On thermal insulation
The regulation of heat transfer.
The scope of thermal insulation.
Control heat transfer, and prevent heat loss or increase inside a building to reduce energy consumption and maintain adequate living comfort.
How does heat transfer occur.
From hot to cold, always.
The heat flow represents the movement of heat and has only one direction, which sees the propagation from hot areas to cold areas.
This movement is what causes a drop in temperature inside buildings in winter, and a rise in temperatures in summer periods.
Heat transmission occurs by one or more of the following mechanisms:
propagation by conduction;
propagation by convection;
propagation by radiation.
Insulating materials are designed to reduce heat transfer.
Propagation by conduction.
Through elements in solid, liquid or gaseous state.
This propagation modality sees the transmission of heat through one or more elements in direct contact (remember that this always happens from the hot element to the cold element).
The amount of heat propagated in a body, in a given time interval, is directly proportional to the thermal conductivity of the material itself and to the temperature differential between the two elements in question.
The lower the conductivity of the material, the greater the heat propagation time, the more the material will be a good insulator.
The lower the λ value of a material, the greater its ability to resist the propagation of heat by conduction.
Propagation by convection.
Through bodies in liquid and gaseous state.
This modality of propagation (non-existent for bodies in a solid state and absent in a vacuum) occurs inside gases or liquids, when the molecules inside them overheat, generating a variation in terms of density (i.e. when the hot air becomes less dense and tends to go upwards, the "natural convection" occurs, but the same result is obtained with wind or artificial means, incurring in "forced convection").
The amount of heat transmitted by a body (liquid or gaseous) depends on the heat difference between the two bodies, the incident speed and the area of the exposed surface. A wall exposed to a cold and strong wind will cool down faster than a wall exposed to a cold but moderate wind.
A material with a closed cell structure inhibits thermal convection within the cell itself. When the cellular structure is very fine and the cells are very small, thermal convection and the propagation of heat to nearby cells is further reduced.
Propagation by radiation.
Also in the absence of solid, liquid or gaseous bodies.
This modality of propagation sees the transmission of heat, in the form of energy, between two bodies through space. It does not require a direct contact between the bodies that exchange heat and does not need a channel to propagate.
It is a phenomenon that does not occur at every temperature, but only at fairly high temperatures, or at very close distances, and only in these cases will its contribution exceed that of the other transmission modes.
Notions to evaluate the performance of an insulating material.
The effectiveness of an insulating material is extrapolated from its ability to limit heat flow. That is from its thermal conductivity or its thermal resistance.
Thermal conductivity, or λ value [W/(mK)]
It serves to identify the ability of a material to transmit heat. Each material has a specific thermal conductivity. It is expressed with the symbol λ (lambda) and represents an intrinsic property of the material. Since λ represents the amount of heat that can pass through the material, the smaller it is, the greater the insulating capacity of the material.
To clarify, we give a practical example. Using 15 centimeters of SUPERCEL BUILDING, which has a λ=0.019 [W/(mK)], we are able to obtain a thermal insulation equal to that obtained with 16 meters of cement, which has an λ=2.00 [W/(mK)], and which is therefore not considered an insulating material.
This example takes into account the concept of thermal resistance explained below.
Thermal Resistance R [m2K/W]
Represents the ability of a material to restrain the flow of heat passing through it. This is calculated through the relationship between the material thickness (in metres) and its thermal conductivity λ.
The greater its value, the greater the insulating capacity of the material. Note that in the case of multi-layer constructions, thermal resistance is calculated to be the sum of the resistances of the individual layers.
Thermal Transmittance U [W/m2K]
Describes the thermal characteristic of an element or set of elements in terms of its capacity to disperse heat. Thus thermal transmittance can be used to define the heat-insulating performance of the entire building envelope.
It is given by the inverse of the thermal resistance R. The smaller the value, the slower is the heat dispersion and the greater is the effectiveness of the insulating material.
The key values of insulating capacity.
Thermal conductivity λ is a quantity that characterizes only homogeneous materials. It is used in fact in the technical sheets to indicate the performances of the single insulating materials.
The thermal resistance R, on the other hand, indicates the ability to oppose the thermal flow by one (i.e. single insulating material) or more materials (i.e. the wall/stratigraphy).
Thermal transmittance U is a quantity that can be applied to heterogeneous materials, as it is calculated from the various Rs of the different layers in analysis.This allows an evaluation of the insulating capacity of a set of elements (e.g. an entire wall).
The design phase in energy-efficiency enhancing projects.
With the values declared by the manufacturer within the Declaration of Performance it is possible for the professional in charge of drafting the project to respond to the specific needs of the case. The ultimate goal is to identify the most suitable materials and thicknesses necessary to be within the required values, thus defining the stratigraphy as a whole.
As example, given the declared thermal conductivity (λD) it is possible to adjust it based on the conditions of use and the climatic characteristics of the climatic zone of interest so as to obtain the thermal conductivity of the project (λP).
“If the external maximum temperature is 35°C, and it is perceived as such around 2.00 pm, how much will this temperature be attenuated passing through the stratigraphy?
And after how much time will I perceive this new reduced heat within the building envelope?"
The design phase and summer air conditioning, a separate note.
Inertia and heat capacity, to oppose temperature variations guaranteeing a thermal offset (lag) and attenuation.
In order to determine and limit the energy requirements for summer air conditioning, it is good to analyze the ability of a wall in obstructing the thermal heat wave, when exposed to sun and high temperatures. The parameters that allow you to perform this analysis are:
The thermal offset (lag) of the thermal wave (s), a sort of time delay, measured in hours. It represents the result of lag in time between the moment in which the external surface and the internal surface of the building reach the respective maximum temperature. That is, the time taken by the thermal wave to pass through the building envelope.
The attenuation of the thermal wave (σ) represents how much the amplitude of the thermal wave is reduced in its passage through a given stratigraphy and can be interpreted as the ratio between the amplitude of the external and internal thermal wave.
Periodic transmittance (Yie) is the ability of a stratigraphy to attenuate and phase out the thermal flow coming from the outside over the 24h period. It represents the fusion of the two previous concepts, and is the result of the product between the thermal transmittance (U) and the attenuation factor (fa).
The attenuation factor (fa) is representative of the phenomenon of thermal inertia, or the ability of a material or structure to vary its temperature more or less slowly as a response to changes in external temperature. It is clear that the more (fa) is small, the greater the attenuation, the better the isolation.