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)
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:
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.
SλΔλ: normalized relative spectral distribution of the UV radiation (part of the standard global solar radiation)
The wavelength range of interest is 300 nm – 380 nm. The term SλΔλ is the weights used in the weighted average of the spectral transmittance. The standard values of SλΔλ are plotted in the figure below.
The peak of the UV radiation distribution is at 375 nm. Glasses with high spectral transmittance near 375 nm are with high UV transmittance.
There are some small differences between the ISO 9050 and EN 410 UV distributions. The UV transmittances calculated according to the two standards could be slightly different.
We are now able to measure water sample UV transmittance (UVT) at 254 nm wavelength. Quartz cells with a 10 mm path length are used. The UV transmittance of tap water in Singapore was measured 97% (very clean).
All typical commercial solar panels can be tested, including panels greater than 1.2 m x 2.4 m in size. The results reported are the same as our standard daylight reflectance testing service (sample report). Due to the extra effort in handling such large and heavy samples, some large-size sample surcharge is applicable.
Step 1. Set the user database as the optical data source
In the “File” -> “Preference” -> “Optical Data” tab, browse the Optics user database file and set it as the optical data source, as shown below.
Step 2. Import glass optical data from Optics user database
In the glass library (“Libraries” -> “Glass”), click “Import” (in the “List” view). In the pop-up window, set the format as “IGDB or Optics User Database”, as shown below. The optical data entries in the Optics user database can then be imported in the next pop-up window.