in the Investigation of Solid Masonry Structures
|This thermal image inset within a digital image shows temperature
variations across a roof using the ‘rainbow’
colour scheme. Each colour
represents a temperature, shown in the scale on the right.
(All images: Historic Scotland).
Thermal imaging (also called
infrared thermography) is a type of
non-destructive investigation (NDI).
It allows the user to obtain data and analyse
an object without the need for a sample to be
removed and without damage to the object.
A wide range of non-destructive techniques
is available for use in building conservation.
Some are ‘active’ techniques where a signal
is sent out by the instrument. Information is
obtained by analysing changes in the returning
signal. Examples of this kind of NDI include:
- 3D laser scanning, which maps the
three-dimensional structure of objects.
- Microwave moisture analysis, where
the instrument projects a beam of
microwaves into a substrate and
quantifies sub-surface moisture.
Other NDI techniques are ‘passive’. They
do not emit a signal themselves, but are
dependent on detecting some emission by the
- Heat flow sensors, used to determine
U-value, are a form of passive NDI. They
detect heat flow through a surface.
- Thermal imaging is, in the main, a
passive form of NDI. It detects infrared
light emitted by objects. However, some
specialist forms of thermal imaging do
involve active heating of surfaces, often to
observe rapid heating and cooling events.
WHAT IS THERMAL IMAGING?
Thermal imaging is a method for visualising
and quantifying temperature variations across
surfaces. Temperature differences as low as
0.1°C can be observed. It is a rapid method
allowing imaging of large surface areas. It is
non-destructive and non-contact, which can
be very important in some applications such as
|It can be useful to combine parts of the temperature
range of a thermal image with a visual image. Here
the technique illustrates the path of a heating pipe
Thermal imaging is a useful technique
for diagnosing the condition and behaviour of
many aspects of buildings, visually revealing
structural deficiencies. Applications include:
detection of moisture and water infiltration,
observing thermal bridges, voids, cracks or
delamination, locating areas of heat loss or
air leakage and assessing the performance
of insulation. Anything that results in a
temperature contrast on a surface can be
usefully imaged by thermography.
Thermal cameras look very like digital
photographic cameras but instead of using
visible light they detect infrared radiation
(IR), a form of light which is invisible to the
human eye as it occurs beyond the red end
of the visible light spectrum (0.4-0.76 μm).
IR cameras work with wavelengths between
2 and 14 μm, although no single camera covers
the entire range. Different types of IR camera
are appropriate for different applications.
Cameras which detect medium wave
IR (2-5 μm) are used in specialist laboratory,
medical and military applications.
for building thermography detect long wave
IR (8-14 μm). Although they are mainly used
to take single images, higher specification IR
cameras can also take images in time lapse
or video format. Specialist lenses allowing
telephoto and wide angle imaging are available.
Wide angle lenses can be particularly useful in
building thermography when working indoors
in small spaces or externally to capture wide
areas of facades in a single image.
The resolution of most thermal cameras is
not as high as that of photographic cameras.
Low-cost IR cameras have a resolution of about
60x60 pixels while some higher specification
cameras achieve 640x480 pixels. IR cameras
also vary in their thermal sensitivity (the
smallest difference in temperature they can
resolve). The more sensitive the detector, the
more detail will be visible in a thermal image,
especially when temperature differences are
Infrared radiation of the wavelengths
we are interested in (8-14 μm) does not
pass through glass. This means that the
camera cannot be used to see objects behind glass. Also, the camera cannot
use glass lenses; the lenses in long wave
IR cameras are made of germanium.
|Always verify the cause of a thermal anomaly. The cold patch on the left is caused by dampness; that on the
right by a cold draught.
Infrared radiation is emitted by all
objects at temperatures above absolute zero
(-273°C). The warmer the object, the more
infrared radiation it emits. The IR camera
converts the intensity of radiation to a visible
image (or ‘thermogram’) in which every pixel
represents a temperature.
schemes are available. The choice depends
on what is being illustrated. Heat losses
are often illustrated with the ‘iron’ colour
scheme as it emphasises the importance
of hot spots, highlighting them in white
and yellow. The ‘rainbow’ colour scheme is
often useful for illustrating dampness or
draughts, emphasising colder areas in blues
and purples. Inconsistencies in thermal
images are called anomalies and they may
indicate a problem with the building fabric.
The thermogram is more than just an
image, it is a ‘radiometric’ image – each pixel
records a spot temperature allowing, for
example, cross-sections or areas of data to be
exported to a spreadsheet for further analysis
or to produce a graphic representation of a
temperature profile. It is useful to have an
ordinary digital image as well as it is not always
easy to tell what is being shown in a complex
thermal image. It can be difficult to locate an
anomaly on an area of wall which is visually
uniform without an accompanying visual
image. Most thermal cameras capture a digital
image at the same time as the thermal image
ensuring that both illustrate the same area in
the same orientation.
GETTING THE CONDITIONS RIGHT
For useful thermal images to be recorded
there needs to be a temperature contrast. In an
unheated building where there is no external
heat source (solar heating, for example)
all surfaces will tend to be at a similar
temperature and no useful information will be
obtained. Without heat to drive evaporation,
a damp patch on a wall will have the same
temperature as an adjacent dry surface.
|Visual, thermal and dewpoint thermal images of this window illustrate how surfaces with a temperature below dewpoint can be highlighted. Condensation risk is
illustrated in green on the right hand image.
useful data we need to heat an object above
ambient background temperature (or cool
it below ambient) and observe it while it is
warming up or cooling down. The bigger the
temperature contrast, the better the thermal
images. For this reason, most thermal imaging
for heat loss is done in the winter months when
the temperature contrast between the interior
and exterior of buildings is greatest.
Heating may come from man-made
sources such as domestic heating or from the sun – solar heating. Domestic heating
is often used in building thermography,
supplemented as necessary by additional
heaters. As a rough rule of thumb, a building
should be heated to at least 10°C above
ambient temperatures for about 24 hours
before thermal imaging commences.
To see structures or voids below the
immediate surface, a longer period of heating
may be needed so that heat has time to
penetrate to the depth required. In massed
stone structures wall thicknesses may be
substantially greater than in a domestic setting
and heating times and intensity may need to
It should be noted that intense
heating of some spaces may not be appropriate
if they contain materials which could be
damaged, for example wooden structures
which could suffer shrinkage, or fragile painted
plaster. In these cases the thermographer will
have to make the best of such heating as can be
|This brick building with areas of harling is fully sunlit. Hot patches on the harling show areas of detachment as heat is trapped on the surface. This thermogram uses the
‘iron’ colour scheme.
The success of thermal imaging outdoors
is dependent on the weather and the time of
day. Wind will tend to chill surfaces, effectively
blowing away any thermal anomalies, reducing
the temperature contrast between hot and
cold areas. Thermal imaging is unlikely to be
successful if the wind is gusting at over 20mph
(approximately 10m/s). Unless the object is
to observe wetting patterns on buildings,
the surface should not be wet (or recently
wet) from rainfall as evaporative cooling will
confuse the thermal emissions the imaging is
intended to observe.
If the object is to observe heat losses from
structures then thermal imaging has to be
carried out during the hours of darkness, when
solar heating will not confuse the issue. The ideal time to conduct a survey to localise heat
losses is in the early morning before sunrise.
This allows time for any solar heating of the
building on the previous day to dissipate.
Thermography is very useful in the detection
of dampness. When surfaces are warmed,
damp patches remain relatively cold due to
evaporative cooling as moisture is lost from
the surface. The situation is more complex
when moisture is held at depth, as may be the
case in a solid masonry structure. Evaporative
cooling can only take place when water is
present on or near the surface. When moisture
is trapped below the surface and heating has
been of sufficient intensity and duration,
trapped water will show up as a hot spot.
This potentially confusing effect is caused
by differences in the thermal capacity of the
water and the wall. Water holds heat longer
than dry stone, so during cooling a mass of
water inside a wall shows up as a warm patch.
Whether cooling down or warming up, a wet
wall will change temperature more slowly
than a dry wall.
|This image combines 3D laser scanning with thermal imaging to produce
a 3D thermogram. Air leakage at the
junction between the original 16th-
century structure and a Victorian addition is the cause of significant heat
loss. High heat loss is also observed on the thin stonework at the end of
the aisle. The thickness of the stone wall
at this point would not have been
easily observed without access to 3D scan data.
|Evaporation of moisture from this masonry wall is the cause of cooler patches in the thermal image (left).
Concentrations of moisture below the surface (right) were confirmed using a microwave moisture sensor which
is able to detect moisture at depths up to 20-30cm.
Not all damp problems are caused by
water ingress, some are due to condensation.
As it displays the temperature of surfaces,
IR thermography can predict areas at risk
of condensation provided the values for air
temperature and relative humidity are input.
Some cameras have software to perform
a dewpoint calculation and will display
condensation risk as a differently coloured
overlay on the thermal image.
The IR camera only sees the temperature of
surfaces, but it can provide information about the deeper structure where this affects surface
temperature. As noted, a mass of warm water
in a masonry wall will be observed as a warm
patch on the surface. In thermal bridging, observation inside a heated building will show
a cold area where heat flow to the outside is
unusually high, while observation outside the
building will show a thermal bridge as a hot
The pattern of variations in heat flow
can often give clues to sub-surface structures.
Sources of heat within or behind a structure
can be located. Thermography can show the
location of warm flues in gable walls, it can be
used to visualise the performance of underfloor
heating and to locate hot water and heating
pipes. It can therefore be useful for locating
problems without having to open up the floor
Although solar heating can often confound
imaging, in some instances it can be very
useful. On sunlit walls heat will flow most readily into a structure with no thermal
barriers. If a wall has voids, blistering,
detached harling or roughcast, heat will
tend to be trapped on the outside of a sunlit
wall, finding it more difficult to flow through
the wall compared to adjacent solid areas.
This method can be used to detect detached
external finishes on sunlit walls.
The ability to visualise heat makes
thermography a powerful tool for studying
energy efficiency. Excessive levels of heat loss
through a structure are easily located. Further
investigation will be required to determine
whether the cause is lack of insulation, air leakage or some other defect.
The insulation value of building materials
is often quoted as a U-value. This is a measure
of the thermal transmittance of a material or
structure. As the thermal camera observes
radiant heat loss from surfaces there are
methods for deriving U-values from thermal
images. The results are dependent on several
assumptions and can be significantly affected
by local conditions. The U-values derived
from thermal imaging data are not as reliable
as those from measurements derived from in
situ heat flow sensors, which average heat flow
over a long period, reducing errors caused by
unstable atmospheric conditions. Thermal
images record only a single point in time.
|Slate roof before (above) and after (below) insulation of the roof space. Heat losses on ridges and around the
skylight have been significantly reduced following insulation.
Air leaks are a problem where they
lead to higher energy consumption or to
condensation. Air leaks can be detected with
a thermal camera, especially when used in
combination with a ‘blower door’ which is used
to reduce the air pressure inside a building.
The IR camera is used on the lower pressure
side (indoors) and air leaks will show up as
cooler areas as air is drawn into the structure.
The camera does not see the air flow itself, it
sees the cooling effect on adjacent surfaces.
CONFIRMING THE CAUSE OF
Imaging can easily locate thermal anomalies
but the cause is not always obvious.
Anomalous cold patches are not always
caused by dampness – draughts can look
very similar. The corners of rooms are
normally colder and this does not indicate a
problem; warm air does not circulate so well
into corners and they have a relatively large
external surface from which to radiate heat.
It is important to ensure that what is being
observed is an anomaly and not a normal
temperature variation. The area in question
should be compared to other similar areas and
potential causes of the temperature anomaly
investigated. Often, confirmation by further
inspection will be required. This may involve
the use of equipment such as moisture sensors
or careful examination of building plans.
In some situations thermal imaging
can also be usefully combined with 3D laser
scanning to produce 3D thermal images. This
is particularly useful in complex buildings
where the relationship between different parts
of the structure may be difficult to visualise.
For example, tracing water leaks through a
complex structure to determine the source of
ingress may be easier if the thermal data can be
visualised in three dimensions.
A thermal image should not be taken at
face value. The diagnosis should always be
confirmed by further inspection.
BINDT, Infrared Thermography Handbook
Vol 1: Principles and Practice, 2004
BINDT, Infrared Thermography Handbook
Vol 2: Applications, 2004
BS EN 13187:1999, Thermal performance of
buildings. Qualitative detection of thermal
irregularities in building envelopes.
FLIR, Infrared guidebook for building
FLIR, Thermal imaging guidebook for building
and renewable energy applications, 2011
M Vollmer and KP Möllmann, Infrared
Thermal Imaging: Fundamentals,
Research and Applications, Wiley-VCH,
T Ward, Information Paper IP1/06, Assessing
the effects of thermal bridging at junctions
and around openings, 2006