Is their a theory that the foil under laminate flooring interferes with WIFI (wireless network) singles ?

Question

Or the foil under the laminate flooring blocks some signals making them weak ?

Answer 1

Almost everything deteriorates the signal.  Including air.

Distance, structures, people, cars, everything.

So yes, the foil would make it weaker, along with the floor itself.  But maybe not enough to notice depending on the distance.

Answer 2

Nope i have foil under my laminate flooring (covers my whole house) and my signal (all my computers on the 2nd floor of the house and the ones in the attic) is always excellent. my floors and walls are full of foil and a whole load of insulation (we have a very cold house).

Answer 3

not that I ever noticed. we have foil on wires to keep baby cat from biting them.

our dsl is still fantastic