Insulation will never interfere with Wi-Fi but rather attenuate the signal. If metal particals are inside the insulation this will generate some kind of cage of faraday effect. Same effect as a elevator shaft. Could you do a site survey placing a temporarily access point? First place the access point and measure the free space path loss (RSSI value). Then measure the same distance from the acces point but with the insulation inbetween. Then you will know the attenuation value.