Serious HH campers need some serious insulation.

World Famous make a sleeping pad with high density foam on one side and foil on the other. It might require a trim to fit into your Expedition. You will feel the heat reflecting from your body the moment you lie on it. To maximize your comfort, especially when camping in the shoulders (spring/fall), wear polyester fleece next to your skin. There is still not a substitute for a good goose down mummy sleeping bag.

Anyone who needs more info should visit my stealth camping journals.