Please provide me with references demonstrated via hard science and research supporting an air space. If possible, I’d like to know if science exists to support any specific space depth for a given IR wavelength.
Answer:
References:
1) Thermal Radiation Heat Transfer by Robert Siegel and John Howell
Chapter 8 “Radiation Exchange in an Enclosure Composed of Diffuse-Gray Surfaces”
2) ASHRAE Handbook of Fundamentals 2013. Pages 26.12-26.15 contain
Discussion of plane air spaces.
3) Materials for energy efficiency and thermal comfort in buildings Edited by
Matthew R. Hall. Chapter 12 “Reflective insulation and radiant barriers for insulation in buildings.
4) ISO 6946 “Building components and building element-Thermal resistance and
Thermal transmittance. (International Standards Organization)
5) Data Base of Measurements
- Robinson and F. J. Powell “The Thermal Insulation Value of Air Spaces”
Housing Research Paper 32, United States National Bureau of Standards (1956)
Andre O. Desjarlais and David W. Yarbrough “Prediction of the Performance of Single and Multi-Airspace Reflective Insulation Materials” ASTM STP 1116”
Pages 24-49 (1991). (American Society for Testing and Materials)
6) W. P. Goss and R. G. Miller, “Literature Review of Measurement and Predictions
Of Reflective Insulation System Performance; 1900-1989”. ASHRAE
Transactions 1989 Vol 95 Part 2
The above references will provide an introduction and some insight on the use of enclosed reflective air spaces as thermal insulation. The spectrum of interest is usually taken to be wavelengths from 0.5 to 50 microns. The radiation emitted by surfaces in the building envelope are in this range. The emittance of aluminum foil and metalized films varies some with wavelength in the above range (but not much). The emittance that is used in design calculations and product evaluations is an average value for the above range. The radiation coming from an element of area includes a spectrum of wavelengths.
The AI-space dimensions tend to be 10 mm and above. Small gaps would not be useful because the resistance to heat transfer by conduction would be small. The radiative component (in one dimension) does not depend on the separation distance (see Stefan-Boltzman Equation)
R = l / ka. Evaluations and design of reflective systems includes heat transport by all mechanisms. Measurements involve total heat transfer across the air space in question. I doubt that “tuning” emittance for a particular set of boundary conditions would be commercially useful. As you will discover, we know a lot about how to adjust spacing. The use of polished aluminum of metalized film to provide low emittance is pretty much universal. There are isolated efforts to use other metals, but cost is an important issue.
There is a vast literature on the subject of radiative transfer between surfaces and the design and performance of reflective insulations. The references listed above will provide an in-depth introduction starting with textbook discussion of radiation.
The key issue from a commercial point of view is cost. Building insulation is an inexpensive material.