5.3 W per threshold	CRITICAL
Door frame verticals	Aluminium extrusion	12-18 W per frame	CRITICAL
Corner posts (4×)	Aluminium extrusion	8-14 W per post	HIGH
Door hinges (4×)	Zinc-plated steel	1.5-3 W per hinge	HIGH
Lock rods & latches	Zinc-plated steel	2-4 W per assembly	HIGH
Floor-to-wall joints	Steel angle / channel	3-6 W per linear metre	HIGH
Floor surface	Aluminium sheet	Distributed — significant	MEDIUM
Evaporator mounting	Direct to roof structure	4-8 W	MEDIUM
Tie-down anchors	Steel through-bolts	0.5-1.5 W each	MEDIUM
Drain holes	Steel grommet	0.3-0.8 W each	LOW
Cable penetrations	Unsealed	Variable	LOW

The cumulative problem: Add it all up. A typical South African loadbox with standard materials accumulates 80-150W of continuous thermal bridge load — before you even account for the insulated panels, door openings, solar radiation, or urban heat island effects. That’s 80-150W of cooling capacity your TRU is burning just to compensate for material choices at connection points. Over 2,500 operating hours per year, at R18/L diesel and 10 kWh/L energy density: R360-R675/year in fuel consumed purely by thermal bridges.

Small numbers per component. Large number when they’re all working against you simultaneously. And they never stop — 24 hours a day, every day the system operates.

The Numbers That Matter: Material Conductivity Comparison

Here’s the table that should be on every bodybuilder’s wall. It isn’t.

Material	Thermal Conductivity (W/m·K)	Relative to Stainless 304
Copper	385-400	24× worse
Aluminium	205-235	13× worse
Mild / Carbon Steel	45-60	3× worse
Stainless Steel 304	15-16	Baseline
Stainless Steel 316	14-16	Similar
Stainless Steel 430 (ferritic)	23-26	1.5× worse
Titanium	15-16	Similar
GRP / Fibreglass	0.3-0.5	30-50× better
Polyurethane foam	0.024-0.028	~600× better

Read that table carefully. Stainless steel 304 has the lowest thermal conductivity of any structural metal commonly available in South Africa. Lower than mild steel by 3×. Lower than aluminium by 13×. It’s not exotic. It’s not expensive at the quantities used in loadbox hardware. It’s available from any stainless supplier in Gauteng or Cape Town.

And almost nobody specifies it at thermal bridge locations.

Why mild steel became “standard”: Mild steel door thresholds aren’t standard because they perform better. They’re standard because they cost less per kilogram and because bodybuilders have always used them. The material choice was made decades ago based on purchase price and never revisited. Nobody ran the thermal calculations. Nobody compared lifecycle cost including fuel consumption. Nobody asked the physics.

The Door Threshold Deep Dive: The Most Critical Thermal Bridge

The door threshold is the single most critical thermal bridge in any loadbox. It spans the full width of the door opening. It connects the interior floor to the exterior chassis. It runs continuously, without interruption, through the insulation envelope. And on most South African builds, it’s made from galvanised mild steel.

Let’s calculate what that choice actually costs.

Door Threshold Heat Transfer Comparison

Given:
- Threshold dimensions: 1,800mm × 50mm × 3mm (typical)
- Cross-sectional area: 50mm × 3mm = 150mm² = 0.00015 m²
- Thermal path length: ~75mm (through insulation thickness)
- ΔT: 53K (-18°C interior to 35°C exterior)

Formula: Q = k × A × ΔT / L

Mild steel (k = 50 W/m·K):
Q = 50 × 0.00015 × 53 / 0.075 = 5.30 W per threshold

Stainless 304 (k = 16 W/m·K):
Q = 16 × 0.00015 × 53 / 0.075 = 1.70 W per threshold

Savings: 3.60W per threshold
Two-door vehicle: 7.20W continuous reduction
Annual fuel saving: ~R150-200 per vehicle

R150-200 per year from one component change doesn’t sound like much. But that’s just the door threshold. Now multiply the same material logic across every thermal bridge location — door frames, hinges, latches, lock rods, floor joints — and the numbers compound.

More importantly, the threshold calculation understates the real impact. It models only conduction through the cross-section. In reality, the threshold also conducts heat along its full 1,800mm length, distributing thermal energy across the door opening. It acts as a condensation surface, collecting moisture that accelerates corrosion in galvanised steel (but not stainless). And it creates a localised warm zone that accelerates product temperature rise at floor level — exactly where your frozen goods sit.

The corrosion alignment: Here’s what makes stainless steel at the door threshold a doubly intelligent specification: the location experiencing the worst thermal bridging is also the location experiencing the worst corrosion. Door thresholds endure constant moisture cycling from condensation, wash-down water, product spills, and humidity infiltration during door openings. Galvanised mild steel corrodes. Stainless 304 doesn’t. The material that conducts 3× less heat is also the material that lasts the life of the vehicle without corrosion. Thermal performance and durability align perfectly at this location.

The Yellow Strip Nobody Noticed: Thermal Break Materials

Look at the door frame on a Foton Miler LOXA loadbox. Running the full height of each vertical, there’s a yellow composite strip separating the stainless steel door frame from the insulated panel edge.

That strip is a thermal break.

A thermal break is a non-conductive material inserted to interrupt a thermal bridge. Where metal must connect to metal for structural reasons, inserting a strip of nylon, GFRP (glass fibre reinforced polymer), HDPE, or engineered composite between the two metal surfaces breaks the conductive pathway. Heat can no longer short-circuit straight through.

Think of it as cutting the bypass wire in our electrical analogy. The circuit isn’t eliminated — you still need the structural connection — but you’ve inserted a high-value resistor into what was previously a dead short.

What this tells us: Someone at the LOXA factory engineered this. Someone calculated the thermal bridge at the door frame, identified it as a problem, and specified a composite thermal break as the solution. This isn’t accidental. This isn’t decorative. This is deliberate thermal bridge management — the kind of engineering analysis that most South African bodybuilders have never performed.

Where Thermal Breaks Work

Thermal breaks are most effective at locations where structural metal must span the insulation envelope: door frame-to-panel junctions, where the frame connects interior and exterior surfaces; floor-to-wall transitions, where metal angle or channel connects floor structure to wall panels; corner post connections, where vertical structural members connect adjacent panels; and evaporator mounting points, where brackets connect the TRU to the roof or wall structure.

Where They Can’t Help

Thermal breaks can’t address hardware that must physically penetrate the envelope — hinges, lock rods, through-bolts, and tie-down anchors. For these locations, the only solution is material selection: choosing the lowest-conductivity structural metal available. Which brings us back to stainless steel 304.

SA Availability

Thermal break materials are readily available in South Africa. Nylon strip, HDPE sheet, GFRP angle — all are manufactured or stocked domestically. The barrier isn’t availability. It’s specification. Nobody asks for it because nobody calculates why it matters.

Stainless Steel in Loadbox Construction: Where It Makes Sense

Stainless 304 isn’t the answer everywhere. It’s more expensive per kilogram than mild steel, harder to weld, and overkill for purely structural members buried within insulated panels. But at thermal bridge locations — where metal connects the cold interior to the warm exterior — it’s the obvious engineering choice.

Where Stainless Makes Sense

Where Stainless Doesn’t Make Sense

• Primary structural frame: The chassis sub-frame and main structural members are fully enclosed within insulation. Thermal bridges at these locations are managed by the insulation itself. The cost premium of stainless for non-bridge structural members isn’t justified.
• Floor surface: This is worth discussing. The Foton Miler uses aluminium floor sheeting — 13× more conductive than stainless. For floor surfaces, the answer isn’t necessarily stainless (weight and cost become significant over 8m²). Better solutions include composite flooring, insulated sub-floor systems, or at minimum, thermal break mounting between the aluminium floor and the chassis. We covered this in detail in our floor thermal load analysis.

Galvanic Corrosion: The Mixed-Metal Warning

When stainless steel contacts aluminium or mild steel in the presence of moisture — which is guaranteed in a refrigerated environment with constant condensation — galvanic corrosion occurs. The less noble metal (aluminium or mild steel) corrodes preferentially.

The solution is isolation: nylon washers, rubber gaskets, or plastic spacers between dissimilar metals. This is standard practice in marine and architectural construction. In loadbox construction, it’s rarely specified — yet the moisture environment demands it. Another detail the Foton Miler addresses with its thermal break strips, which also serve as galvanic isolation between dissimilar metal components.

What Chinese Manufacturing Understood That South African Builders Didn’t

We recently inspected a new Foton Miler with LOXA loadbox for our fleet assessment. The material audit was revealing:

Location	Material Used	Thermal Bridge Management
Door threshold	Stainless 304	3× reduction vs mild steel
Door frame verticals	Stainless + composite thermal break	Deliberate thermal discontinuity
Door hinges	Stainless 304	3× reduction vs zinc-plated steel
Lock rods & latches	Stainless 304	3× reduction
Panel joining channels	Stainless vertical channels	13× better than aluminium alternative
Door gasket	Black rubber seal	Standard — adequate
Floor surface	Aluminium sheet	No thermal bridge management
Kick plates	Aluminium checkerplate	No thermal bridge management

Stainless at the threshold, hinges, latches, lock rods, and panel joints. Composite thermal break at the door frame. Rubber gasket seals. The only weakness: aluminium flooring, which remains the industry default regardless of origin.

This isn’t a premium upgrade package. This is the standard LOXA specification. The factory calculated thermal bridge severity, chose appropriate materials, and built it in at the production line. It costs marginally more than mild steel and zinc-plated hardware. It performs dramatically better.

The question for SA bodybuilders: If a Chinese factory building loadboxes at volume can justify stainless hardware and composite thermal breaks at standard specification, why are South African custom builders — charging significantly more per unit — still specifying galvanised mild steel and aluminium at every thermal bridge location?

Specification Guidance: What to Request from Your Bodybuilder

If you’re specifying a new refrigerated loadbox build, these are the thermal bridge management requirements that separate engineering from guesswork.

Minimum Specification

• Door thresholds: Stainless steel 304 (not galvanised mild steel). Cost premium: R200-500 per threshold. Thermal benefit: 3× reduction in conduction. Corrosion benefit: lifetime versus 2-3 year galvanised lifespan.
• Door hinges: Stainless steel 304. Standard stainless hinges cost marginally more than zinc-plated equivalents. No reason not to specify.
• Lock rods and latches: Stainless steel 304. Again, marginal cost premium for 3× thermal improvement plus zero corrosion.
• Door gaskets: Continuous rubber seal with compression fit. Verify gasket condition at every service interval.

Enhanced Specification

• Thermal breaks at door frame-to-panel junctions: Composite or GFRP strip inserted between door frame and panel edge. This is the detail that separates adequate from engineered.
• Floor-to-wall thermal breaks: Non-conductive spacer between metal floor structure and wall panel connections.
• Stainless panel joining channels: Replace aluminium extrusions with stainless 304 at all panel joints.

Questions That Expose Thermal Bridge Awareness

Ask your bodybuilder these questions during specification. The answers tell you whether they’ve considered thermal bridge management or whether they’re building from habit:

“What material do you use for the door threshold?” — If the answer is “galvanised steel,” ask why they chose a material that conducts heat 3× faster than the alternative.

“Do you install thermal breaks at the door frame-to-panel junction?” — If they ask what a thermal break is, you have your answer.

“What grade of stainless do you use for door hardware?” — If they don’t use stainless at all, ask why they specified the highest-conductivity option at every thermal bridge location.

“Have you calculated the thermal bridge load for this build?” — If they haven’t, they’re sizing the TRU based on insulation performance alone and ignoring the thermal highways through it.

The Full Inventory: How Thermal Bridges Compare to Panel Heat Loss

Let’s put it all in context. For a typical 12m³ loadbox operating at -18°C in 35°C ambient conditions:

Total Heat Load Breakdown

INSULATED PANELS (6 surfaces):
Total panel area: ~24 m²
Average U-value: 0.35 W/m²·K (75mm PUR)
Panel heat load: 24 × 0.35 × 53 = 445W

THERMAL BRIDGES (standard SA mild steel/aluminium):
Door thresholds (2×):           10.6W
Door frames:                    30.0W
Corner posts (4×):              48.0W
Door hinges (8×):               18.0W
Lock rods/latches:               8.0W
Floor-wall joints:              24.0W
Evaporator mount:                6.0W
Tie-downs/penetrations:          5.0W
                               ------
Total thermal bridge load:     ~150W

Thermal bridges as percentage of envelope load:
150W / (445W + 150W) = 25% of total envelope heat transfer

WITH STAINLESS 304 + THERMAL BREAKS:
Estimated bridge load: ~55W
Percentage: 55W / (445W + 55W) = 11% of total envelope heat transfer

RESULT:
Material selection at connection points reduces thermal bridge load by 63%
Total envelope heat load reduced from 595W to 500W — a 16% improvement
Without changing a single insulation panel.

A 16% reduction in envelope heat load. No change to panel thickness. No change to insulation material. No structural modifications. Just choosing better materials at the points where heat bypasses your insulation.

That 16% translates to reduced TRU runtime, lower fuel consumption, faster temperature recovery after door openings, and less stress on refrigeration equipment. Over a 10-year vehicle life at 2,500 operating hours per year, the cumulative fuel saving is R3,000-R6,000. The stainless and thermal break materials cost R2,000-R4,000 more than standard specification. The payback period is typically 3-5 years, with the remaining 5-7 years as pure saving.

And that’s before accounting for reduced corrosion maintenance, improved temperature stability reducing product loss risk, and longer equipment life from reduced thermal cycling.

The Challenge

Ask your bodybuilder what material they use at the door threshold.

If they say “galvanised steel,” ask why they chose a material that conducts heat 3× faster than the alternative. Ask if they’ve calculated the thermal bridge load. Ask if they install thermal breaks at door frame junctions. Ask if they know what a ψ-value is.

If a Chinese factory building LOXA loadboxes at volume can specify stainless hardware and composite thermal breaks as standard, what’s the excuse for a custom South African build that costs more and performs worse?

Your insulation is only as good as the weakest point in your thermal envelope. And right now, on most South African builds, those weak points are the connection hardware that nobody specified, nobody calculated, and nobody questioned.

Until now.

Related Technical Articles

• Six Surfaces of Failure — How every panel of your refrigerated vehicle is losing the temperature war
• Radiating Upward — The thermal load nobody calculated and why your floor is the weakest link
• Composite Materials — Carbon fibre is building billion-dollar yachts but your freezer body is still made of aluminium
• Seven Cost Levers — Where to spend your rands on a refrigerated vehicle build
• Insulation Materials Guide — Why your refrigerated vehicle can’t maintain -15°C
• The Pre-Cooling Myth — Contains steel thermal bridge calculation (1,350W example)

Technical Reference

• Technical Formulas Reference — Thermal bridge heat transfer and material conductivity calculations