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This might help, on page 58 table 2.1 CWNA official guide 4th edition shows the RF absorption rate by common material.

If you need more I found a very detailed study on RF absorbtion of construction material at the NIST site:

http://www.fire.nist.gov/bfrlpubs/build97/PDF/b97123.pdf

(it is a 200 page pdf about 5 Mbytes, will cure your insomnia!)

thanks