Energy Efficiency and Properties of Building Materials
Energy efficiency in building construction is an extremely important component in the planning of future and the renovation of existing buildings.
It is important for reducing CO2 emissions, energy consumption for heating and cooling, but also for ensuring our comfort in indoor spaces.
n one of our earlier posts, we mentioned that the walls of houses would need to be several meters thick if we didn’t use thermal insulation and still wanted the house or building to be energy-efficient.
Today, we will demonstrate the impact of the first few centimeters of insulation and how the effect changes as the thickness of the insulation increases.
Concrete and XPS as an Example
It is well known that concrete is used in construction (foundations, walls, load-bearing columns, floors, ceilings), and it is known that concrete is strong, fire-resistant, and durable. However, concrete is a very poor thermal insulator and easily transfers energy.
The number that describes how well a material conducts thermal energy is denoted by lambda (λ).
The lower the lambda value, the better the material is as an insulator.
Thermal Conductivity of Concrete (λ): Approximately 1.0–2.6 W/m·K, depending on its composition and density.
XPS
Extruded polystyrene or XPS is a foam insulation material, sometimes colloquially referred to as rigid styrofoam to distinguish it from EPS (expanded polystyrene). Due to its strength, XPS is most commonly used for insulating foundations, floors, and similar applications.
Thermal Conductivity of XPS (λ): approximately 0.035 W/m·K.
Conclusion of the First Part:
We can see from the λ values that XPS has insulation properties that are several dozen times better than concrete.
Just 1 cm of XPS thermal insulation (λ = 0.035 W/m·K) would have the same ability to retain heat as approximately 42 cm of concrete (λ = 1.5 W/m·K).
In this case, XPS is 42 times better as an insulator than concrete!
Therefore, concrete needs to be insulated.
In this post, we will not delve into the specifics of how much insulation is needed for walls, floors, ceilings, etc., to meet today’s energy conservation standards. However, we will demonstrate the impact of the first few centimeters of insulation and how this effect changes as the thickness of the insulation increases.
Ceiling Slab Insulation and the Effect of 1 cm of XPS
Let’s say we have an old house with a 20 cm thick concrete slab under an unheated attic, and we want to insulate it. Our goal is to estimate how many centimeters of insulation we need to reduce heat losses, while not overspending.
The 20 cm thick concrete, with a λ coefficient of 1.5 W/m·K, will have a thermal conductivity coefficient U = 7.5 W/(m²·K).
(*The U-value is the heat transfer coefficient through a construction assembly and includes all layers and materials that make up that assembly. U = d/λ (material thickness/lambda). Lower U-value = better insulation properties.)
With just 1 cm of insulation, the heat transfer coefficient will drop from 7.5 to 2.39 W/m²·K, which is a reduction of 68.18%. The second centimeter will reduce it by 81.08%, and 3 cm of insulation will cumulatively reduce thermal conductivity by 86.54%.
For a complete picture and how insulation behaves up to a thickness of 20 cm, please refer to the attached table:
Savings Curve and Conclusion
Thermal insulation is crucial for energy efficiency, and even small additions can lead to significant savings. By analyzing the thermal properties of materials and their impact on energy efficiency, it can be observed that the savings curve shows the steepest slope when adding the first few centimeters of insulation. This means that savings are most significant at the beginning, gradually decreasing as the thickness of the insulation layer increases.
In applying our analysis to 20 cm of concrete with the addition of XPS, we see that the savings curve is steepest when adding the first and second centimeters of insulation. This is the period when adding insulation is most cost-effective per unit of material.
After approximately 7-8 cm of insulation thickness, the savings curve becomes more gradual, flattening out as it approaches 20 cm. This means that each additional centimeter of insulation provides diminishing returns, reaching a point where the savings per additional centimeter of insulation become negligible.
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