Home is 15 years old. Note the new vapor barrier lapped and taped 2’ up all the walls and piers. Insulation was new also. This is NOT a conditioned crawl space.
(1st time for me seeing this type of coating on floor joists. Thought others may learn from it. I’ll tell you what I said about it later.)
Sure beats what I see when I get them.
Hey Joe
Out here they paint joists white or gray after a fire to seal the smoke smell. Did you see any evidince of charing or burn areas. Or smoke staining on any other areas.
Possible cause Might be becuase of the extension cords there in the pic. LOL
Not a fire.
Let’s see, evidence of new vapour barrier on floor, painted joists, new insulation, and painted joists, I say it was probably MOLD!
The mold remediation companies around here typically paint treated surfaces after CO2 or soda blasting them with a calcium carbonate based paint. I would see if there was ever any issues with mold in the past. They also use a white paint like Kilz or Zizbar after fire restoration work, but you aready said there wasn’t a fire.
I would say MOLD !!!
Why only 3" of the bottom? I was going to say intumescent paint but the job is not done properly. 
MOLD is my guess also, especially since the lot grade sloped toward the house on 2 different sides and since the crawl space vents were at grade level in places.
"The floor joists have been coated with a substance we suspect is an anti-microbial compound, which is consistent with previous work by a mold remediation company. We suggest that you ask for full disclosure by the current owner as to the nature, purpose and scope of the work and obtain copies of any guarantees, permits, etc. related to this activity. Note that mold growth is symptomatic of high moisture conditions. Any remediation should have taken moisture control into consideration to prevent recurrence of fungi growth. More information about mold can be obtained on our website. (Note also that the sub-floor insulation in the crawl space appeared new, also consistent with mold remediation.)
The vapor barrier in the crawl space appeared new and it was unusual in that it lapped 2 feet up all perimeter walls and piers. We suggest that you ask for full disclosure by the current owner as to the nature, purpose and scope of the work related to this activity. Our experience is that vapor barriers may conceal but not correct a larger problem related to water intrusion in the crawl space."
Well shucks I thought it was whitewash. I wonder why nobody ever uses whitewash anymore.
Whitewash = Calcium Carbonate
Mold remediation contractors use it all the time after CO2 (dry ice) or Soda (sodium bicabonate aka baking soda) blasting.
LOL i know so why not call it White wash .