A measure of thermal resistance indicating how effectively a material or assembly resists heat transfer, expressed as the temperature difference required to drive one unit of heat flow through one unit area. Higher R-values indicate better insulation performance—but R-value specifications for refrigerated bodies often misrepresent actual performance by ignoring thermal bridges, age degradation, and real-world operating conditions.
Understanding R-Value
R-value quantifies insulation effectiveness:
- Higher R-value = Better insulation = Less heat transfer = Lower refrigeration load
- Lower R-value = Poorer insulation = More heat transfer = Higher refrigeration load
The relationship is linear: doubling R-value halves heat transfer (all else equal).
Typical R-Values in Refrigerated Transport
| Material | R-Value per inch | Application |
|---|---|---|
| Polyurethane foam | R-6 to R-7 | Primary loadbox insulation |
| Extruded polystyrene | R-5 | Floor insulation, rigid panels |
| Expanded polystyrene | R-3.8 to R-4.4 | Lower-cost applications |
| Fiberglass | R-3 to R-4 | Rarely used in transport |
| Air gap | R-1 | Minimal insulating value |
Panel vs Assembly R-Value
Manufacturers often specify panel R-value rather than assembly R-value:
Panel R-Value (What’s quoted)
- Pure insulation material performance
- Laboratory conditions, no thermal bridges
- Ignores joints, fasteners, structural members
Assembly R-Value (What actually matters)
- Complete wall/floor/roof performance
- Includes all thermal bridges and penetrations
- Reflects actual heat infiltration rate
Typical assembly R-value is 60-80% of panel R-value due to thermal bridging—a gap manufacturers prefer not to discuss.
South African R-Value Considerations
South African conditions demand higher effective R-values:
Temperature Differential
- European design: 32°C ambient to -20°C cargo = 52K differential
- SA summer: 40°C ambient to -18°C cargo = 58K differential
- 12% higher differential requires proportionally better insulation
Age Degradation
- Polyurethane loses R-value over time (outgassing of blowing agent)
- UV exposure accelerates degradation
- Moisture infiltration reduces effectiveness
- 10-year-old loadbox may have 20-30% reduced R-value
Construction Quality
- Local bodybuilders may use lower-density foam (lower R-value per inch)
- Panel thickness specifications don’t guarantee R-value
- Quality control variation affects actual performance
R-Value Requirements for Transport Refrigeration
Minimum recommendations for frozen food transport:
| Surface | Minimum R-Value | Recommended R-Value |
|---|---|---|
| Walls | R-14 | R-20+ |
| Roof | R-16 | R-22+ |
| Floor | R-12 | R-18+ |
| Doors | R-12 | R-16+ |
Floor R-value is particularly critical due to urban heat island pavement radiation and chassis thermal bridging.
Evaluating Insulation Claims
When assessing loadbox insulation:
- Request assembly R-value, not just panel R-value
- Inquire about thermal bridge mitigation at joints and fasteners
- Check insulation thickness AND density
- Ask about long-term R-value retention
- Consider floor construction particularly carefully
Related Terms: Thermal Bridge, Thermal Load, Cold Storage Facility