That should be better since the oil and salt from a chip bag would be not so good for electronics. These insulated sandwich bags (I have a few) are plastic lined and appear to have an aluminized exterior.

Test 1: Put my iPhone in said bag while it was connected to a cell signal (2 bars) from outside and then I called the phone. The iPhone lost the 2 bar signal, so it started searching and finally found my network extender at which point it had 4 bars and the call went through on the final ring before it kicked over to voice mail. So the bag alone seems to block weak incoming cell signals, but not strong signals.

As for GPS, I placed my Garmin Oregon 600 in the same bag and left it outside long enough that it should have had a solid location and after 5 minutes it still had trouble locating satellites (zero satellites showing). Outside the bag but inside the house under a steel roof, the Oregon GPS took a few minutes to find 4 satellites; in 30 seconds back outside it had 9 satellites. Normally this receiver locates satellites and determines its location to within 20’ very quickly even from a cold start, so I’d say the bag was successful at blocking GPS.

Bottom line: the bag alone seems to work for weak cell signals and GPS, but not for strong cell signals. Next test will be to redo the strong cell signal with a layer of bubble wrap between the bag and an outer layer of aluminum foil. Film at 11.

For the record, no affiliation with any company named other than as a customer. The sandwich bags were purchased for my lunch.