You don't even need a theoretical blanket to do this, just a hot/humid enough environment. When the air is hot and humid enough that evaporative cooling no longer works human bodies can no longer dissipate heat and bad things start happening. This is called the wet-bulb temperature. Basically when the wet bulb temperature (thermometer whose measuring point is covered by a pocket of water) is equal to the dry bulb temperature (thermometer in open air) it means the air is fully saturated. A human in the shade with unlimited water could not survived more than a few hours when wet bulb reaches 35 °C (95 °F).