Question: I have two thermometers in my attic but one is always reading about fifteen degrees hotter than the other. When I bring the thermometers inside, both read the same temperature. What is going on?
Measuring Air Temperature and Surface Temperatures
This example illustrates perfectly how radiant heat can affect surface temperatures. There is a huge difference between the air temperature inside your attic and the surface temperatures inside your attic.In your attic, radiant heat being absorbed by your roof is re-radiated across the attic space until it is either reflected or absorbed. Assuming you do not have a radiant barrier installed, your attic can measure two different temperature readings: the air and the surface. Generally surface temperatures will be hotter than air temperatures. An example of this is parking your car in the sun. You can leave the windows down and have great air flow (ventilation) and the air temperature inside the car will be pretty close the outside air temperature. However, if your seats are in direct sun, then they will increase in temperature well above the ambient air temperature.
Why Do the Thermometers Read Different Temperatures?
Most likely this customer had the thermometers placed in two different areas in that attic. One of the probes was probably in the direct line of the radiant heat (causing it to read a higher temperature) coming off the roof. The other probe might have been behind a board, or under another material, causing it to not absorb as much of the radiant heat off the roof.
How to Measure Temperature Accurately
In order to get an accurate reading, you need to make sure your thermometer is placed in an area that is out of the line of direct radiant heat. A simple way to do this is to get a piece of aluminum foil (kitchen foil will work) and create an upside down cone that you can place around the probe/thermometer. Doing this will block the line of direct radiant heat that is heating up the probe's surface, causing it to read a high temperature. In this way, the probe can accurately measure the air temperature alone, and not the radiant heat.
Another way to think of this is to consider you had a glass of ice outside and set it in the direct sunlight – the ice cubes would melt quickly, because they are heating up from direct absorption of radiant heat. If you were to place the glass of ice in the shade instead, it would take a lot longer for the ice to melt because the surface temperature in the shade is cooler than the surface temperature in the sunlight.
Adding a Radiant Barrier to Your Attic
Just like adding the foil around the probe prevents it from absorbing radiant heat and increasing its surface temperature, the same is true for your attic. Adding a radiant barrier will block 97% of the radiant heat from entering the attic via the roof. This easy addition will create a cooler attic overall as well as decrease the amount of heat being absorbed by your insulation, which in turn affects the comfort of your living space. By placing a radiant barrier in between the radiant heat source and the insulation on your attic floor, you can regulate the surface temperature of the insulation and help it stay cooler, making your home much more comfortable in the summertime. In the winter it works the same way by keeping the heat inside your home from escaping to the cold attic and eventually outside. For more information on installing a radiant barrier, check out the most popular installation methods.