INSULATION BASICS

INSULATION BASICS

CONDUCTION

The transfer of energy between objects that are in physical contact.


Heat Conduction is the flow of internal energy from a region of higher temperature to one of lower temperature by the interaction of the adjacent particles (atoms, molecules, etc.) in the intervening space.


Factors affecting the rate of heat transfer in a material by conduction are: 

 a) temperature difference

 b) length

 c) cross-sectional area

 d) type of material

The rate of heat flow between two regions is proportional to the temperature difference between them and the heat conductivity of the material.

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CONVECTION

The transfer of energy between an object and it's environment, due to fluid motion.


Convection is the transfer of internal energy into or out of an object by the physical movement of a surrounding gas or fluid that transfers the internal energy along with it's mass. Although the heat is initially transferred between the object and the gas or fluid by conduction, the bulk transfer of energy comes from the motion of the gas or fluid.

RADIATION

The transfer of energy to or from a body by means of the emission or absorption of electromagnetic radiation.


Thermal radiation is energy emitted by matter as electromagnetic waves, due to the pool of thermal energy in all matter with a temperature above absolute zero. Thermal radiation propagates without the presence of matter through the vacuum of space. 


Radiant energy from the sun is an example. where the energy may be absorbed as heat by a building wall or a motor vehicle. The understanding of the concepts of Heat transfer of materials is then used in the design and construction of Controlled Temperature structures. 

FUNDAMENTAL HEAT TRANSFER CONCEPTS FOR INSULATION MATERIALS

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy and heat between physical systems. Heat transfer is classified into various mechanisms, such as physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convention, thermal radiation, and transfer of energy by phase changes. We will address the first three.

THERMAL CONDUCTIVITY

A standard measure of the rate at which heat is transmitted through any specific material independent of the material thickness. Measured in Watts(W) per metre(m) Kelvin(K). Units expressed as W/mK. Thermal Conductivity values are listed in the table below.

Material Density (kg/m3) Thermal Conductivity (W/mK)
Air (at 0°C) 1.2 0.024
Aluminium 2800 204.00
Brick 1600 0.600
Concrete 1600 0.800
Cork 200 0.065
EPS sheet (S grade) 16 0.038
Glass 2500 0.800
Plasterboard (gypsum) 1000 0.230
Rock Wool 35 - 200 0.040
Steel 7800 52.00
Water (at 20°C) 0.600
Wood (hardwood) 800 0.170
Wood (softwood) 550 0.143

The lower the k value, the better the thermal performance

THERMAL TRANSMITTANCE

A measure of how much heat is lost through any specific material (building element), which includes factors such as condition, convection, and radiation. 

 

The U value of a material is calculated from the reciprocal of the combined thermal resistances of the materials in the element, air spaces and surfaces, also taken into account is the effect of thermal bridges (wall ties), air gaps and fixings. 

 

U = t / λ where  -t is the thickness in metre of the material   -λ is the thermal conductivty in W/mK

 

Measured in Watts (W) per metre squared (m2) Kelvin (K). Units expressed as m2K/W

THERMAL RESISTANCE

A measure of how much heat loss is reduced through a given thickness of any specific material. The thermal resistance of a material is calculated as follows:

 

R = t / λ where -t is the thickness in metre of the material -λ is the thermal conductivty in W/mK

 

Measured in metres squared (m2) Kelvin (K) per Watt (W). Units expressed as m2K/W

 

Using materials listed in Table 5 we can calculate the Thermal Resistance for a 50mm sample. These are listed and ranked in the table below.

Material Ranking Thermal Resistance (m2K/W)
Brick 5 0.08
Concrete 6 0.06
EPS sheet (S grade) 1 1.32
Fibreglass 2 1.00
Glass 6 0.06
Plasterboard (gypsum) 4 0.22
Steel 8 0.0009
Wood (softwood) 3 0.35

The higher the R value, the better the thermal performance

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