The problem with WiFi reception isn't so much signal strength, rather that all the channels are in use by people in the neighborhood. The WiFi analyzer picks up a vast number of signals from houses up to 250 meters away, so the best way to improve the signal is to block all these other WiFi signals. That's why I'm considering using aluminium foil wallpaper, my own WiFi signal comes from below, so this will move through the ceiling and the floor of the room I'm in.
What I'm concerned about is that aluminium foil will block water vapor, so humidity may build up on the wall causing fungus.