Thermal resistance. Material vs Heating

When we start building or reforming, a multitude of materials with insulating capacities appear before us. Their mission will be to protect our home against the weather, preventing the entry of moisture and cold currents from the outside and maintaining a stable temperature.

These materials show different characteristics and behaviour. Generally, the choice between one and the other will depend on different variables including the location and orientation of the building, its climatic conditions, price, etc.

But how did these materials come to be considered as insulators? To do this, we need to know the thermal resistance, its parameters and the methods used to define when a material is a good insulator. You’ve guessed it, today we’re talking about thermal resistance.

The thermal resistance determines the insulation

In short, we can say that thermal resistance is the ability of a material to oppose the flow of heat.

  • If the materials are homogeneous, the thermal resistance will represent the relationship between the thickness and the thermal conductivity of the material.
  • Between non-homogeneous materials, it will suppose the inverse of the thermal conductance, that is, the heat transfer that will be given between both.

The thermal resistance of a material layer is measured in W/(K.m), which takes into account thickness and thermal conductivity. It differs from the total thermal surface, in which both sides of the material to be evaluated are taken into account.

The thermal resistance (r) is calculated using the following parameters:

  • Thermal conductivity -λ
  • Material thickness – e
  • The relationship between conductivity – λ and thickness is used to determine -R, the thermal resistance. In this way: R = e/λ

Thus, the higher the R-value, the greater the insulating capacity of the material because of its resistance to the flow of temperature through it.

Heat transfer coefficient (U)

This coefficient is used to determine the insulation level of a wall. It is directly related to the -R value we have just seen and indicates the amount of heat that passes, per unit of time, through the surface unit of the material to be analysed.

As with the calculation of the thermal conductivity, an analysis in a stationary state is required, since the values can vary, for example, in the presence of moisture.

Some of the best U materials used in construction are

Material Thermal Conductivity (W/(m·K))
Polyurethane 0,018-0,025
Expanded polystyrene 0,025-0,045
Fiberglass 0,03-0,071​
Cork 0,04-0,301​
Wood 0,131​
Fire brick 0,47-1,051​
Glass 0,6 – 11​
Brick 0,801​
Stainless steel 16,32​
Aluminium 209,31​
Alpaca 29,11​
Steel 47 – 581​

Spanish UNE-EN 12667:2002

The test methods for determining both the resistance and the thermal conductivity of materials are set out, in Spain, by the UNE-EN 12667:2002 standard. This standard establishes the minimum results for considering a material as an insulator.

In order to determine the thermal conductivity and resistance of a material, a number of requirements must be met with regard to temperature, heat flow and the stability of the measurement signal.

These values must remain stable for the study to be carried out correctly. Let us now look at the methods approved by the standard:

Hot plate test

The hot plate test consists of a plate that supplies heat to the samples to be tested, which are placed on both sides of the plate. The heat then flows through the samples, and the measurement of the values seen above begins. This is an absolute method.

Heat flow test

Heat flow is a relative method based on a previous calibration made with a material whose conductivity has been certified. It is currently the most widely used method because of its speed.

Different variants are available for the measurement, such as single-sample, two-sample or asymmetric with a sample.

The test consists of two plates, one fixed and one mobile in contact with the material to be analysed. By means of a temperature gradient, a heat flow will be produced that will measure the conductivity of the material.

The great insulators in construction

As we saw in our post on thermal insulation, there are a large number of materials on the insulation market, from natural and biodegradable products such as cork or wool, to compounds such as expanded polyurethane.

And if you want to go deeper into the world of energy efficiency, you can visit the following posts:

What is the energy efficiency?

Passivhaus, in search of maximum energy efficiency