I found this Cummins document to be informative about DEF: https://www.cummins.com/news/2016/01...-exhaust-fluid
Below are a couple of points from the Cummins document:
Q: What’s the shelf life of DEF?
DEF shelf life depends on the temperature of your storage facility. It is recommended that DEF be stored between 12°F and 86°F, but when stored below 65°F, shelf life is extended to 2 years. It is recommended to store DEF in a climate-controlled environment and out of direct sunlight to extend shelf life.
Q: What happens if DEF freezes?
While DEF does freeze at 12 degrees F, frozen DEF does not impact the start-up or operation of the vehicle. Upon engine start-up, the SCR system provides heat to the DEF tank and lines which allows theDEF to thaw rapidly and keeps it flowing to the aftertreatment system regardless of the outside temperature.
Diesel Exhaust Fluid is formulated with 32.5 percent urea and 67.5 percent deionized water. In storage or when the engine is not in use, DEF will freeze at 12°F (-11°C). At this concentration level, the urea and the water freeze and thaw at the same rate – ensuring that you always have the correct amount of each. Engines with SCR are specifically programmed for optimal performance at this ratio, which is why it is important to make sure you get a quality brand of DEF that meets ISO specifications.
There is one operational difference to note: Like any water-based fluid, DEF expands when it is frozen (by approximately 7 percent). In cold weather, when the operator shuts the vehicle down, waiting 60 seconds before turning off the battery will allow the fluid to drain back out of the hoses and into the DEF tank.
No anti-gelling additive or freeze point improver should ever be added to the DEF, as they will impede its ability to perform correctly and may damage SCR system components.
At a Spartan Academy class in June of 2018, it was suggested that if you're concerned about DEF aging while your coach is in storage, it might be good to plan your DEF usage so that your tank is as close to empty as practicable when you put your coach into storage. Then, immediately before you start the engine as you get ready to take the coach out of storage, top off the DEF tank with fresh DEF.