I picked up some frozen food at the grocery and need it to survive a 2 hour car ride. so I got a couple of those insulated thermal grocery bags for a few bucks. do I need to add ice to the bags for the trip or will the bags alone keep the food frozen for 2 hours?
It depends on the actuall temp of the items when you put them in the bag(s) and the temp of your car where you carry the bag(s) - backseat or trunk.
If the items are to be prepared/consumed upon your arrival, I say go w/o ice.
You can keep the items colder by putting a blanket on/around the bag(s).
Be careful about where you store the food. Obviously it will be warmish inside your car, but items in the trunk may be warmed more than you’d think. In the last car I owned, the exhaust system running under the car would definitely leave warm areas in the floor of the trunk, and would warm items left there. You might want to experiment a bit to ensure safe transport.
The bags should work pretty well for a couple of hours, as long as there’s enough frozen food in them, and you squeeze all the air out. But it doesn’t hurt to add more ice. Try to keep the bag on top of other insulating materials, and not in direct contact with a highly conductive surface like metal, or up against a window.
FWIW, I did end up putting about 3.5 pounds of ice in each bag (the ice was wrapped in a plastic bag) along with the food and the trip only took 1.5 hours with the bags in the trunk. However when I got to my destination the ice hadn’t even started to melt. So I probably could’ve gone 3-4 hours with that setup. These thermal bags are awesome, I’ve seen them in stores for years but never bought one before now.