Some years back I was working for Panasonic on their giant LED screens, trying to fix EMI issues. At one point I decided to try a graphite based acrylic EMI paint on the back of the panels.
It was without doubt THE best EMI bandaid I've ever tried - broadband attenuation of 25-30dB. The coating has a sheet resistance of ~100ohms/square, so I suspect its effectiveness comes from being dissipative, compared to metal coatings which are more likely to be reflective.
So the paint definitely works, and works well. Though the only effects from painting your home with it are going to be total lack of cell/wifi/gps coverage, and a deep hole in your wallet.