R-value and U-value in Building Insulation

Knowing the difference between the R-Value and U-Value helps you make the most effective decision in selecting the best building products suitable to your project, and with the correct thermal insulation for the specific building climate.

R-value vs U-value

R-value tells builders how well a construction material insulates. It refers to a product’s or material’s resistance to heat. The higher the R-value, the better insulation and energy efficiency.

The R-value rating refers to materials and not systems. As a result, R-value is employed in connection with construction systems made of one building material.

While R-value measures material resistance to heat, the U-value indicates the rate of heat transfer. It shows how effective a material is as an insulator. In this case, a lower U-value is preferable as it indicates more energy efficiency.

The U-value is mostly used to rate systems such as window and door units, which may include different materials to ensure various functions (ventilation, filtration, heat transfer).

The Isowall structural composite panel consists of a core of thermally efficient insulation material sandwiched between two outer pre-stressed skins and bonded to them. This gives the panel its impressive strength characteristics. Various facing materials and finishes are available.

What is the connection between the two? At a glance, the R-value only represents resistance to heat transfer, while the U-value addresses the heat transfer that happens via the insulation system through radiation and conduction.

Mathematically speaking, U-value is a low number and inversely proportional to R-value as it measures how much heat energy is lost or gained, hence U = 1/R. Therefore, a high R-value translates into a low U-value. A material with R-value of 10 will have a U-value of 0.1.

The U-value describes thermal performance as opposed to the R-value. However, the relation between these two values is more complex than just a mathematical equation.

Further to the above, the R-value can be calculated by adding individual R-values of various components made of the same material. The total R-value of a wall cavity will then be the sum of individual R-values for the wall sheathing, wall insulation, and the internal dry wall.

By comparison, U-value calculations are not as straight-forward since the systems which require insulation are comprised of different materials, as seen above, sometimes with different R-value heat resistance numbers. Adding individual U-value is not possible.

A simple calculation of U-value, also known as thermal transmittance, uses the rate of transfer of heat through a structure, which can be a single material or a composite, divided by the difference in temperature across that structure. The units of measurement are W/m²K. Thermal transmittance takes heat loss due to conduction, convection and radiation into account.


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