I built my own recording room using chicken wire to cover the whole room (roof/walls/floor) before I laid down the floor/put up the wall plates.
I also connected a ground wire to this "Faraday" cage, and ran it to the main water intake (copper) and connected it there.
If, in case this wasn't enough, I also ran a ground wire outside the house to be connected to a dedicated ground spear driven into the ground in the backyard. Haven't had the need to do this yet though.
Another thing to consider, if you are using a robot lawn mower, the control wire generates a ton of noise.
I had to put mine on a timer to get rid of all the static noise...