The method described in the post is only applicable to one single layer of coating. For a surface coated with two or more layers of coatings, the method can only measure the top layer, but cannot determine the apparent thermal conductivity of multiple layers.
To determine the apparent thermal conductivity, the calculation method needs to be used. In this post, we use a two-layer coating system as an example:
Measure the thermal conductivity of each layer individually (k1 and k2). Note: the samples need to be prepared individually too, as the method can measure the top layer only.
Calculate the overall thermal resistance: R = d1/k1 + d2/k2, where d1 and d2 are the thickness of each layer.
Calculate the apparent thermal conductivity: k = (d1 + d2)/R
For the calculation of the overall thermal resistance, our online ETTV U-value calculator can be used. The R-value result reported is the overall thermal resistance of all layers in the system.
As an alternative to glasses, several transparent plastic materials are utilized as window glazing panels. Transparent polycarbonate sheets and transparent acrylic sheets are two examples of such transparent plastic materials.
The NFRC/EN/SIO glass test methods are for glazing materials, which are not limited to glasses. Plastics are a type of glazing material. Transparent plastic sheets can be tested by the NFRC/EN/ISO methods when the following conditions are met:
With specular transmission and reflection only: materials with significant diffuse transmission/reflection are out of the scope (for example, frosted glasses, glasses with ceramic frits, and hazy plastic sheets).
Note: in the latest NFRC methods (2020 version), diffuse materials are supported, but our lab is not ready to test such diffuse materials.
Homogenous and flat sheet: corrugated plastic sheets and double-wall (multiple-wall) polycarbonate sheets are out of the scope.
Without far-infrared transmission: plastic sheets with significant far-infrared transmission in the 5 µm to 50 µm range are out of the scope.
Note: in the NFRC methods, it is possible to test materials with far-infrared transmission, but our lab is not ready to test such materials.
In principle, the scope of the NFRC/EN/ISO glass optical & thermal property test methods is based on the optical characteristics, but not on the material type. Transparent plastic sheets with the same optical characteristics as transparent glasses are within the scope.
The requirements on daylight reflectance by BCA are available in this circular dated 27/06/2016. Please refer to Section P.3.2 of the document (pages 5-6) for the requirements.
Below is a flowchart prepared by us for your easy reference:
For a specific scenario, you may click one of the blocks below for more elaborations.
My material is glass
For glasses, the requirement is that the daylight reflectance shall be less than 20%.
Please note the following:
For glasses, the concept of total/diffuse/specular daylight reflectance is not applicable, as glass is a material with specular reflection only (without diffuse reflection).
Daylight reflectance is also called visible light reflectance in some contexts. The “visible light reflectance, front” result in our glass test report is the same as the daylight reflectance result.
My material is not glass and is installed on a facade or horizontal/low-slope roof
For non-glass materials installed on facades or horizontal/low-slope roofs, the requirement is that the specular daylight reflectance shall be less than 10%.
Please note the following:
Specular daylight reflectance is also called specular reflectance in some contexts. They are the same.
Low-slope roof refers to a roof with less than 20 degrees of inclination angle (a horizontal roof is with 0 degrees of inclination angle).
My material is not glass and is installed on a steep-slope roof
For non-glass materials installed on steep-slope roofs, the requirement is that the total daylight reflectance shall be less than 20%.
Please note the following:
Total daylight reflectance is also called daylight reflectance in some contexts. They are the same.
Steep-slope roof refers to a roof with greater than 20 degrees of inclination angle (a horizontal roof is with 0 degrees of inclination angle).
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)
Glass
1 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.
Notes
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.
After testing, a test sample is retained by the lab for a certain period. The following sample retention periods are implemented at OTM:
Sample type
Size range
Retention period
Small samples
Smaller than 10 cm × 10 cm × 3 cm
3 years
Large samples
Larger than 10 cm × 10 cm × 3 cm Smaller than 30 cm × 30 cm × 5 cm
1 year
Oversize samples
Larger than 30 cm × 30 cm × 5 cm
3 months
At the end of the retention period, the sample will be disposed of by the lab. The lab may further retain some samples for internal quality control purposes.
Upon the customer’s request (indicated in the test request form or in writing), a test sample can be returned to the customer.