AFIK, the rules for RF screening a room require the aperture between the conductors(left to right AND up to down) to be a small fraction of a wavelength at the cutoff frequency. Since most rebar I have seen is about a foot apart, it might work at frequencies below about 100Mhz (3m wavelength) , but not well at 1GHz (0.1m wavelength). Another problem is that rebar is usually rusty, and probably does not make good electrical contact to the crossing piece of rebar.