The Science of Insulation Explained
How heat flows
To understand how insulation works, it is
first necessary to explain the different ways heat flows through a
construction. Physically, heat always moves from areas of high
temperature to those at a lower temperature, which is why, in the
cold external temperatures of winter, the warmth inside a building
will try to escape through walls, windows, roof and floor.
This heat flow takes place by conduction,
convection and radiation.
Conduction
Conduction is the transmission of heat through
a material, or from one material to another, through direct
contact. Conduction can take place in solids, liquids and
gases.
In relation to construction materials, metals
are the best conductors of heat, followed by concrete and masonry.
In contrast, wood and insulating materials are poor conductors, as
are air and other gases.
Convection
Convection occurs in gases and liquids. If a
hot surface is in contact with cooler air, heat is conducted to the
air. This air then becomes warmer and therefore less dense than the
adjacent cooler air. The warmer, lighter air rises upwards and is
replaced by cooler air, causing a continuous flow of air by natural
convection – gradually removing heat from the hot surface to the
air. The process is reversed if warm air comes into contact with a
cold surface.
In constructions, the convective heat transfer
we are interested in occurs mainly in wall and roof cavities.
Radiation
Radiation is the transmission of infra-red
radiant energy from a ‘hot’ surface to a ‘cold’ surface through air
or a vacuum. Radiant energy moves through space without heating
anything in between – the energy is only absorbed when its path is
blocked by an object which absorbs the energy and converts it to
heat. All materials emit radiant energy to a greater or lesser
extent according to their surface characteristics and the
temperature of the surface. The higher the temperature of a surface
the greater the amount of radiant energy emitted.
The most common example of this is the radiant
heat from the sun, which travels millions of miles through space,
and only has any effect when it is blocked by an object, e.g.
people, buildings or the earth itself.
How to reduce heat flow by the use of insulating materials in
buildings
In order to perform effectively, an insulation
material must reduce heat flow.
How conduction is reduced
To reduce heat transfer by conduction, an
insulating material should have a very small amount of solid
material in relation to void. Additionally, the solid material
should consist of thin connecting walls, or discontinuous
fibres.
How convection is reduced
To reduce heat transfer by convection, an
insulating material should contain small voids or air pockets
within which air movement is minimised. Similarly, within a
construction, convection can be reduced by having small
self-contained air spaces, rather than large ventilated air
spaces.
How radiation is reduced
The
transmission of heat by radiation is stopped when it is absorbed
into the surface of a material, this results in a rise in
temperature of the material. However that material will in turn
emit radiant energy. The most effective surface is a “low
emissivity” surface that emits very little radiant energy and
absorbs a very small percentage of the radiant energy falling on
it. A “low emissivity” surface is characterised by a shiny metallic
finish. In a building the transmission of heat by radiation from
one internal surface to another is not regarded as heat loss
however the transmission of heat from external elements of the
building away from the building is.
Conversely the unit used to describe the
thermal insulation characteristic of a material actually is a
measure of how much heat the material allows to flow, this unit is
thermal conductivity (units W/mK) this is also known as lambda
value (λ).
Thermal conductivity by material type
The graph below shows the classical curve type associated
with the thermal conductivity performance of traditional bulk
insulation materials.
This particular graph shows the curve for
glasswool products, as can be seen the thermal conductivity of the
product is improved as the density of the product increases,
however the rate of change diminishes as density increases and
ultimately, at higher densities, the thermal conductivity starts to
increase.
The basic trend of this graph holds true for
all bulk insulation materials, and its shape is a function of the
varying efficiency of the material at restricting the three
different methods of heat flow at different densities.
Thermal conductivity also varies with
temperature. As temperature rises then the thermal conductivity of
materials generally increases. This is not a phenomenon that is of
concern in buildings because the variance only becomes significant
at temperatures which would not be experienced in normal
conditions. It is a consideration when insulating building services
and high temperature processes.
Measurement of thermal conductivity
All insulation products have an inherent
variability when it comes to thermal conductivity. This is
basically dependent on the method by which the insulation is made
and actually ‘works’. Put simply, the lambda value for building
insulation products must be such that 90% of the results obtained
are within 90% of the quoted value – hence ‘Lambda 90/90’. The aim
is to ensure that the values quoted for insulation performance are
consistent and give both users and building designers confidence in
the products and solutions that are being specified.
Lambda 90/90 effectively means that all
thermal insulation products manufactured in accordance with
harmonised European Standards have their lambda value tested and
declared to the same methodology, establishing a level playing
field for all materials.
Thermal conductivity (K value or λ value)
The measure of a material’s ability to
transmit heat. Units: W/mK. Also called lambda (λ) value.
Thermal Resistance (R value)
The measure of a material’s ability to resist
the transfer of heat, it is specific to a particular thickness of
material. Units: m2K/W.
R value = thickness (m)/Thermal
Conductivity (W/mK)
Thermal Resistance is the most important
material characteristic that should to be defined when specifying
insulation. From the formula for calculating, it is apparent that
there are two factors affecting the thermal resistance: the
thickness of the insulation and the thermal conductivity of the
material. Simply specifying thickness of material is not enough
Thermal Transmittance (U-value)
Commonly known as the U-value, it is a measure
of the rate of conductive heat loss of a building element or
component. Units: W/m2K.
The actual thermal transmittance of a building
element is a function of the thermal resistance of the materials
that are used in the construction and the way they are
assembled.
U-values of building elements can be
established by laboratory testing, but the process is costly, time
consuming and size limited. Furthermore, the result would only hold
true for an identical construction or element. Testing is widely
used to establish the thermal transmittance of glazing and doors,
but for other construction elements it is more normal to use
numerical and mathematical models to predict the U-value.
In its simplest form, a U-value is calculated
by establishing the thermal resistance of each layer in the
construction element and adding them together to provide a total
resistance (TR) value. The U-value is calculated from the
reciprocal of the combined resistances of the materials in the
element, including any airspaces and surface resistance values.
TR = Rsi + Ra + Rb + Rc +
Rso
Rsi is the internal surface
resistance
Rso is the outer surface
resistance
Therefore U-value = 1/TR
For instance in external wall with a total
thermal resistance of
3.50m2K/W would have a U-value of
1/3.50 or 0.29W/m2K.
This method of calculating U-values, however,
does not allow for nonuniformities that exist in real constructions
and therefore will not enable a realistic model to be calculated.
The non-uniformities require factors to include allowance for the
effect of repeating thermal bridges, (e.g. timber studs in timber
frame construction, mortar joints in lightweight and aircrete
masonry or metal rails and clips in twin metal skin constructions),
fasteners that penetrate the construction and the possibility of
the imperfection of fit of layers that might allow air movement
around insulation layers. These factors are included in more
sophisticated numerical and mathematical models. These methods are
defined by international standards such as BS EN ISO 6946 ‘Building
components and building elements – Thermal resistance and thermal
transmittance – Calculation method’ and guidance is given regarding
the suitability of each method for the proposed construction.
Additionally, reference should also be made to BR443:2006
Conventions for U-value calculations 2006 edition which sets
conventions for and gives guidance on the calculation of
U-values.
Generally, the Combined Method is suitable for
most elements of construction except where there are metal
repeating thermal bridges in the insulation layer.
If the example above is taken to be a timber
frame panel, then it becomes apparent that the insulation is
bridged by the timber studs. In these circumstances, the combined
method is appropriate.
When the combined U value method is applied to
the calculation of the U-value of this construction it becomes
0.32.
In this calculation, the proportion of
insulation replaced by timber is 15%. This proportion is identified
as the default timber fraction in BR443, and a level 0 correction
for air gaps in the insulation layer has been applied because
mineral wool is deemed to be cut with a positive tolerance so that
it has to be compressed between the timber studs to be fitted and
cross joints are compressed together. If a rigid foam board has
been used, it might be considered necessary to apply a level 1 air
gap correction because the board has to be cut with a negative
tolerance to enable fitting and there may well be air gaps greater
than 5mm in width.
Clearly the accurate calculation of U-values
requires detailed knowledge of product characteristics, calculation
methodologies and standards, and construction techniques. The
accurate calculation of U-values is a fundamental building block in
the development of whole building energy models and Building
Regulations submissions.