The number written most often on panel quotations is the thermal conductivity: 0.022 W/mK. On its own it is not enough for a comparison, because it is the value of the core alone. What decides how much heat escapes from a building is the thermal transmittance of the section.
Two separate numbers
Thermal conductivity (λ) is the value of the material itself, in W/mK, and is independent of thickness.
| Core | Thermal conductivity |
|---|---|
| PUR, PIR | 0.022 W/mK |
| Rock wool, hybrid | 0.045 W/mK |
| EPS | 0.038 W/mK |
Thermal transmittance (U) is the value of the section, in W/m²K, and falls with thickness. To EN ISO 6946 it is found from U = 1 / (Rsi + ΣR + Rse), where Rsi and Rse are the surface resistances.
The comparison is made on U. A 40 mm panel at 0.022 W/mK does not insulate better than a 100 mm panel at 0.045 W/mK; without the calculation this is not known. The result for the section is read from the U-value calculator.
Who sets the target value?
In Türkiye the thermal insulation rules for buildings are defined in TS 825. The region and the type of use set the target U-value, and the envelope component is chosen to meet it.
In cold stores the target comes not from a regulation but from the operating temperature: as the difference between inside and outside grows, the section thickens.
What thickness returns is an annual figure
The cost of going up one step in thickness is paid once and returned over the life of the building. That return is not estimated but calculated: the heat loss and savings calculator gives the annual difference between two sections in kWh, and shows its money value and the payback period with your own unit price.
The assumptions of the calculation are written on the page itself: it is the conduction loss of a single envelope component, not a building energy calculation. Internal gains, solar gain, ventilation and thermal bridges are outside it, so the result is an upper bound.
The section can be good and the envelope poor
Insulation is usually lost not in the middle of the panel but at its edge.
- Joints: a joint left open takes away what the section gained
- Thermal bridges: purlins, columns and door frames are the weakest points of the envelope
- Continuity of the vapour barrier: in cooled spaces, condensation begins where that continuity breaks
- Moisture: a rock wool core that gets wet loses its insulation value and does not recover it
The details are drawn in the Library section.
The sustainability side
In the environmental balance of a heated building, the largest item is not production but operating energy. The thickness decision is therefore also an environmental decision: fuel not burned every year accumulates for longer than material produced once.
The steel faces of the panel enter the recycling chain at the end of their life; a screwed envelope, unlike a cast one, can be taken apart and separated.
In short
Insulation performance is discussed with the U-value of the section, not with the name of the core. A comparison made without writing the number down is a guess based on thickness, and it usually turns out wrong.