Stone wool efficiency
In this article we will try to help you choose the right insulation by explaining what factors the effectiveness of the insulation depends on, how they are calculated and what their values mean. The insulation material that we have chosen in this case is stone wool. Each manufacturer presents a technical card for the specific type of stone wool, but what do the indicators and their values actually mean and which ones are essential for determining the insulation effectiveness.
Calculating the R-value of rock wool
The R-value is determined by dividing the thickness of the rock wool by the thermal conductivity coefficient λ declared by the manufacturer. The R-value is a measure of how effectively the material resists heat flow.
Example 1:
Let’s calculate the R-value of stone wool with thickness = 100mm (0.1m) with declared thermal conductivity by the manufacturer λ = 0.035 (at 10°C).
The calculated R-value is only for the rock wool, if we want to more accurately calculate this value for an entire wall system, we must sum the R-values of all layers of materials in it. The total sum of the R-values of all layers of materials participating in the wall system is denoted by RSI.
The higher the R-value, the better the insulation. As a rule of thumb, the lower the temperature difference between the surface of the material and the interior wall, the higher its R-value. A higher R-value means a more energy-efficient home. So when comparing the R-value of rock wool insulation to other materials, it’s important to remember that a higher R-value means a more efficient and comfortable space.
U-value calculation
U-value is a general thermal transmittance coefficient that describes how well an insulation material conducts heat or the rate of heat transfer (in watts) through one square meter of insulation system, where the temperature difference between the inner and outer surfaces is 1 degree Kelvin. The system is usually a collection of many layers of components such as those that make up walls / floors / roofs etc. The lower the U-value, the less heat loss and therefore the better the insulation system. It is calculated using the following formula:
Where: RSI – The total sum of R of all layers of materials involved in the wall system.
The thickness and the manufacturer’s declared thermal conductivity coefficient were taken into account in calculating the R-value. Let’s look at them too.
Thermal conductivity coefficient λ of stone wool
Typical thermal conductivity values for rock wool insulation are between 0.0031 and 0.0045 W/mK. These values are declared by the manufacturer based on tests in certified laboratories and the lower this value, the better the insulator the material is. In other words, this coefficient shows how easily heat passes through the material. Dense materials are better conductors of heat, since heat transfer occurs from an area of higher to an area of lower temperature through the kinetic energy of the atoms in the material.
If we go back to the R-value again, we will see that thickness affects the thermal conductivity R. Changing the thickness will also change the R-values.
It is very important to note here that stone wool does not change its insulation properties over time and accordingly the thermal conductivity coefficient λ and the R-value do not change either, unlike other insulation materials that age and change their insulation efficiency.
Example 2:
In example 1, we calculated the R value for stone wool with a thickness of = 100mm (0.1m), let’s see what will happen if we lay stone wool with a thickness of = 120mm (0.12m), with the same declared thermal conductivity by the manufacturer λ = 0.035 (at 10°C).
From example 2 it becomes clear that the thickness of the stone wool plays a significant role in its insulation qualities.
To illustrate the importance of the thickness of the stone wool, we will complicate the task a little and at the same time debunk another myth that stone wool with a higher density does not necessarily have better insulation properties. Let’s compare 100 kg. of stone wool, which has a declared thermal conductivity by the manufacturer λ = 0.0370 (at 10° C) and is 100 mm (0.1 m) thick, and stone wool 70 kg., with a declared thermal conductivity by the manufacturer λ = 0.035 (at 10° C) with the same thickness of 100 mm (0.1 m).
Stone wool 100 kg.
Stone wool 70kg.
From the calculations made, it is clear that at equal thicknesses, the lighter wool has better characteristics than the heavier stone wool, but this can of course be applicable to applications where compressive strength is not required, or more specifically for contact facades and flat roofs.
Given a given R or RSI in architectural designs, you can calculate the thickness of the rock wool.
Example 3:
For example, with a set value of R – 4.05, the calculations will look like this:
i.e. the required thickness is 150mm.
These are general rules, but each case is individual and other factors must be taken into account. There are cases in which the architecture does not allow for a large thickness of the wadding and in order to achieve the specified R – value, different methods are used. It is no coincidence that the specialists of ZDesign Bulgaria approach each case individually to find the most optimal solution.
Density and compressive strength of stone wool
The density of a material is the weight of a given substance in one cubic meter of material. Stone wool with a higher density is characterized by greater moisture resistance and the ability to withstand high loads. A high degree of strength can be an advantage in some types of construction, while for others it is a bad solution. Therefore, for example, for different types of roofs there are several classes of stone wool based on compressive strength, tailored to its use. Compressive strength is defined in kPa as 1kPa = 1000 Pa. Pa (Pascal) is a unit of pressure that creates a force of one newton applied to a surface area of one square meter.
1 Pa = 1 N/m2
Stone wool has values from 10 to 70kPa.
Most often, stone wool with a high compressive strength value is placed on flat roofs, on which additional equipment will be installed.
The requirements for compressive strength values are set out in the structural design of each building and are subject to calculations by a structural designer.
The range of different densities, thicknesses and applications of stone wool is large and it is impossible to cover them all, so we have tried to describe the basic principles for determining the appropriate material and its effectiveness.
The ZDesign team is available for professional consultations and will be happy to answer any questions you may have. You can contact us by phone:+359 888 807 510 , as well as by e-mail: office@zdesign.bg
