Thermal conductivity, thermal resistance, and thermal transmittance are the 3 commonly used properties related to material or system thermal insulation performance. The 3 properties and their differences are explained below.
Thermal conductivity (K-value)
Thermal conductivity represents the ability of a material to conduct heat.
Thermal conductivity is also called K-value and its unit is W/(m⋅K). The smaller the thermal conductivity, the better the thermal insulation performance.
Listed in the table below are the typical thermal conductivity ranges of selected building material types.
|Building material type||Typical thermal conductivity range|
|Insulation material (e.g. polyurethane/polystyrene foam, mineral wool)||0.02 – 0.04 W/(m⋅K)|
|Wood, plywood, and gypsum board||0.1 – 0.5 W/(m⋅K)|
|Coating (e.g. paint), plastics, and rubber||0.1 – 0.5 W/(m⋅K)|
|Concrete (light weight or heavy weight), brick, and tile||0.5 – 2.5 W/(m⋅K)|
|Metal (e.g. stainless steel or aluminum)||15 – 200 W/(m⋅K)|
Thermal conductivity is typically for homogeneous materials. For inhomogeneous materials (e.g. concretes or composite panels), their average thermal conductivity is referred to as the apparent thermal conductivity.
Thermal resistance (R-value)
Thermal resistance represents the ability of a material layer to resist heat transmission. Thermal resistance is calculated as:
Thermal resistance is also called R-value and its unit is (m2K)/W. The greater the thermal resistance, the better the thermal insulation performance.
Thermal resistance is always in terms of one or multiple material layers with fixed thicknesses:
- Material layer: thermal resistance is applicable to layer-by-layer structures only.
- Example 1: for a wall system with 3 layers: concrete + rock wool insulation + plaster, there is a thermal resistance for each material layer (or multiple layers combined together).
- Example 2: for studs, frames and fasteners, and other materials without layered structure, there is no thermal resistance for such materials
- Fixed thickness: thermal resistance is dependent on thickness.
- Example: the insulation performance of a thin insulation material could be worse than a thick conductive material (e.g. 1 mm thick polystyrene foam vs. 100 mm thick glass)
- It is therefore not practical to reduce the thermal conductivity of thin coatings for better insulation.
Thermal transmittance (U-value)
Thermal transmittance represents the ability of a wall, roof or fenestration system to transmit heat. Thermal transmittance is calculated as:
Thermal transmittance is also called U-value and its unit is W/(m2K). The smaller the thermal transmittance, the better the thermal insulation performance.
Thermal transmittance is always in terms of a complete wall/roof/fenestration system and air-to-air thermal transmission.
- Complete wall/roof/fenestration system: thermal transmittance is applicable to a complete wall/roof/fenestration system only.
- There is no thermal transmittance of an individual material layer unless the wall/roof/fenestration system is formed by 1 material layer only
- Example 1: for a wall system with 3 layers: concrete + rock wool insulation + plaster, there is a thermal transmittance of the complete system only, but no thermal transmittance of each material layer.
- Example 2: for a double glazing unit (DGU) system with 3 layers: glass + air gap + glass, there is a thermal transmittance of the complete DGU system only, but no thermal transmittance of each layer.
- Air-to-air thermal transmission: thermal transmittance includes thermal transmission through both indoor and outdoor air layers.
- The indoor and outdoor air layers adjacent to a wall/roof/fenestration system introduce some additional thermal resistance too (called surface film resistance). The additional thermal resistances by air layers are always included in thermal transmittance calculations.
- Thermal transmittance is therefore always dependent on the environmental conditions (such as wall/roof/fenestration orientation, indoor/outdoor airflow speed, and indoor/outdoor surface emissivity).
- Example: a wall system and a roof system made of the same materials may be with different thermal transmittances, due to the different airflow speeds along a vertical surface (for a wall) and a horizontal surface (for a roof).
OTM provides two online thermal transmittance (U-value) calculators:
- For wall or roof systems: online ETTV U-value calculator
- For glasses: online glass U-value calculator
Thermal conductivity (K-value) vs. thermal resistance (R-value)
- Thermal conductivity is independent of thickness; thermal resistance is dependent on thickness.
- Thermal conductivity is for homogeneous materials (or averaged for inhomogeneous materials); thermal resistance is for material layers.
Thermal resistance (R-value) vs. thermal transmittance (U-value)
- Thermal resistance can be in terms of an individual material layer in a wall/roof system; thermal transmittance is always in terms of a complete wall/roof/fenestration system and there is no thermal transmittance of an individual material layer (unless the wall/roof system is made of a single material layer only).
- Thermal resistance excludes the effect of indoor and outdoor air layers; thermal transmittance includes the effects of indoor and outdoor air layers (air-to-air thermal transmission).
- Thermal resistance is independent of environmental conditions; thermal transmittance is dependent on environmental conditions.
The discussions above follow typical engineering practices. In the academic context, the practices could be different.
If we use double glazing unit (DGU) glasses as an example, the engineering practice is to use its thermal transmittance (U-value), instead of the other two, for performance evaluations, though in the academic context it is still correct to calculate the apparent thermal conductivity and thermal resistance of a DGU glass.