1. In the first case, the aluminum fins do a good job of transferring more of the available energy from the hot water out of the pipe. In particular, aluminum fins place thermal energy directly on the underside of the floor to be heated, whereas ducts without aluminum fins primarily heat the air in the cavity and then transfer the air to the subfloor through inefficient mechanisms of warm air convection and conduction . In the case of 5/8″ tubing, a heat output of 15,124 btu/HR was observed at elevated temperature, while the heat output for 1/2″ tubing was only 10,955, or a 38% increase. However, at lower temperatures (118F vs. 145F), the improvement was less pronounced, and the improvement dropped to about 11 percent. It’s important to note that the comparisons aren’t quite the same because the 5/8″ tubing is slightly larger than the 1/2″ tubing, but other studies comparing tubing only show that the positive results are mostly attributable to aluminum.
2. Another very important factor in the design of radiant heat is to control the so-called “return loss”, ie the heat energy in the wrong direction. Those who would say that heat loss downwards are irrelevant because heat always rises are misguided. The joisted floor application does control heat losses to convection and conduction to near zero in a natural and efficient manner. However, radiant heat can travel in all directions.
Practical experience and these data strongly suggest that if the radiative heat loss in the downward direction is not well controlled, the results will be unsatisfactory. When heat is lost down to another heated space, that heated space may actually be overheated and the intended space is not sufficiently heated. These data strongly suggest that most of the thermal energy provided by the 1/2″ PEX tube without aluminum fins is going in the wrong direction. This situation must be corrected by adding some kind of extra insulation expense. Unfortunately, this This remedy usually does not occur.
An underappreciated advantage of aluminum thermal pads is its low emissivity (nearly zero). This means that when aluminum is heated, it does not emit radiant energy like other materials. In the present case, this characteristic is well used to control the radiation “return loss” which, if left unchecked, can seriously damage the system. The superior effect of the low emissivity is clearly demonstrated by the thermal image, which shows that there is almost no downward radiant heat loss when aluminum is used.
In particular, the thermograms (below and left) are instructive. Infrared cameras simply “see” the radiative heating spectrum, which is then mathematically corrected to predict molecular temperatures from the measured radiation. This method works for almost all materials except aluminum. It can be seen that the aluminum thermal fins are blue in the temperature record, and the falsely reported temperature is only 67 degrees, when in fact it is 106 degrees. It is worth noting that the actual infrared energy emitted by aluminum is almost zero. The camera is recording reflected radiation from the environment below. These effects reduce the radiation loss of aluminum to near zero.
3. Another useful property of aluminum is that it reflects radiant energy that strikes it from another source. An additional layer of aluminium foil placed under the radiant floor will reflect any misdirected radiant energy. Fortunately, this property can be achieved with extremely thin aluminum foil, which is often bonded to reinforcing paper for strength. This work demonstrated early on the value of foil-faced reflective barriers. Radiant energy that would otherwise be misled as return loss is reflected upwards to the subfloor, where it is useful to the occupants. The infrared thermal image clearly shows the improvement of the foil-backed material compared to plain paper.
