Sustainable Charlbury

Thermal imaging explanatory notes


We’re grateful to West Oxfordshire District Council’s Community Services Department for providing their thermal imaging equipment to enable Sustainable Charlbury to survey local properties, as well as Charlbury Beer Festival Fund to facilitate this as a free service for local residents.

These notes provide some detailed information to help you interpret the images of your property. Towards the end we also has some general advice that may be helpful for saving heating costs.

We hope the images we’ve taken, in combination with this advice, proves valuable to Charlbury residents in reducing your heating bills in winters to come.

General Background to our surveys

The thermal imaging surveys we have provided images were qualitative rather than quantitative. That is to say, the objective for taking images was to locate possible issues or trouble spots where heat was released from inside to the outside of buildings, not to try to measure exactly how much heat was being lost. We can only offer information regarding issues worthy of further investigation by professional analysis, which could, if you wish to do so, provide an actual measure of degree of heat loss from your building.

We hope you find what is revealed interesting and useful. Sometimes more detailed study may be needed to understand what the image truly reveals, but it may be the case that the persons living in, or paying the bills, may already have a good idea of the likely causes of heat loss. The images may, then, be a strong visual reminder to investigate further.

We therefore can’t providing all the answers, but information to help in asking the right questions!

Understanding what Thermal Images reveal

Thermal imagery (thermography) of buildings can reveal three main types of fault:

  • Thermal bridging
    • A thermal bridge is an area where the building has a lower thermal resistance, caused by construction constraints, such as cramping or ledges between elements situated on both sides of an insulated layer.
  • Air leaks:
    • An air leak is a non-controlled movement of air through walls, doors, windows and roof.
  • Insulation deficiencies:
    • An insulation deficiency is a area having a lower thermal resistance that is not due to the design of the building. Causes can include
      • Insulating material being compressed.
      • Insulating material having moved or slipped.
      • Insulating material having been altered.
      • Poor workmanship.
Example Thermal Image

Example Thermal Image

A Thermal imaging camera converts invisible infrared (IR) radiation into an image we can understand. This thermal image is an accurate image of IR radiation received, not a true image of temperature, but with some caveats mentioned later, and some clever tecnhnology in the camera to take account of the variables, is a very good approximation so is the best way to measure temperature from a distance.

We’ve noted in conversation in speaking to some residents an apparent misunderstanding of thermal images show that we’d like to address.

The different colours in the images – a heat map if you will – represent different temperatures measured across the surfaces in the view of the camera at one moment; from coolest (deepest blue) through the spectrum to the red and then white for the highest temperatures measured.

These visual representations of temperate are, of course, artificial constructs of something we cannot see with our eyes (we cannot see heat, although we can sense it by touch).

The actual choice of colours shown by the camera; from blue for coldest and up represents the range of temperatures in view of the camera at the particular moment an image is taken. These colours enable us to pick out and enhance temperature differences across that view, so the choice of colour seen depends on the range of temperatures in view rather being fixed at any one temperature. So red in one image could represent a lower temperature than blue in another – and vice versa. It all depends on what temperatures are in view and where the camera’s pointing!

To clear up any potential confusion, a temperature scale at the side of each image makes it possible to compare the temperatures measured.

The important point to note in the images is not the precise temperature measurements (which depend on how cold it was outside, and how much heat was leaking out) but to see where heat from inside the building is being lost, if it’s significant and to try to interpret why.

Some technicalities do complicate simple interpretation. They should ideally have been done on a cold dry night, but there are also issues about the intrinsic characteristics of the building materials. In some circumstances, the colours will be influenced by differences in emissivity of the materials used on the outside of your house. For instance, different type of brick or stone will be highlighted as different colours not necessarily due to them being at different temperatures but variations in how efficiently those materials emit the heat passed from the warmer side to the cooler side.

More often that not, the context of these materials, say if they are side-by-side, makes this obvious.

Another important point is heat always is lost to some degree; it’s a fundamental law of nature that heat moves from hot places to cooler places. There’s no point even trying to achieve a perfectly insulated box – it’s impossible.

Anyway, we’re interested in real properties in Charlbury, that you’ll want to be comfortable to live in and efficient to heat!

Thermal imaging is best performed in cold, still, dark and dry conditions, otherwise the camera’s ability to accurately provide measurements of temperature is compromised.

For instance if walls are damp, measurements could be inaccurate, so it’s best if it hadn’t rained immediately prior. Likewise, it shouldn’t be windy and there should be no residual daytime heat from the sun (the imagery should have been done at least 4 hours after sunset); and the outside temperature should to be at least 10°C lower than the temperature inside the house so areas of heat loss could be clearly seen.

You can see the British winter – especially this last winter – won’t often provide the ideal conditions for thermography!

However, the photos do display the time and date that each image was taken, and there’s also usually other information displayed such as the outside temperature and the outside humidity. This should help put these things into context and explain some unexpected or inconclusive images.


Things to look out for in the thermal images of your property – plus some remedies

  • See if the variation of temperature measurements of the exterior of your house is significant.
      • Check the scale at the side of the photo to gauge the temperature range that the different colours represent, a small difference of the outside temperatures of your house might not mean much, whereas, a hot spot with a difference of 5°C compared to the cooler parts might be significant.
      • Photos including the cold sky in the image background will have a very low value at the bottom dark blue end and the building will look relatively warm at the red end of the scale. This may mean the image of the house may be compromised in pointing out any hot spots.
      • Close-up photos might have a narrower temperature range on the scale, emphasising the differences of temperature. These may then emphasise fine differences, the temperature ranges will show wether these are fine differences so insignificant or wider differences highlighting specific areas of heat loss.
  • Note construction details:
      • The architectural details on the underlying the surface of your house may be revealed in the thermal image. The emissivity of materials may vary; such as painted wood, limestone, concrete, glass, or brass. Differences in construction methods – say with the main body of the house and an extension – may also show up.
      • Once the house has been designed and built, issues affecting heat loss such as thermal bridging will be difficult to change. If you are extending or modifying your house, then you could take that opportunity to improve the insulation and reduce air leakage.
      • Houses often have solid lintels above the windows which show up warmer than the surrounding cavity walls because they create a break in the insulation layer.
      • Floor slabs might be constructed so that they breach the insulation layer of the walls providing a thermal ‘bridge’ through which heat is lost.
      • Bay windows might show heat loss at the sides because the way that they are joined to the main wall breaches the insulation layer and allows air leakage.
      • Dormer windows lose heat through the (usually uninsulated) thin cheeks and where they are joined to the roof.
      • Where neighbouring houses of a similar construction are shown in the photo there can be interesting comparisons. To understand what’s gong on you need to know the circumstances; differences in colour might be because the houses are heated to different temperatures or that they have been insulated to different degrees.
  • Windows reflect infrared
    • Window glass is like a mirror to infrared and reflect what is incident to the camera. So, it’s difficult to draw conclusions about heat loss through windows. Metal door knobs and letterboxes, the aluminium spacer in double glazed windows, will all show up as reflections which will mask actual heat losses.
  • Roofs – a different perspective required
      • A thermal image, like any photo, is a two dimensional representation of a three dimensional object. Interpretation requires translating the flat 2D image to a 3D world.
      • From the ground the camera views a roof at a narrower angle than the walls in front. Remember the camera measures IR radiation not heat. IR is radiated from a surface mostly in straight lines so a lesser proportion of that heat (compared to the walls) will arrive from the angle of view of the camera, so will be falsely represented as being cooler than the rest of the house.
      • Therefore you would need closer examination before concluded the roof really is cooler, even if it does appear so in thermal images. It does, to some extent, depend on your viewpoint!
  • Corners
    • Corners of buildings produce convection patterns. Outside corners get more exposure to winds and will appear cooler than the rest of the walls, while inside corners will appear warmer than the rest of the walls because they are more sheltered – moving air currents are curved and don’t go into corners.
    • This applies to horizontal corners as well, like under eaves, under window sills, and under any angled lengths of downpipes. These all show up as warmer, not just because of the sheltered corner effect but also because warm air rises and gets trapped.
  • Eaves
    • Eaves usually have a ventilation gap into the loft space and warmer air from the loft will be spilling out through that gap.
    • The degree of that heat loss will depend on:
      • The amount of insulation in the loft;
      • How well it has been fitted.
      • Insulation works by trapping air within its structure. It’s important to reduce convection currents between the layers or underneath the insulation material, or it will cease to act as effectively. When top-up insulation has been fitted to bring it up to a depth of 300mm (12″), the angle of the eaves makes it difficult to fit into the corners without blocking the ventilation gap at the eaves. Special L-shaped ventilation trays may to be fitted between the rafters to preserve the air gap and allow the insulation to be pushed down into the angle between the roof and the eaves. The top of the cavity wall should also be blocked off to stop the movement of air from the walls into the insulation.
  • Walls
    • Sometimes the images reveal where former windows and doors have been blocked up and where the insulation is different from the surrounding wall. Cavity walls which have been filled with insulation might appear patchy. Cavities often have obstructions within them which affect how well they can be filled with blown fibre. The width of the cavity is also significant: 50mm cavities are too narrow to retrofit cellulose insulation and need particles like Vermiculite to be used; 75mm is the minimum for blown-fibre. It might also be difficult to get the insulation up into difficult places like under windowsills. Modern houses have cavities of 100mm and the wider the cavity the better the insulation will be, but even with solid insulation batts, there might be unsealed gaps between them. Metal wall ties will also reduce the effectiveness of cavity wall insulation by creating small thermal bridges.
    • Often the loft space at the top of the gable will be cooler because the loft is unheated and the ceiling below is well-insulated. However, this also shows that heat is being lost through the walls from the heated rooms on the floors below not just for solid walls but also if there is cavity wall insulation. While insulation slows down the rate of heat loss, if the heating has been on a long time, then heat loss will still be visible through the wall.
    • Brickwork below the damp proof course usually shows up as ‘warmer’ than above the line which could be because of heat loss from the ground.
    • Walls of recent extensions built to improved building regulation insulation standards might be cooler (ie losing less heat) than the walls of the original house.
  • Chimneys
    • Chimneys are likely to show up very white (i.e. hot) when there is a fire alight in them and the line of the chimney will be very visible on a gable wall. If the chimneys are built on internal walls this will be useful heat gained by the adjoining room. Chimneys without fires alight in them will still appear warm as heat is lost through draughts up the chimney from any open fireplaces. This heat loss will be reduced by blocking the chimney with a chimney balloon or something more permanent, but even then, there will still be heat loss from the room through the chimney breast into the chimney space.
  • Windows
    • If several windows are visible, you might see that some windows appear cooler than others.  This might well indicate the windows that are double glazed or rooms that are kept cooler.
    • Some windows might have heavy curtains drawn over them without any gaps and these might also show up as cooler.
    • Trickle vents in windows will show up as warm patches, but for successful cross-ventilation of the building these need to be kept open, except on windows facing the prevailing wind in very windy weather. It will not be possible to use the photos to get an accurate feel of heat loss from conservatories, if it’s single-glazed then there may be considerable heat loss. You may already be aware of frames or windows are draughty, so, if you take steps to reduce these draughts, then you are bound to benefit.
  • Doors
    • The construction of external doors can often be revealed in the thermal image. Low energy doors contain a layer of insulation. Sometimes it is easy to remedy this by packing the inside of the door with insulation and then covering it with something that looks OK. Heavy curtains that cover the whole door will reduce heat loss by trapping a layer of still air between the curtain and the door and reducing draughts.
    • The fit of a door in the frame is also important and can often be improved by an experienced joiner. Doors might well fit tighter into their frame and cause fewer draughts when they are locked. Mortice lock keyholes and letter boxes need to be covered to reduce draughts. Heat loss from under the door onto the outside step is sometimes visible in photos. Draughts at the bottom of a door can be reduced by fitting a brush strip and soft ‘sausage dogs’.
    • Cat flaps can be a significant cause of heat loss and draughts. Airtight cat flaps are now available.
  • Personal Knowledge
      • Your knowledge of your house should help explain much of what is shown in the thermal images. Open windows will show up with a bright white line, so will boiler vents and air vents. Radiators on outside walls are likely to show up as warm patches; if some have reflective foil behind them then they might not be so visible. Hot water tanks located near outside walls are likely to be visible, as are fridges and freezers because they produce heat, but, if they are on internal walls then this heat will help warm the house. The external walls of any rooms that are kept cooler than others are likely to show a cooler temperature.
      • Your knowledge of your own house will help you identify the measures you have taken which do reduce heat losses. In addition, you might well know parts of your house which are normally inaccessible and are not insulated, e.g. knee-high side walls of converted lofts, spaces between floors.
      • It might be possible to see heat loss from these places in the photos, however, if you know part of your house is not insulated, then you can be sure that heat will be escaping.

General advice and follow up actions you can take

Knowing your property – Thermal mass

Wall insulation slows down the rate of heat loss through walls. When a room is heated, both the air and the fabric of the building warms up; the construction of the walls affect its thermal mass and its rate of heat gain/loss.

If a wall has a high thermal mass, like solid stone walls, they tend to lose heat slowly and regulate the temperature by dampening the variations in temperature, so the room doesn’t get cold so quickly when a heat source is turned off. Similarly, the fabric will take longer to heat up so will be cooler in summer.

For comfort, it is best to have the fabric of the building and the space air temperature within 3°C of each other. This is why it can be more fuel efficient and yet still comfortable to maintain the temperature of the house at a lower constant temperature for 24 hours a day rather than putting the heating on at a higher temperature when you are home morning and evening. It is also the reason external wall insulation is better than internal insulation so that the thermal mass of the wall can still be used to regulate the temperature. Solid plaster on the inside of external walls is also preferable to using plasterboard, as it allows the effect of the thermal mass of the wall to regulate temperature in the room.

Radiator reflectors

Reflective foils behind radiators reflect more heat back into a room so less is lost through the wall.


Apart from around doors and windows, draughts also are caused by gaps in the fabric of the house around service pipes etc.

Indeed, draughts have the equivalent effect of having a large window open all the time.

To take this further, air pressure tests can reveal the extent of the problem so that the draughts can be identified easily and then the gaps filled. Even at normal pressure, a good way to detect draughts is to wet the back of your hand and place it near any gaps; your hand will feel particularly cold in a draught as the moisture evaporates

Indoor thermal images

For this project we images the outside of properties. However, indoor thermography can be another further test to identify cold air draughts around doors and window frames – and these are problems that are easily remedied.

Sometimes the door might be modern and well insulated but the draught comes from where the door frame fits the wall or floor.

Photos of radiators can also be revealing, radiators that contain air and need bleeding will be cold at the top, and radiators that contain sludge and need flushing will be cold at the bottom, either problem reduces the efficiency of the whole central heating system.

Patchiness in loft insulation can be visible as cold patches on the ceiling, often insulation is missing above recessed spot lights but special protective covers are now available which can be fitted above the light fittings to safely support the insulation.

Loft hatches might also show up as cold, ie heat being lost into the loft space, these can easily be insulated and the edges also draught-proofed.

Convection currents don’t reach into the corners of rooms, so corners always be cooler than the rest of the walls and ceilings and this is why moulds grow in the still air conditions in the corners of rooms.


While thermal images can reveal issues, it takes knowledge about the characteristics of your home to understand what is really happening when you heat your home.

Draughts are particularly important to fix because you will feel more comfortable in still air at a lower ambient temperature.

Lifestyle factors can also affect comfort and fuel bills: your personal insulation, as in the clothes you wear to reduce your own heat loss, will enable you to be more comfortable at lower room temperatures. Interestingly, we generate about 100W when resting and about 160W when moderately active, and it is very simple to trap this heat next to the skin by a close-fitting insulation layer of thermal clothing such as fine merino wool.

Thanks again to our partners and volunteers, and we hope we’ve been able to help you stay warm and comfortable, while saving fuel bills for next winter.

One Comment

  1. Building codes effecting insulation levels wouldn’t really start to take affect until early 1980’s. If your household was built previous to 1984, there is a reasonably good chance that an attic has small attic insulation. Builders in the 1940’s would not insulate much of anything, builders in the particular 1960’s filled the place between the ceiling rafters with concerning 4 inches of insulation. Builders in the actual 1990’s installed 8 in . (R-25 in order to R-30) regarding loose-fill fiberglass insulation and by the year 2000, insulation levels received reached 12 inches (R-38). ‘

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