Insulation does not trap cold; it slows the heat that moves from the warm side to the cold side. To see why a container warms up, identify the path the heat takes.
Three paths and where insulation cuts them
| Path | What carries the energy | Where insulation cuts it |
|---|---|---|
| Conduction | Energy passes through the solid wall | Low-conductivity material and the air cells inside it |
| Convection | Moving air or liquid carries the energy | Closed cells and a structure that limits air circulation |
| Radiation | Energy passes between surfaces as electromagnetic waves | Surface facing and layers |
According to the ASHRAE Handbook, heat transfer in dry insulation can be approximated by adding gas conduction, conduction through the solid matrix and radiation inside the pores.
Foam conductivity depends on the gas in its cells
NIST's gas table puts still air at about 26.4 mW/(m·K) at 300 K. New polyurethane foam with more than 90% closed cells shows about 0.022 W/(m·K) at 24 °C, and according to ASHRAE the value rises as air diffuses into the cells. For PU boards, the nominal value declared under EN 13165 is 0.022 W/(m·K) and the design value 0.023 W/(m·K), as declared in the IVPU EPD. So the age of the panel counts too.
Calculation: K, surface area and temperature difference. Total heat gain of a box is Q = K × S × ΔT: K is the overall heat transfer coefficient in W/(m²·K), S the surface area in m², and ΔT the inside-outside temperature difference in K. The ATP test-report formula gives K as (W1 - W2) / (S × ΔT). ATP limits: heavily insulated (IR) equipment at most 0.40 and normally insulated (IN) equipment at most 0.70 W/(m²·K) (ATP Annex 1, Appendix 1, para 1). Turkey's MEB refrigerated-vehicle module also gives 0.40 W/(m²·K) for container insulation. The Isotec TBX has a K value of at most 0.40 W/(m²·K).
Example, hypothetical: outside 30 °C, inside 0 °C, ΔT = 30 K. A 1 m² wall passes Q = 0.40 × 1 × 30 = 12 W. For a 1 m × 1 m × 1 m box with 6 m² of surface, Q = 0.40 × 6 × 30 = 72 W; over six hours (72 × 21,600 s) that is 1.56 MJ. This is an upper estimate: a K below 0.40 lowers the gain in proportion.
1.56 MJ would melt 4.7 kg of ice at 0 °C, since the heat of fusion of ice is 333.5 kJ/kg, per the 1939 NBS measurement (RP1260). A eutectic plate takes in leaking heat the same way, by melting; how much heat it can absorb before it warms depends on its mass.
Lid, gasket and leakage. When a lid opens, warm air enters and carries heat by convection. According to ASHRAE, air leakage and air circulation can increase heat transfer in insulation systems, so lid openings and the time they stay open are logged separately. Gasket gaps and metal fittings can also act as paths that bypass the insulation.
Thickness and age read together. A 40 mm board with λ 0.022 W/(m·K) gives R = d / λ = 0.040 / 0.022 = 1.82 (m²·K)/W. If λ rises to 0.026, close to still air, the same board drops to 1.54 (m²·K)/W. More thickness raises R, but thickness does not close the leakage paths at the lid and fittings.
What to measure
Under the same load, logging product temperature together with the number of lid openings separates wall performance from handling practice. Wall design and heat gain in the field are not the same thing.
