Slow Rise Spray Foam Existing Walls
How blind filling a wall really goes. Spray Foam Insulation is the best because it provides a tight air seal and high R-value per inch. Whenever possible, try to see where you are spraying. Existing walls can still have foam. This video shows the pluses and minuses of blind filling a wall with Foam It Green Slow Rise kits.
Safety first. Make sure to protect your eyes and skin, have ventilation, and use the suggested p100 cartridge particulate respirator.