Just bought some foil backed foam underlay to put under laminate flooring upstairs. Then realised the wifi router is in the room below. I’m sure foil will block a bit of the wifi signal, but is that 1% or 99%? Anyone done this?

It will attenuate the direct signal, but the wifi will still be able to propagate around it. Exactly how much attenuation is impossible to say without modelling your house. You can always add another wifi ap, or use a power line repeater to improve coverage upstairs if necessary.

Thanks – I’ll just go ahead and use the underlay. I’ve realised I already have a Cat 5 cable into the room so an AP would be easy.