Thermal conductivity
Thermal conductivity is a physical property that describes how well a material can conduct heat. It’s the rate at which heat energy is conducted through a unit area of a material when there is a temperature difference across that material. In other words, it quantifies how efficiently a material transfers heat from a hot region to a cold region.
Thermal resistivity
Thermal resistivity is the reciprocal of thermal conductivity. While thermal conductivity measures how well a material conducts heat, thermal resistivity measures how much a material resists the flow of heat. It’s a property that indicates the degree to which a material inhibits the transfer of heat across it. The higher the thermal resistivity, the more insulating the material is.
Thermal resistivity is particularly useful when dealing with insulating materials. Insulators are materials that have low thermal conductivity and therefore provide high thermal resistance to the flow of heat. For example, materials like fiberglass, foam, and mineral wool are commonly used as insulation where heat conservation or temperature control is essential.
Thermal resistivity is typically measured in square meters per watt (m²/W). Square meters represent the area through which heat is conducted and watts represent the heat flow rate. The measure represents the resistance of a material to the flow of heat per unit area for a given temperature difference.
R-values
Perhaps a more accessible measure is an R-value. The R-value of an insulating material is calculated based on its thermal resistance and thickness. It’s a straightforward calculation that involves dividing the thickness of the material by its thermal resistivity.
The following table compares typical R-values of common insulation materials based on a standard thickness of 50mm:
| Insulating Material | R-value (m2·K/W) per 50mm thickness |
| Glasswool / Fibreglass | 1.0 |
| Mineral Wool / Rockwool | 1.3 |
| Polystyrene | 1.75 |
Increasing the thickness of an insulating layer increases the thermal resistance. For example, doubling the thickness of insulation will double its R-value, perhaps from 1.5 m2⋅K/W for 50 mm of thickness, up to 3.0 m2⋅K/W for 100 mm of thickness.
It’s important to note that R-values are typically additive when calculating the overall insulation performance of a multi-layered system. For instance, if you have two layers of insulation with R-values of R1 and R2, the total R-value of the combined system is the sum of R1 and R2.
While higher R-values generally indicate better insulation performance, when selecting insulation materials for specific applications, A&G engineers will consider other factors like cost, density, moisture resistance, and fire safety, and solutions will often involve the use of combination insulation to ensure the most cost-effective outcomes for customers.
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