Thermal Imaging

West Oxfordshire District Council and the Oxfordshire CAG network both have thermal imaging cameras that we can borrow. We have used one of these cameras each year in Charlbury since 2011/12 but are very dependent on the weather being suitable during our loan period. In 2012/3 it was generally too wet, too warm, too snowy or too windy to take many pictures while we had the camera. In 2013/4, January proved impossible with record breaking continuous rain, and even though it dried out in March we had to work around warm conditions with lots of sunshine warming up the exterior stonework well into the evening. Since then, we have managed to do some surveys each winter at the request of individual householders. So if you would like images taken of your house this winter, then please contact us so we can arrange to borrow one of the cameras.

We have learnt some general lessons about heat loss from houses and produced the following guidelines which can be understood by anyone, even if they don't have specific photos of their own house.

Reducing fuel bills

People are usually already aware about which rooms are difficult to heat, where the draughts are coming from and how much insulation they have. Thermal imaging usually confirms what they already know about their home, so there is no need to wait before carrying out simple remedial action.

  • Removing draughts and improving insulation will always make people feel more comfortable and reduce their fuel bills.
  • Draughts are particularly important to fix because people feel more comfortable at lower temperatures if the air is still.
  • Fuel bills are also directly affected by the clothes people wear, i.e. their own personal insulation which reduces the heat lost from their body. Heat is generated both from the food they eat and the activity they are doing - we generate about 100W when resting and about 160W when moderately active. It is very simple to trap this heat next to the skin with a close-fitting insulated layer of clothing such as fine merino wool, invisibly worn under ordinary clothing and enabling people to be much more comfortable at lower room temperatures.
  • When a room is heated, both the air and the fabric of the building warm up; the construction type of the walls affects their thermal mass and their rate of heat gain/loss. If the walls have a high thermal mass, like solid stone walls, they will lose heat slowly and regulate the temperature so that the room doesn't feel cold so quickly when the heat source is turned off. Similarly, the fabric will take longer to heat up in hot weather so will feel 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.

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The architectural details on the outside of buildings are revealed as different colours. This is partially to do with the surface emissivity of different building materials such as painted wood, limestone, concrete, glass, or brass rather than differences in temperature. However, it can also be due to differences in temperature caused by variations in heat transmission through different building materials and by different construction methods. Once the house has been designed and built, these factors are more difficult to change. If alterations are being carried out, then that would be a good 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 often show heat loss at the sides because the way 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.
  • Sloping roofs and side walls of converted lofts are often inaccessible and inadequately insulated.
  • Often there are inaccessible spaces which can be impossible to insulate.

Thermal images of a row of neighbouring houses with a similar construction can be interesting - differences in colour might be because the houses are heated to different temperatures or that they have been insulated differently since the houses were originally built. Talking to neighbours can be useful to find out about their fuel bills and about what steps they've taken to reduce them.

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It is important not to take photos in windy weather because heat lost from buildings will get blown away and will not get picked up on the images. In reasonable weather the following conditions will often be visible in thermal images.

  • Outside corners of walls get more exposure and will appear cooler than the rest of the walls.
  • The opposite of this, i.e. inverted corners where two walls join each other, 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.
  • Horizontal corners are more sheltered 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. This is particularly noticeable in porches which trap the warmth from the day and also from outside lights that might have been on several hours beforehand.
  • Trellises and vegetation on the walls of houses will also trap heat and show up as warmer.
  • Cotswold limestone has a very uneven surface - full of bumps and crevices and these act as tiny corners, so the limestone can look warmer than a smooth flat concrete surface.

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Since the photos are taken from the ground, the angle of most roofs means that they reflect the night sky and appear colder than the rest of the house - so thermal images of roofs can be very misleading. Patches of moss growing on a roof are usually clearly visible.

  • Depending on the angle of view it can be possible to detect patchiness in insulation of the sloping roof where loft space has been converted into living accommodation.
  • Effectiveness of insulation in an unheated roof space can be better detected by looking at heat escaping through the ventilated eaves than through the roof (see below).
  • Take the opportunity when it snows to have a look at your roof and assess your insulation by comparing rates of melting with neighbouring roofs. January 2013 was perfect for this because the snow stayed for several days and there was no sunshine to affect the rate of melting. This can be far more revealing than thermal images of your roof.

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In addition to being sheltered, eaves also look warmer because there is usually a ventilation gap into the loft space and warmer air from the loft will be spilling out through that gap. The warmth of that air will depend on two things: the amount of insulation in the loft and also how well it has been fitted.

  • Insulation works by trapping air within the insulation material and it is important to reduce any convection currents within these layers, or it will stop being effective.
  • 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 need 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. Thermal images of ceilings taken inside the building, frequently show cold uneven patches where insulation isn't fitted properly into 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.

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Blank walls of a house, such as the gable wall, often reveal the most interesting thermal images because, in the absence of distracting architectural details, any differences in colour have to relate to real temperature differences.

  • Sometimes the colours reveal where former windows and doors have been blocked up and where the insulation is better or worse than the surrounding wall.
  • Cavity walls filled with insulation often appeared 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 instead; 75mm is the minimum gap suitable 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 looks cooler because the loft is unheated and the ceiling below is well-insulated. However, this also shows that heat is lost through the walls from the heated rooms on the floors below, not just through solid walls but also through insulated cavity walls. While insulation slows down the rate of heat loss, the wall will become heat saturated over time and heat will continue to be lost through the wall. Neighbouring semi-detached and terraced houses mutually benefit each other.
  • Brickwork below the damp proof course often shows up as "warmer" than above the line which could be because it is damper.
  • Chimneys are likely to show up very white (i.e. hot) when there is a fire alight in them making the line of the chimney very visible on a gable wall. Even chimneys without fires alight in them still appear warmer than the rest of the gable because heat is lost by draughts blowing up the chimney through the open fireplace. 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 cavity. If chimneys are built within the house on internal walls, heat from fires will usefully heat rooms adjoining the chimney space.
  • Walls of recent extensions tend to look cooler (i.e. losing less heat) than the walls of the original house, this reflects the improvement in building regulation standards of insulation over the years.

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 an external wall is preferable to using plasterboard, not just because of the thermal mass, but also to avoid heat loss from air currents behind the plasterboard.

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Understanding heat loss from windows with thermal imaging from the outside is generally difficult because of reflections, windows are like mirrors and reflect the cold sky, or the hot images of people or outside lights. Aluminium spacers in double glazed windows are also highly reflective and their thermal image can therefore be misleading.

  • At some angles it is possible to view several windows in the same picture and detect that single glazed windows do lose more heat than double glazed windows.
  • Heavy curtains drawn carefully without gaps and touching the ground also reduce heat lost from windows.
  • Trickle vents in windows show up as warm patches, but for successful cross-ventilation of the building they do need to be kept open and only closed on windows facing the prevailing wind in very windy weather.
  • The expansive glass windows of conservatories are highly reflective of both hot and cold images according to the angle of view, so it is not be possible to use the photos to detect heat loss from conservatories. However, people with single glazed conservatories know that they suffer considerable heat loss.
  • Internal photos of windows can reveal where draughts are coming in and often it is from a crack between the plaster and the window frame rather than where the windows actually open. An easy way to detect draughts without a thermal camera is to lick the back of your hand and hold it near the window frame, any draught will evaporate the moisture and make your hand feel much cooler.
  • Reducing draughts will definitely bring benefits.

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The construction of external doors are easily revealed by thermal images, but reflections of the cold night sky in shiny metal door knobs and letterboxes can mask actual heat loss.

  • Thinner sections lose more heat than the thicker parts - the thicker the door the better.
  • Low energy doors contain a layer of insulation.
  • Sometimes it is easy to improve a thin door by packing the inside of the door with insulation and then covering it with something that looks OK.
  • Heavy curtains covering the entire door and door frame will reduce heat loss by trapping a layer of still air between the curtain and the door and reduce 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 also soft "sausage dogs".
  • Cat flaps are a significant cause of heat loss and draughts. Airtight cat flaps are now available. If there is no longer a cat living in the house, it is a good idea to block up the hole.
  • Internal photos of doors can reveal that draughts often come from a crack between the plaster and the door frame rather than where the door actually opens. An easy way to detect draughts without a thermal camera is to lick the back of your hand and hold it near the window frame, any draught will evaporate the moisture and make your hand feel much cooler.

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Personal knowledge of the property

People living in a house can often explain various sources of heat loss which show up very conspicuously on thermal images.

  • Open windows show up as a bright white line, so do boiler and air vents.
  • Radiators on outside walls are likely to show up as warm patches but, 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 a lot of heat from their cooling panels, this shows why it is so much better to position these on internal walls so that they help to heat the house.
  • The external walls of rooms that are kept cooler than other rooms in the house are likely to show up cooler from outside, and warmer rooms will show up warmer.
  • Improvement work done to a house to reduce heat losses are sometimes clearly visible.
  • Inaccessible parts of the house which aren't insulated, e.g. knee-high side walls of converted lofts and spaces between floors - it is sometimes possible to see heat loss from these places in the photos.

If you know that part of your house is not well-insulated, then you can be sure that heat will be escaping - you don't need a thermal image to confirm this.

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Indoor photos

Photos taken indoors are often more revealing than outside photos - cold draughts and cold patches can show up very clearly as dark areas on a photo.

  • Draughts of cold air around doors and window frames can be visible, sometimes quite dramatically - and these are problems that are easily remedied.
  • Sometimes the door or window might be modern and well insulated but the draught comes from where the door or window frame fits the wall or floor.
  • Photos of radiators can also be revealing, radiators that contain air and need bleeding will look (and feel) cold at the top, and radiators that contain sludge and need flushing will look (and feel) 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, i.e. 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 look 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.
  • Apart from around doors and windows, draughts are also caused by hidden gaps in the fabric of the house around service pipes etc - draughts can be the equivalent of having a large window open all the time. Houses can be given air pressure tests to reveal the extent of the problem so that the draughts can be identified easily and then the gaps can be filled.
  • 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.

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Weather conditions for thermal imaging

Thermal images can only be taken under certain conditions or the images will be misleading:

  • walls must be dry so it must not have been raining recently;
  • it must not be windy or the wind will blow the heat away from the walls;
  • there must be no residual daytime heat from the sun so it should be at least 4 hours after sunset;
  • and the outside temperature needs to be at least 10°C lower than the temperature inside the house.

The photos display the time and date that they were taken, and there is also usually other information displayed such as the outside temperature and the outside humidity.

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Interpretation of thermal images

Interpretation of the thermal images is not straightforward. The photos taken are "qualitative" rather than "quantitative", this means they are for identifying patterns and differences rather than making deductions from exact temperatures - the latter type of survey would require considerably time and expertise.

Colours on different photos don't necessarily mean the same temperature, it all depends on the temperature range of things visible within the picture. Photos display a temperature scale at the right hand side ranging from very dark blue being the coldest through green, yellow, red to white being the warmest, and the scale displays the highest and lowest temperatures visible within the photo. It is important to check the temperature scale when interpreting the image.

  • Photos including the cold sky in the image 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.
  • Photos without sky in the image means the coldest temperature won't be so low and the building will be coloured in the cooler midrange part of the colour scale.
  • Close-up photos might have a narrow temperature range on the scale even though they still show the full range of colours.
  • A small temperature variation on different parts of a building won't really be important, however, a difference of 5°C might be significant - but it will always depend upon circumstance.
  • The colours will also change according to the emissivity of the different buildings materials used on the outside of the house, i.e. the ability of the surface of the material to emit radiated energy, so again, a change in colour might well not be significant.

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