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What Does the Surface Temperature Really Mean?

We know the direction of heat transfer is from hotter to cooler, but sometimes the temperatures we measure can be misleading. Particularly, when the process of transfer is still in transition, the temperatures will not have peaked yet. Also when a surface is being strongly influenced by outside forces—such as evaporation, sun, wind or clear sky cooling—the temperatures we measure will probably not represent the true nature of transfer through or from the materials involved. If we don’t recognize these influences or understand their affects, we can get into trouble fast!

In both images the top of the wall is being shaded by a large overhang and the lower part of the wall is in a thermal transition due to solar loading. In the left image the insulated (3.5” dense-packed cellulose) cavity has warmed up more than the framing. A few hours later the framing has warmed more than the insulated cavities. Although the framing is more conductive, and by rights should have warmed before the insulated cavities, it also has a much higher thermal capacitance and thus requires more energy to reach a higher temperature.

We encounter this sort of dilemma in buildings during both the summer and the winter. For example, when solar loading puts a wall into a transient condition, temperatures and thermal patterns often reverse themselves. Long ago, I learned this lesson rather painfully when I told a contractor that the north wall of building was insulated while the south wall was not.

What happened?

I had assumed the all the walls were in steady state, but they were not. It turns out that during any time of the year, as the sun heats the exterior, the direction of transfer can fairly quickly change move toward the interior. When this happens, an uninsulated cavity will heat rapidly, but surprisingly in most situations that will be followed by  the insulated cavity warming next. Last to warm will be the framing, a fact that at first seems counterintuitive because it is much more conductive than the insulated cavity.

The explanation, of course, is that the thermal capacitance of the framing is much greater than the insulation, i.e. we have to wait for more heat to be transferred before the same temperature is reached. So depending on when we show up to inspect the wall, it may appear insulated or uninsulated. Even more insidious is the fact that in some instances the wall will be so uniform in temperature as to appear to have no framing!

What can you do to avoid trouble?

I find it useful to watch temperatures. If they are changing, it can give me an indication that transfer is still transient or that unseen influences are at work. A “cooling curve” can be a great way to portray and see these sorts of things.

Try this simple demonstration. Pour a cup of coffee into a thick mug and, using your imager, every 60 seconds measure two different temperatures—the surface of the mug and the surface of the water. When you plot a cooling curve graph of both temperatures over time, you’ll see some interesting thermal relationships.

The mug will increase in temperature until it reaches steady state, at which point it will begin to cool to the temperature of the ambient surroundings. The hot water begins cooling immediately and will continue to do so until it too reaches the temperature of the surroundings. But the interplay of the two curves with each other is both fascinating and educational and can help us learn how to better understand temperature and heat transfer in our real work. If you have a wall in your house or office that faces the sun, you can learn even more by watching it (from the interior) as it goes through a complete thermal cycle!

Temperatures can be either meaningful or misleading. If you understand heat transfer conditions and direction of flow, temperature is your friend. If you just blithely measure temperature and think it means something, you’ll likely end up in hot water, no pun intended. Next week we’ll talk more about transient heat flow, especially in buildings, and the particularly disturbing trend among some in the industry who believe they can calculate R-values from thermal images.

Thinking Thermally,
John Snell—The Snell Group, a
Fluke Thermal Imaging Blog content partner

3 comments to What Does the Surface Temperature Really Mean?

  • In Ukraine we have many houses constructed of bricks. And most of new houses are constructed with concrete and bricks. Sometimes the walls are quite thick. As a consequense, they have quite high heat capasitance and change their state quite slowly. For example, in the house, where I live, the thickness of the walls is about 60 cm, and the wall itself is made of shell limestone (shell rock). So though during long periods of stable temperatures I can not boast about very high insulation properties of the wall itself, but the changes in outside temperature becomes noticeable inside in about one day or more. As I observed the behavior of my appartmen, this is mostly due to high capacitance and significant mass of the wall, as all heating tubes and radiators display themselves quite good on thermal image of external wall made from outside.
    So almost every time during survey I usually ask myself, if I will be lucky enough to make right conclusion about direction of heat transfer in every case :). And definitely recommended two-three hours to heat up (or cool down) the walls to make defects visible are not enough if you make survey during summer time. Sometimes it is necessary actually day or two untill the direction become as it expected.
    Sometimes I had situations, when after quick change of weather, though I could see defects, but the walls of the buildings were COLDER, than air, which couses some confusions by the customers.

  • Many thanks for your fine comments. They serve to remind me I need to broaden my thinking to include other types of buildings than the wood frame ones I generally deal with.

    You are correct that the “thermal lag” for a 60cm masonry wall can be substantial and a cause of great confusion! What kinds of defects might be in such a wall? Is there insulation in the wall?

  • First, 60 cm of brick is not enough for appropriate thermal insulation according to modern regulations. But old buildings usually have no additional insulation.
    Then, there can be cracks, where air can come into the house.
    Most defects are usually inside the building: bad windows, the doors which are not tight enough, wrong plastic window mounting without insulation on perimeter (almost all, to be honest), cold air coming through floor and ceilings, old “natural” ventilation, which leads to high heet losses (if you give me an e-mail, where to send images, I can collect some of them and send to you), heating radiators without shield from inside, heating tubes installed directly within walls… And of course, just poor quality of construction works. It’s common in our country, that people, who purchase or get an appartment, have to put many costs to correct the problems which were not eliminated by construction workers, like gaps between walls, wetween wall and ceiling etc.
    Many of these problems originate from wrong construction or wrong work done many years ago, and correction requires large costs.
    By the way, most of our houses are made of brick or concrete, so sometimes the correction is not so easy…
    Due to such large amount of problems, sometimes I even think, that thermal imaging is not critical yet, as most of problems are obvious. What I can do is to work with people, who build new houses (very often with the same problems and same mistakes) or tried to correct existing problems and want to check how effective were the corrections.

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