Randomly browsing, found an OMAFRA page reporting on some "Eastern Ontario Poultry Conference, 2007" that gives 55-65% as a target humidity to try not to exceed. Probably this is for commercial barns. But still. I know some people have been looking for A Number. So, there ya go
Pat, aware there may be other numbers out there too, but passing this on as a data point

Pat, aware there may be other numbers out there too, but passing this on as a data point