About 7 years ago we had open cell retrofitted in our 1940s attic. At the time we were given the option of OC vs CC and went with OC based on cost. Sounds like that isn’t recommended any more due to possible moisture rotting the sheathing. 

Based on some research here, some have success with it, some don’t. Some attribute it to cheap OSB, maybe I got lucky with my 80 year old sheathing?

What dew points are considered acceptable in the attic, and how long does it take for wood to rot? I’ve been seeing DPs of 30-40F right now in the unconditioned storage area at the ridge peak, would that offset summer months? 

kevin