Can I see Radiant Energy…?

No, but you sure can feel it. The following “hot box” will help!

Short of finding the sun and setting up the experiment in the winter outside; the following review should help identify the dramatic amount of heat that is created by radiant energy, and how it conducts to all adjacent areas quickly. Simply, heat seeking cold, quickly!

First, the heat source is “ignited” and aimed at the thermometer on the right side. The heat source is pointing down, so little to no heat is conducted, and little to no heat is convected to the box area. The only material between the heat source and the thermometer is a radiant barrier (AKA: 98.7% pure shiny aluminum, on a double bubble film. The product is 3/8” thick).

Since this is a test, the sun like bright “spot” is a 200 watt “heat lamp” that emits radiant energy that can raise the temp of any surface it “hits” to 350ºF in about 10 seconds. The timer is set on zero, and the room temp is 70ºF. The white’ish material is the bottom of the double bubble foil. The “pile” of pink is Fiberglas, waiting for the next steps

In the opposing picture the only thing that has changed is the timer….. 58 minutes passed and the thermometers read about 70.5ºF. So, no heat was conducted to the thermometer, no heat was convected to the thermometer, and no heat to speak of was transmitted radiantly. You guessed it. The radiant barrier did its job; reflected about 97% of the radiant energy “back to sender”! What is interesting is the foil surface reads about 145-160ºF, more on this later.

	This Picture is the beginning of the experiment. The steady state is: both boxes at 70ºF, and the timer at zero. The pink Fiberglas has filled the left box, two 6½“ blankets. Ready, set, go!
Off we go. After 20 minutes the pink thermometer is just below 100ºF, and the Foil blanket thermometer is at 74ºF, Recall that some of the heat in the pink box is conductively “creeping” into the foil box, they are connected, the foil box thermometer is “reflecting” this phenomenon. Ok, let’s see where we are after …..
	40 minutes….. The thermometer under the pink Fiberglas reads 104ºF ( and it is worth noting that the surface of the Fiberglas remains at 350ºF) and the foil thermometer reads 75ºF. A bit more conductive creep.

Well, what does all this mean? First, the source of heat is the conversion of energy to heat, radiant to heat, fuel oil to heat, gas to heat, wood to heat, etc.. The first law of physics is you cannot loose energy, you can only change it! So, the radiant energy that is present in all buildings and from the sun is going to hit something and be converted to heat. At that point, the more familiar heat effects begin. Second, the conduction of heat begins, just as we observed in the experiment on the pink side. And third, the convection of heat occurs as air washes against warm surfaces and rises. This kind of sounds like … weather. 

Materials that are present in most building systems, wood, brick, glass, steel, aluminum, and Fiberglas, etc. have a characteristic called emissivity. This is a term for “does the material absorb or reflect heat”. The larger the number the more energy the material absorbs which is converted to heat. Well, Brick, wood, steel, shingles, concrete, stucco, rock, tile, have an emissivity of .95+/- a few points. AND, aluminum has an emissivity of .03. This means that all except aluminum absorb 95% of the energy and converts it to heat, while aluminum only absorbs 3%. Aluminum reflects 97% of the energy that hits the surface. Conclusion: why not manage the energy, rather than the heat it produces…? In the experiment, this is what the foil did; it reflected the heat from the source back to the source. If this was the inside of your house, the heat would be reflected back into the living space; and if this was the outside of your house the sun’s “rays” would be reflected back to …. Space.

This leads to R-values. All materials have an R-Value. The R stands for Resistance to heat flow from hot to cold (Physics again: Heat always seeks cold, and warm air rises). In order to engage R-Values in materials, the building must produce heat and the heat is properly conducted to colder places, I.E. Outside. In the process the heat being conducted to colder places warms everything it is in contact with, studs, walls, ceiling, floor joists, and the insulation. Since all of the building materials have an emissivity of .93 -.95 the entire house is absorbing and moving the heat to cold. The experiment showed that Fiberglas did a nice job conducting heat to the Bottom of the box. The heat could only have been conducted since hot air rises and the heat source was above the Fiberglas.

Now we move from winter to spring and summer. We notice that the sun is higher in the sky, better for a direct hit on the earth and its objects. Your roof is a great target. The roof surface like your cars hood can easily reach something south of 200ºF ( Don’t we all have foil windshield reflectors stuffed between the wheel and the glass…?), and that heat is conducted to all parts of the house connected to the roof. The attic heats up, the Fiberglas heats up, the framing heats up… from top to bottom! This is why at 3AM you are still benefiting from suns absorbed heat in your second floor bedrooms, or through out the house in single story structures. The absorbed heat is the gift that keeps on giving… What is interesting is, the top of your house is heated up, and like the experiment, the only way the heat gets to heat the room to an uncomfortable level is by radiant heat formation.

At this point, the folks in the southern warm climates are wondering: Air Conditioning or Foil, AC or foil, AC or foil?…..  And the folks in the northern cold climates are wondering: more Fiberglas or foil, more Fiberglas or foil, more Fiberglas or foil. Well, we observed in the experiment, 13 inches of Fiberglas (approx. R 38) reached 100ºF plus in 20 minutes, while the radiant barrier reached just 74 ºF in the same period of time. "R-Value" describes the "resistance" to heat and measures the rate at which the heat flows through the insulation products. Many of these insulation products absorb a very high percentage of radiant heat (emitting about 90%). Radiant barriers on the other hand, completely block a very high percentage of radiant heat (reflecting over 90%). Is it any wonder that radiant barrier technology is used by NASA and has been recommended in residential construction for over 60 years? The next discussion is all about … well, if I can better reduce the heat entering my home in the summer, why do I need a higher tonnage HVAC?? And if I can better reduce heat loss in the winter, why do I need such a big furnace/boiler??? AND tell me more about the cost of cool roofs. This discussion has legs…!

Ain’t life grand!