I'm going to repeat myself.
With common board materials (FR4), and without active guarding, the actual board leakage will be orders of magnitude higher.
There used to be an EXCELLENT app note by National Semiconductor that thoroughly explained this.
Personally, I once saw an electrometer design, whereas the board had a teflon insert for the opamp's input terminals. The opamp itself was a TO39 package (metal case with a bottom glass-frit seal), to prevent humidity absorption in the DIP8's plastic moulding.