The nature of radiative transfer changes drastically as the gap gets close to
the wavelength of the peak of the distribution. There are many references on
these the enhanced transfer takes place due to tunneling. You can check out my
dissertation "Thermal applications of microbridges" chap8 or read:

 1. D. Polder and M. Van Hove, "theory of radiative heat transfer between
    closely spaced bodies",
 2. Phys. Rev. B, vol 4, no. 10, pp. 3303-3314, Nov 15, 1971
 3. C. M. Hargreaves, "Anomalous radiative transfer between closely spaced
    bodies",
 4. Phys. Lett., vol 30A(9), pp. 491-492, Dec. 1969.

the heat transfer may be enhanced by several (3) orders of magnitude.

Other weird things that happen is that the emissivity is difficult to predict
of you are dealing with a dielectric body.

C. H. Mastrangelo
University of Michigan