Vapor barrier or no vapor barrier with ISO board?

[NavyBlaq]

I'm planning to add 3" ISO board for better insulation, but I'm a little stuck on what to do with what's already in place. Currently, there's fiberglass insulation with a vapor barrier, and I'm not sure if I should completely remove that layer or just cut it open so it doesn't trap moisture once the ISO goes up and you see moisture buildup is the main thing I'm worried about. I don't want to go through the effort of upgrading only to end up with condensation again.
Anyone of us here layered ISO board over existing fiberglass? and did you keep the vapor barrier intact, slice it open, or strip it out completely?
Thanks

 

[Ghost_Nephilim]

If you leave the old barrier untouched, you're basically sandwiching the fiberglass between two plastic bags. That's asking for condensation. Either cut it or rip it out, just don't leave it sealed. ISO board with foil facer will do all the blocking you need.

 

[slvr22_bd]

I'm planning to add 3" ISO board for better insulation, but I'm a little stuck on what to do with what's already in place. Currently, there's fiberglass insulation with a vapor barrier, and I'm not sure if I should completely remove that layer or just cut it open so it doesn't trap moisture once the ISO goes up and you see moisture buildup is the main thing I'm worried about. I don't want to go through the effort of upgrading only to end up with condensation again.
Anyone of us here layered ISO board over existing fiberglass? and did you keep the vapor barrier intact, slice it open, or strip it out completely?
Thanks

I agree with @Ghost_Nephilim. Leaving the old plastic intact would be a major mistake, as it will trap moisture and cause the very condensation issues you're trying to avoid. The best and safest thing to do is to completely remove the existing fiberglass and its vapor barrier. If for some reason you can't remove the old insulation, you should slice the existing vapor barrier open in several places so moisture can escape. But really, the best practice is to remove it completely.