It shouldn't have much effect, if any. There will be some degradation of the signal, since any radio wave will lose power going through a solid medium, but it would be such a tiny amount that I doubt the effect could be measured.
Lets face it, your wifi signal is already travelling through walls etc. Right now, mine is travelling through a double thickness wooden wall, an insulated steel exterior wall and a truck window. The router (a not very new Linksys unit) is around 100 yards from where I'm sitting, and I have a good signal.