Mold on floor joist in crawl space.

Household bleach is already diluted, commonly 3 to 6 per cent sodium hypochlorite: so 20% bleach in water applies less than

For killing bleach a 50% solution is recommended. I frequently use 100%. You might add a wetting agent. Perhaps dishwashing soap.

Maybe its doing its job the runoff is excess, did the wood look alot different after an hour. Maybe some soap like dish soap

The CDC endorses a mixture of 10 parts water, one part bleach. Assuming that a person is gonna be using 5% sodium hypochlorite as the base solution. Half cup per gallon of solution is close, then apply it with a misting sprayer, and give it plenty of contact time.

Mold thrives best when the humidity is 55% or higher. Given good thought towards correction in that direction.

give your mix a shot of dish soap. Or buy some anabek and do it properly.

Soap. I use about 3/1 dilution when using bleach for mold as well. I don't care what the cc says works. Get your self a good vapor mask too if you're ding this in a crawl space. And get the heck out as soon as you finish spraying.

Dry out the crawl space and the mold will die. Kill the mold without drying the crawl space, and the mold will come back as soon as spores blow in, which will be tomorrow. Someone else mentioned 55% RH; I heard 70% when I had issues. Buy a remote-reading thermometer-hygrometer (there are at least a couple available) so you can track it easily. Consider sealing the crawl space to keep it dry

-- good in many circumstances but not all.

Edward