Any one have insight on why a higher mash efficiency raises IBU's?
But to go back to his original question -
We have only answered why a higher efficiency decreases hop utilization and therefore IBU's. I do not know of any correlation between increased mash efficiency and INCREASED IBU's. Can you give us a specific example with your software? How much of an increase in efficiency is needed to bump the IBU's? I am on the same thinking with Spider and Dirk, that if you increase your efficiency then the IBU's decrease by a touch (usually not much, but a tad). At least that is what I have noticed on my brewing software too (ProMash).