R measures resistance to heat flow; higher is better. For a uniform layer, R = thickness / conductivity.
U measures heat passing through the complete assembly; lower is better. U = 1 / total R and transmission is Q = U × area × temperature difference.
In short: use the manufacturer's complete-panel U-value, not a foam estimate.
R-value — thermal resistance (higher is better)
R-value measures how strongly a layer resists heat flow. It depends on both conductivity and thickness:
R = d ÷ λ
where d = thickness in metres and λ = conductivity in W/m·K, giving R in m²·K/W.
Worked example: 100 mm of PUR at λ = 0.022 W/m·K → R = 0.100 ÷ 0.022 = 4.55 m²·K/W. Double the foam to 200 mm and you double the resistance — thickness buys performance almost linearly.
U-value — thermal transmittance (lower is better)
U-value is the inverse of the assembly's total R-value. It states how much heat (watts) crosses one square metre per degree of temperature difference:
U = 1 ÷ R_total
For a real panel you add the surface air films and the (negligible) skin resistances to the foam:
R_total = R_si + R_steel + R_foam + R_steel + R_se
where R_si (internal surface film) ≈ 0.13 and R_se (external) ≈ 0.04 m²·K/W. The steel skins contribute almost nothing; the foam dominates. A 100 mm PUR panel therefore lands at roughly U ≈ 0.22 W/m²·K — the value used directly in heat-load sums.
This U-value feeds the transmission term of a heat-load calculation (Q = U × A × ΔT). For the full method, see Heat load calculation.
Choosing thickness for temperature
Frostbox standard range: The Algor/Frostbox catalogue is built around 80 mm and 100 mm panels for chill/cold rooms and 100 / 120 mm for freezer rooms. Broader engineering tables sometimes list 60, 150 or 200 mm — those are generic industry context, not Frostbox product options (60 and 150 mm were dropped). As a rule of thumb, step up one thickness for roughly every −10 °C of target temperature, and one more step for high ambient or strong solar gain.
A thicker panel raises R-value and lowers U-value almost linearly, which directly cuts the transmission heat load and lets you specify a smaller refrigeration unit.
Related articles
- How insulation keeps a cold room cold
- Thermal bridges in cold rooms
- Heat load calculation
- Panel specifications
- Using the Frostbox size calculator
R and U figures above are typical PUR engineering values for design and comparison; project-critical specifications for the MTH Algor panel system are available on request — contact [email protected].
