If you have ever entered an attic in the middle of summer, you have experienced first hand the sauna created by ordinary roofing materials under the hot sun. This is especially true in Houston, one of the most air-conditioned and energy hungry cities on the planet. Even with several inches of fiberglass insulation on the attic floor, there must be considerable heat flowing into the living space below. I decided to run some tests with the help of a thermocouple meter and several thermocouples, a styrofoam board and some sample roofing shingles.
I used a Doric Trendicator Thermocouple meter and Thermocouple switching box (Figure 1). This particular unit displays from below freezing to over 400 degrees F using "type K" thermocouple wire. Type "K" is really a pair of wires made from two dissimilar metals, pure iron and a nickel alloy, Constantan. The ends of the two wires are bared of insulation and the bright and shiny ends twisted tightly together to form the measuring junction. The other end of the thermocouple wire pair is connected to two of the terminals on the Thermocouple switch box. Two thermocouples are prepared to allow a comparison between differing roofing materials at the same time. A third thermocouple can be suspended in the shade to give an ambient temperature reading. Each thermocouple wire junction should be tested first in ice water and then boiling water to calibrate. The accuracy is within one degree F for this meter. Blue dots mark the sensing ends of the thermocouples.
I taped the thermocouple wire to the broad face of the white styrofoam board along a central axis with clear packing tape, leaving the junction ends free of tape.
The pieces of roofing I used were approximately 8 inches on a side and were laid flat against the styrofoam centered over the thermocouple junction.
I tried the following roofing material:
SCA 121 degrees F
WTA 114 The coolest is Bone White.
It is easy to see that there is a 50 degree F (28 C) difference just underneath these almost identical aluminum shingles. The surface coating is the only variable. There is a 7 degree F difference between the shiny aluminum and the white, the white being cooler.
Those who want to do the heat transport equations to determine how much of this heat is transferred to the habitable areas of a building and who wish to figure the savings in cooling costs can follow up with a good text on heat flow. Others can determine why the current economic and political structure discourages these simple discoveries from being implemented on a wide scale to save many megawatt-hours of energy.
A recent flight over Houston in a commercial aircraft has shown the vast majority of roofs to be made of dark, solar heat absorbing materials.
To be tested:
Flat sheet aluminum spray painted with ordinary flat white paint with titania or titanium dioxide being the white pigment of choice.
Also to be tried are both new and aged black composition tile, black "tar-paper", white "gravel" composition tile, galvanized iron and "galvalum" sheet, both new and tarnished. Other white roof coating membranes used by industrial roofing contractors on large facilities can be tested for comparison.
The coating of these surfaces with dust and "mildew" and methods of keeping these and other surfaces clean as they age to prevent increases in solar gain are areas of improvement.
The lowest heat gain in roofing material is achieved with a metal
roof with a "bone white" Teflon (Tm) or Tedlar (Tm) coating with inorganic
titanium dioxide white pigment. These fluoropolymer coatings have a very
long life and do not encourage mildew.
For further discussion contact:
713 523 0515