charcoal and powdered iron if I can find it cheap.
the cheapest i have seen from browsing around is $1/ 25kg graphite, $10-ish /1kg FeO4. there are also ferrite powders about $16-ish / kg iirc. as usual, the source i browse from is china.
have you started? if not i thought i would persuade you not to do it. instead make a active-climatized equipment rack, far smaller and could be moved to other locations.
I agree with 3roomlab, I have done shielding not one layer but two layer (steel and copper) the improvement with noise etc under 500Khz is not worthy compare to cost.
My primary concern for the room is blocking AM and FM broadcast signals. The stuff below 500 kHz is primarily local devices and hopefully any I have not squelched will be too far away to matter.
For a variety of reasons, I'm going to do the initial build out of the room with just the steel studs, 26 gauge steel covered with 1/4" dry wall and paint and ESD vinyl flooring. I'm also going to cover the bench top with the ESD vinyl flooring. The bench will be a pair of 7' x7' x 4' sections on casters, each with 3 toggle supports that will lift the bench off the casters and level it.
Magnetic fields below 500 kHz really require mu metal or thick, low carbon iron. That's only practical for a small enclosure because of cost. I have one which is still a WIP. At present it's a pair of steel chassis with piano hinge, EMI gaskets and a pull down hasp. Still to be added is the Peltier heating and cooling, the PCB mount, thermal insulation, feed thrus and such. The steel box is 4" x 7" x 11". Once my new lab is set up I'll be able to test various shielding options fairly easily. For example, I can get 1/2"- 3/4" low carbon steel plate to cover the surfaces and measure the performance. Then toss the plate in the steel stock pile and substitute the equivalent amount of mu metal if it proves beneficial.
With respect to internal room reflections, I plan to do TDFD modeling to evaluate room noise with various levels of attenuation on the walls and also to test the effect of building the room out of square.
I found powdered iron for $7/lb which is 1/3 of what I found before. But it's still expensive enough to merit testing the actual performance before spending the money. While I can't get accurate measurements of dielectric and permeability constants with a DIY fixture, I can make relative measurements which will suffice.
Henry Ott's EMC book has lots of data from experiments he made on the effectiveness of various shielding materials. His writing is atrocious, but there is a lot of good information.
Reg