Using a thermal imaging camera to detect heat loss
A thermal imaging camera is like an ordinary camera except that it is sensitive to infrared waves rather than ordinary light waves. The thermal camera is most sensitive to radiation with a wavelength in the range from 5-14 micrometres, just the predominant range for heat radiated from normal environments at normal temperatures (say between 0 and 50 degrees centigrade) such as houses, people, trees, grass etc. The digital camera actually measures the rate at which infrared energy falls on each of its pixels and uses a bit of basic physics (the black-body thermal radiation theory) to calculate the temperature of the part of the view that exposes that pixel. The inferred temperature is often expressed to an accuracy of 0.1 degrees centigrade.
The images produced are very useful and appealing. The software within the camera or in the computer to which the images can be downloaded allows various different colour schemes to be used to present ‘false-colour’ portrayals of the calculated surface temperatures. You can usually also calculate the average temperature of a defined line or area of the image. The image instantly shows up hotter or colder regions. For example, in the picture below, the region below the bay window looks blue, indicating it is quite a lot colder than the regions to left and right of the bay window that look greenish or even reddish. The temperature scale on the picture suggests a difference of as much as 2-3 degrees centigrade between these areas. This seems reasonable as we had thick mineral wool insulation installed on the inside of the wall below the bay window but the solid walls to right and left have no insulation and therefore leak more heat from the warm room to the outside world, showing up hotter in the process.
Notes for Geeks:
Queries when Analysing Images from the Thermal Camera:
1. What is the effective U value of the solid brick wall.
2. Are there any obvious thermal leaks associated with air bricks?
The image (top of this page) shows up 3 air bricks under the bay window. They are only ~1 degree centigrade above the temperature of the surrounding bay window wall. The air temperature on this occasion was 2 degrees centigrade, apparently hotter than some of the surfaces. This may be explained by windchill factors. The warm regions to the left of the bay window may be due to shelter from the prevailing north wind at the time (see also the gap next to the gas meter housing).
3. Are there variations in the heat leak through the windows?
Some images show a warm strip around the middle bar of the bathroom window which might indicate an air leak. Or does it represent shading from radiation to a cold sky? Is the warm bar above or below the wooden bar?
4. Can we estimate the U values of the double glazed windows?
5. How effective are the thermal blinds?
6. How effective are the thermal linings of the curtains?
7. How effective are the foil door covers?
8. How effective are the foil sheets behind radiators?
They seem to produce a reduction in the outer brick wall temperature of about 1oC when the inside temperature is 19oC and the outside is 7oC (27/12/2009)
9. Can we find air leaks with the thermal camera?
10. Does the front door sausage have a significant effect?
Avoid making measurements when the sun has been shining on surfaces within recent time (hours?)
Why do smooth surfaces pointing at the clear sky show extremely low temperatures when it’s dark?
The apparent temperature of the brick walls can be lower than air temperature.