Thermal conductivity of refractory glass fibres
J Therm Anal Calorim
Thermal conductivity of refractory glass fibres
Farid Modarresifar 0 1
Paul A. Bingham 0 1
Gary A. Jubb 0 1
0 Morgan Advanced Materials , Thermal Ceramics, Tebay Road, Bromborough, Wirral CH62 3PH , UK
1 Materials and Engineering Research Institute, Sheffield Hallam University , City Campus, Howard Street, Sheffield S1 1WB , UK
In the present study, the current international standards and corresponding apparatus for measuring the thermal conductivity of refractory glass fibre products have been reviewed. Refractory glass fibres are normally produced in the form of low-density needled mats. A major issue with thermal conductivity measurements of these materials is lack of reproducibility in the test results due to transformation of the test material during the test. Also needled mats are inherently inhomogeneous, and this poses additional problems. To be able to compare the various methods of thermal conductivity measurement, a refractory reference material was designed which is capable of withstanding maximum test temperatures (1673 K) with minimum transformation. The thermal conductivity of this reference material was then measured using various methods according to the different standards surveyed. In order to compare different materials, samples have been acquired from major refractory glass fibre manufacturers and the results have been compared against the newly introduced reference material. Materials manufactured by melt spinning, melt blowing and sol-gel have been studied, and results compared with literature values.
Glass fibre; Thermal conductivity calorimeter method; High-temperature insulation; Measurement; Method; Panel; Standard reference material
One of the most important properties of a thermal
insulation material is its thermal conductivity, and this is often
the sole selection criterion for furnace constructions.
Depending on the application, the insulation material could
function for up to 15 years in an industrial furnace.
Therefore, only a 5–10 % advantage in insulating
properties (lower thermal conductivity) could contribute greatly
to energy savings over the service lifetime [1, 2]. Thermal
conductivity values are used regularly in furnace design
and to calculate the required thickness of insulation in a
furnace to achieve a reasonable and safe temperature on the
outer shell [3, 4]. Therefore, accurate thermal conductivity
measurement of these materials is crucial.
The insulating materials under investigation in this study
are designed for long-term applications at temperatures
ranging from 1173 to 1773 K and produced in various
different forms such as monolithic, insulating fire bricks
and glass fibres . These products have a wide range of
applications in many industries as furnace insulation, fire
protection or functional products . Fibres are normally
the preferred technology in applications for which a high
resistance to thermal shock is required. Glass fibres are
usually manufactured and supplied in different forms such
as bulk, needled blanket or blocks, encapsulated fibres and
vacuum-formed panels. Fibre insulation can have a number
of advantages over other forms of insulation including low
cost, flexibility, ease of installation, low thermal mass and
very high resistance to thermal shock which allows rapid
firing rates to save energy . However, fibres are not
suitable for application in environments with very high gas
velocities or in contact with certain molten metals or
glasses. A known disadvantage of some types of glass
fibres is the health hazards associated with them [8–10].
Conventional fibre insulation materials are commonly
known as refractory ceramic fibres (RCF), and these fibres
are classified as category 1A or 1B (definite and possible
human carcinogen, respectively) [11, 12]. In recent years,
major efforts from academic and industrial researchers
have led to the development of new generations of
refractory glass fibres which are biosoluble and minimise
the health risks which were formerly associated with these
types of materials. An example of these new fibre
insulation materials is the Superwool product range developed by
Morgan advanced materials [13, 14]. These products are
normally alkaline-earth silicate (AES) glass fibre products
which have low biopersistence proven through
intra-tracheal (IT) animal testing [15, 16].
Glass fibre insulation products are normally manufactured
using noncontinuous methods such as melt spinning, melt
blowing or extrusion/spun sol–gel, with melt spinning
being the most common method due to its very high
throughput and low cost [17–19]. The spinning method
incorporates an open electric furnace which forces flow of
electricity through the molten glass using high electric
current and low voltage. A continuous melt stream is
generated by flow of the melt through a nozzle at the
bottom of the furnace; the melt stream then hits rapidly
rotating spinners which results in fibre formation, and these
are blown off the edge of the spinners onto a continuous
belt for further downstream processing. The generated
fibres are then formed into various shapes and products
such as nonwoven blankets, paper or vacuum-formed
Due to the industrial nature of this method, the produced
materials are not homogeneous. Glass fibre materials which
are produced by melt spinning methods always contain
some un-fiberised material which normally occurs in the
form of spherical glass particles of various sizes. These are
commonly known as ‘shot’ particles. This inhomogeneity
impacts on thermal conductivity of the fibrous product and
therefore on measured thermal conductivities, limiting the
comparability of different measurement techniques.
Thermal properties, heat transfer mechanisms
Glass fibre insulation is manufactured with various bulk
densities, usually in range 60–350 kg m-3 [1, 4, 20]. Bulk
densities can significantly affect measured thermal
conductivities [21, 22].
The total thermal conductivity of a material can be
explained as in (1), from :
ktotal ¼ kradiation þ kconvection þ kconduction
A few studies suggest that convection effects in porous
materials with bulk densities of [20 kg m-3 or with small
pores are usually negligible . However, whereas this
may be accurate for natural convection, low-density fibrous
insulation materials are usually gas permeable; therefore,
heat transfer due to forced convention  is not
improbable. Miller  developed one of the earliest models to
explain the heat transfer mechanism in fibrous insulation.
In this model, apparent thermal conductivity was predicted
as a function of fibre diameter, bulk density and mean
Numerous studies in recent years [22, 27–31] have led
to improved understanding of heat transfer mechanisms
through fibrous insulation materials. Several models have
been developed [32–36] to predict the thermal conductivity
through different mechanisms. In these models, radiation is
believed to have a minimal effect at temperatures below
773 K, but it becomes the dominant heat transfer
mechanism at high temperatures ([1073 K) . Conduction
through the solid structure of the material, which can be
considered constant if the material has no transformation,
must also be considered [36, 37]. Gas conduction may also
occur through the free gas in open porosity which is
dependent on pressure, or through isolated gases in the
structure of the fibres .
The present study aims to identify the most reproducible
and accurate method of thermal conductivity
measurements for fibre-type insulation materials and to investigate
the source of variability in the measurements.
Materials and methods
There are several well-established methods for measuring
thermal conductivity of insulation materials [39, 40], but
only a few of these methods are capable of conducting
measurements at temperatures greater than 1273 K [21, 41, 42].
Generally, test methods are based on either transient or
steady-state measurements. Transient methods generally
have a simple set-up and can be performed rapidly; however,
these methods are known to have difficulty measuring
nonisotropic and low-density samples . Conventional laser
flash methods also have issues with scattering effects in
lowdensity fibre materials which could lead into underestimation
of the thermal conductivity . On the other hand,
steadystate methods require much more effort in experimental
setup and are consequently expensive and time-consuming to
carry out. However, these methods are well known for their
unidirectional heat flow which allows for high-accuracy
measurements, even for nonisotropic materials, and panel
calorimeter methods offer the highest precision in
measurement amongst these methods . The guarded hot plate
method  has the capability to measure thermal
conductivities at temperatures below 273 K. It is normally limited to
upper measurement temperatures of 773 K, but a few
instruments equipped with alumina heaters can conduct
measurements at temperatures of up to 1273 K . In this
study, thermal conductivity measurements have been taken
using laser flash and hot wire transient methods and a panel
calorimeter steady-state method.
Hot wire cross-array method
In this method, a furnace is used to heat the specimen to a
specified temperature. An additional local heating source
(hot wire) which is embedded in the specimen is used to
introduce a local temperature rise at the centre of the
specimen, and the increase in temperature as a function of
time is recorded. The thermal conductivity (k) value can be
calculated using the known electrical power input to the hot
wire per unit length (Pi), elapsed times (t1, t2) and
temperature change at t1 and t1 (Dh1; Dh2) as shown in (2).
Laser flash method
The hot wire standard method used for measurements in
this study was ISO 8894 .
In this method, one face of a thin specimen is subjected to a
pulse from a high-intensity laser. This short-duration radiant
energy is absorbed by the specimen and results in a
temperature rise on the other face of the specimen, the
temperature curve as a function of time is recorded and used to
calculate the thermal diffusivity (a). The thermal
conductivity (k) can then be calculated using the specimen’s specific
heat capacity (Cp) and density (q) as shown in (3).
The measurement should be repeated at each temperature
of interest in order to produce a thermal conductivity
profile as a function of temperature. The standard method
used for the laser flash measurement in this study was
ASTM E1461 .
Panel calorimeter method
Panel calorimeter instruments normally consist of a furnace
that uniformly heats one face of the sample, and a
calorimeter and guards assembly on the other side of the
sample. Temperature sensors are placed on either side of
the test specimen (in some instruments, additional sensors
are also placed through the thickness of the specimen).
During the test, one side of the sample is heated to a set
temperature and the control guards on the other side of the
sample ensure that heat flow is unidirectional from the hot
face to the cold face. This is achieved by monitoring the
water temperature rise in each zone and varying the water
flow rate through individual guards. Measurements are
taken once the system has reached thermal equilibrium for
a set continuous period of time.
The calorimeter measures the differential water
temperature. These measurements are used in a simple Eq. (4)
[48, 49] to calculate the heat flow rate (Q_ ) from the
measured temperature difference (DTw), specific heat capacity
of water (C) and mass (m) from water flow rate per unit
time (v_) and density (q). Then, the thermal conductivity (k)
between each thermocouple pair with known separation (x)
and known mean temperature (DTtc) can be calculated for
the sample placed above the central calorimeter area (A).
BS 1902-5.5  and ASTM C201  standard methods
were used in this study for the panel calorimeter thermal
conductivity measurements. The main differences between
these standards are specimen size and number of specimen
temperature sensors; these are considered further in the
Many international standards are associated with
hightemperature thermal conductivity testing. These include hot
wire (parallel and cross-array) [46, 51, 52] and laser flash
 among transient methods and guarded hot plate 
and panel calorimeter [48, 50] among the steady-state
methods. These standard methods call for specimens in a
variety of different shapes and sizes, and high-temperature
capability differs greatly between methods and instruments.
A selection of these specifications is presented in Table 1.
Transient hot wire
Table 1 International standards for thermal conductivity testing
BS EN 993-15  (parallel)
Transient laser flash
Steady-state guarded hot plate
ASTM C177 
Steady-state panel calorimeter
BS 1902-5.5 
ASTM C201 
Standard reference materials
To the best of the authors’ knowledge, there are currently
no standard reference materials or samples for fibre-type
insulation that are available for calibration, benchmarking
purposes or comparative studies on high-temperature
thermal conductivity. Consequently, there have been few
comparative studies published on the thermal conductivity
of high-temperature insulation materials.
An account of early comparative thermal conductivity
studies on fibrous thermal insulation is given in .
Transient methods have been reported to measure
consistently *15–20 % higher thermal conductivity values
compared to steady-state panel methods. The source of
these differences has been attributed to the anisotropy of
the material [54, 55].
Thermal conductivities of calcium silicate refractory
boards were measured by plate, cylinder and hot wire
methods in different directions, and the results were
reported to be within ±10 % of one another for the
different methods . Thermal conductivity of alumina fibre
mats measured through the thermal diffusivity method has
been reported to be significantly lower than panel
calorimeter method . However, in almost all of the
previous studies, difficulties have been reported in terms of
reproducibility of results. This has largely been due to
anisotropy of the samples, for example, variations in
density and structure of samples obtained even from the same
batch of product from the same manufacturer. In addition
to sample-to-sample variability, there is also the issue of
transformations within the sample during measurement at
high temperature, which will result in changes in the nature
and structure of the material. Due to the nature and
production methods of these types of materials, the structure
and properties exhibit small variabilities, not only from
sample to sample but even within different parts of the
same sample. Additionally, these materials have different
thermal properties in three dimensions due to orientation of
the fibres during manufacture. Therefore, development of a
standard sample is crucial to conducting thermal
conductivity studies of such materials.
Development of standard reference material
The standard reference material considered in this study
was selected to satisfy certain requirements including
refractoriness to 1673 K and no more than 4 % permanent
linear change in dimensions after heat treatment [57–59].
The standard reference material had to be representative of
fibre-type insulation, so similar density and total porosity
as well as similar structure were essential to ensure that the
thermal properties and heat transfer mechanisms involved
at various different temperatures closely matched those of
typical fibre insulation materials. More importantly, the
material was required to have a good after-service
mechanical strength in order to allow for easy handling.
This was a particular challenge as strength of fibrous
insulation materials normally decreases significantly after
they have been exposed to high temperatures. Moreover,
the material was required to exhibit minimal
transformations during short-term exposure to high temperatures, to
ensure that the key physical and chemical properties of the
materials would remain unchanged during thermal
conductivity measurement at different temperatures and also
from one test to another. Finally, a material with minimal
health hazard was preferable. The issue with anisotropy of
the material can be negligible if the sample is tested in the
same direction in all measurements.
The first candidate material which was investigated for
development of the standard reference material was
97 mass% Al2O3–3 mass% SiO2 fibres manufactured using
the sol–gel method . Alumina-rich fibres are known for
their excellent refractoriness, high strength and high
temperature stability . To improve the mechanical strength,
vacuum-formed panels had been made from these fibres.
This material demonstrated very good high-temperature
thermal performance as well as stability in terms of
transformations. However, the mechanical strength,
modulus of rupture (MOR), of the sample after heat treatment
was not desirable (MOR \ 250 kPa). Several trials were
conducted to improve the mechanical strength by
increasing the amount of inorganic binders in the vacuum-formed
panel. The strength of the board was increased using
various treatments, and the maximum measured MOR was
400 kPa for the fired sample. Although this exceeded the
industry standards , it was considered unlikely to meet
the strength requirements for the standard material, and
hence, it was not investigated further.
The next candidate material investigated was a new
generation of biosoluble high-temperature insulation fibre
manufactured by Morgan Advanced Materials using the
melt spinning process. It has been exonerated based on
European regulations (REACH) . The fibres were made
of an alkali aluminosilicate glass. Long-term stability of
this material has been confirmed during the development
process. To achieve maximum strength, the as-made glass
fibres were formed into a rigid vacuum-formed panel using
colloidal silica as an inorganic binder. The material was
heat-treated for 24 h at 1473 K to ensure that the inorganic
binder was fully activated, crystallisation was completed
and change in dimensions was minimised. Mechanical
testing was carried out on the heat-treated sample, and the
material demonstrated an average MOR exceeding 3 MPa
tested by standard method . A small sample of the final
material was gold-coated by a sputter coater and mounted
on conductive carbon sticker for electron microscopy.
Figure 1 presents backscattered scanning electron
microscopy (SEM) images showing the microstructure of the
final sample. On the basis of the above test results which
demonstrated the stability of this material, it was decided to
select this material as the standard reference material.
Final materials were produced and heat-treated as
described above and subsequently ground and machined to
achieve flat surfaces and the required dimensions (as
described in Table 1) for testing. As mentioned previously,
different test methods require completely different
specimen dimensions. In order to cover at least the most
common test methods, two sets of samples (named A and B)
Fig. 1 Backscattered SEM images showing microstructure of the
final standard reference material sample
Fig. 2 Schematic of the introduced standard reference material,
consisting of three layers of 25 mm thick modular specimen
were produced in the largest specimen dimensions (ASTM
C201 ) and one was cut into several smaller pieces as
shown in Fig. 2 so that it would be compatible with a
number of different instruments and methods.
Testing was carried out using two custom-built panel
calorimeter thermal conductivity instruments based on the
ASTM C201 standard for castable refractories  (no
standard method currently exists for fibre insulation
materials). These instruments are capable of carrying out
thermal conductivity measurements at temperatures of up
to 1673 K. The 76 9 76 mm centre calorimeter and guards
are constructed of copper with a very thin layer of black
paint on the surface for optimised radiation absorption. A
thick SiC plate is placed between the heating source and
sample to ensure uniform heating. A total of four
thermocouples are placed on either side and through the
thickness of the sample which will produce three adjacent
and three nonadjacent thermocouple pairs per measurement
that can all be used to calculate the thermal conductivity as
described in panel calorimeter method.
Intra-laboratory single method reproducibility
Both sets of developed samples were tested using both of
these instruments separately. Additionally, the samples were
tested on the same instrument three times to ensure
reproducibility of the results. Measurement results are presented
in Fig. 3. Thermal conductivity measurements of the same
set of samples measured with the same instrument were
reproducible to within ±0.6 %; test results of the same set of
samples measured on different instruments were
reproducible to within ±5.3 %; test results of different sets of
samples measured on the same instrument were reproducible
to within ±5.4 %; and test results of different sets of samples
measured on the two different instruments were reproducible
to within ±10 % as summarised in Table 2.
Sample B - Internal Rig #1 (ASTM C201) Run #1
Sample B - Internal Rig #1 (ASTM C201) Run #2
Sample B - Internal Rig #2 (ASTM C201) Run #1
Sample B - Internal Rig #2 (ASTM C201) Run #2
Sample A - Internal Rig #1 (ASTM C201) Run #1
Sample A - Internal Rig #1 (ASTM C201) Run #2
Sample A - Internal Rig #2 (ASTM C201)
Fig. 3 Initial testing and reproducibility of measurements. The
thermal conductivity in all tests was calculated for linear mean
temperature of only adjacent sample thermocouple pairs
Inter-laboratory comparison of different methods
In the next stage of this work, standard samples were sent
to a selection of established analytical laboratories for
thermal conductivity testing using different methods.
Results from both transient and steady-state methods were
obtained and are presented in Table 2 and Fig. 4.
No results are presented here for thermal conductivity
measurements obtained using the laser flash diffusivity
method. Two attempts were made to measure the
introduced standard reference sample using an ANTER
LineTM 3000 Thermal Diffusivity System at an external
laboratory. The instrument required very small sample
dimensions (10 9 10 9 3 mm) compared to other
methods. The faces of the sample were gold-coated using a
sputter coater and were subsequently spray-coated with
carbon in order to achieve a uniform emissivity on the
sample face which will be the laser’s target. The instrument
failed to measure the thermal diffusivity after repeated
attempts. This was probably due to the very high level of
porosity (89 %) in this material causing scattering of the
Comparison of different insulation materials
Thermal conductivities of several different fibre insulation
materials were measured using the internal panel
calorimeter rigs, and results are presented in Fig. 5. Total
porosity of these products varied between 87 and 97 %.
Maximum use (also commonly known as classification
temperature) range of these materials is 1373–1873 K .
Thermal conductivity in the classification temperature
range has been measured in between 0.02 and
0.70 W m-1 K-1. The effect of bulk density on thermal
conductivity of fibre mats and blankets is evident and in
good agreement with the literature [21, 42]. For example,
the thermal conductivity at 1380 K for a fibre blanket with
bulk density of 81 kg m-3 is approximately 29 % greater
than that of a blanket made of same type of fibre but with a
bulk density of 139 kg m-3. Fibre products normally have
low thermal conductivities at temperatures below 773 K,
and in most cases, the thermal conductivity is a function of
temperature; therefore, testing at high temperatures is
necessary to differentiate between the high-temperature
performances of these materials.
In agreement with several previous studies, the most
reproducible and accurate measurements were obtained using the
adapted ASTM C201 steady-state panel calorimeter method.
The variability in the intra-laboratory and inter-laboratory
Table 2 Intra-laboratory and inter-laboratory comparison of results reproducibility
Standard reference material
Thermal conductivity measurement method(s)
Single specimen (sample A)
Single specimen (sample A)
2 specimens (samples A and B)
2 specimens (samples A and B)
Single specimen (sample A)
2 Specimens (samples A and B)
Panel calorimeter (ASTM C201) single instrument
Panel calorimeter (ASTM C201) multiple instruments
Panel calorimeter (ASTM C201) single instrument
Panel calorimeter (ASTM C201) multiple instruments
Panel calorimeter (ASTM C201) versus hot wire (ISO
Panel calorimeter (ASTM C201 vs. BS 1902-5.5)
Polycrystalline Al2O3 Sol–Gel Fibre Felt (68 kg m–3)
Fig. 4 Inter-laboratory comparison of thermal conductivity
measurement methods. Thermal conductivity in the methods using more than
two sample thermocouples (TCs) has been calculated for linear mean
temperature of only adjacent sample thermocouple pairs
Fig. 5 Thermal conductivities of various different thermal insulation
materials as a function of temperature, measured by an adapted
ASTM C201 panel calorimeter method calculated for linear mean
temperature of only adjacent sample thermocouple pairs
results generated in this study using the ASTM C201 method
was in good agreement with previous round-robin testing
experiments which were published in the standard method
; however, this variability has been achieved using a
more representative anisotropic fibrous material rather than a
dense castable isotropic refractory material with a different
thermal conductivity profile, as was used in the previous
round robin in .
There are several instrumental errors that could explain
the observed variability in results. As explained in the
panel calorimeter experimental method, the thermal
conductivity could be calculated for a mean temperature from
any adjacent or nonadjacent thermocouple pair. One
concern here is that the temperature profile through the
thickness of the sample is usually nonlinear for this type of
material. According to all standard methods, it is common
practice that the mean temperature between a thermocouple
pair is calculated on a linear basis. However, in reality this
could deviate from the actual mean temperature. The
temperature profile through the thickness of the introduced
standard reference sample in this study, measured by four
thermocouples during the panel calorimeter test, is
presented in Fig. 6. In Fig. 6, the nonlinear regressions for the
temperature readings from four sample thermocouples
have been calculated using a combination of thermal
modelling software  and a curve-fitting program .
The best fit to the data points was achieved using an
inverted hyperbola function (5):
y ¼ A þ B x þ C ð5Þ
Furnace temperature: 673 K
Furnace temperature: 873 K
Furnace temperature: 1073 K
Furnace temperature: 1273 K
Furnace temperature: 1373 K
Furnace temperature: 1473 K
Fig. 6 Temperature profile through the sample thickness during the
panel calorimeter test. Solid lines represent the linear regression, and
broken lines represent nonlinear regression (inverted hyperbola)
between the data points
Fig. 7 Variation in thermal conductivity values calculated for linear
or integral mean temperature between adjacent thermocouple pairs.
Broken line is guide for the eye only
The calculated integral mean temperatures deviated by up
to 8.4 % from linear mean temperatures in the regions of
sample thickness which were farthest from the heating
source, especially measurements at higher temperatures.
However, this variation was considerably lower
(0.3–0.7 %) at lower temperatures and closer to the heating
source. Subsequently, the fitted function was used to
recalculate the thermal conductivity values based on
integral mean temperature between each adjacent
thermocouple pair, as shown in Fig. 7. Nonlinear regression 
was used to compare the data. Variations in calculated
thermal conductivity values were in range 2.5–4 % at
The above issue with the nonlinear temperature profile
could explain the variabilities in the results obtained from
the BS 1902:5.5 panel method; this method uses only two
temperature sensors on each side of a thick (75 mm)
sample and uses linear mean temperature to report thermal
Other instrumental uncertainties involved in
calculations include change in water properties due to
temperature such as change in specific heat capacity of water
from 4181 J kg-1 K-1 at 298 K to 4179 J kg-1 K-1 at
303 K and change in water density from 997.1 kg m-3 at
298 K to 995.7 kg m-3 at 303 K. Additionally,
measurement of water temperature rise with the most precise
and reasonably priced platinum resistance temperature
sensors is accurate to within ±0.03 K. Moreover, the
water flow through the calorimeter is not always perfectly
constant, and the flow rate measurement normally
includes ±1 % error, even using precise digital flow
Furthermore, uncertainty of the thermocouple pair
distance (separation) can introduce substantial errors into the
measurements. This could be caused by thermocouple
movements during the test or by operator error. Finally,
sample thermocouples are only accurate to ±3 K. All of
the above instrumental errors above would accumulate (in
sum of quadrature) to a ±3 % uncertainty for the ASTM
C201 panel calorimeter method.
This study has established good inter-laboratory
reproducibility of high-temperature thermal conductivity
measurement of a standard high-temperature fibre insulation
material using an adapted panel calorimeter method based
on the ASTM C201 standard. Some variability has been
established in measured thermal conductivities between
different test methods including ASTM and BS panel
calorimeters, hot wire, and thermal diffusivity by laser
flash. Results support the use of adapted methods based on
the ASTM C201 standard being the most precise, and this
method is suggested for wider use in fibrous insulation
thermal conductivity measurement.
The new biosoluble alkali aluminosilicate fibre
insulation panel has been introduced as a standard reference
material for calibration and benchmarking purposes, and
for comparative studies of high-temperature thermal
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