Overall this vehicle I believe is very reliable however I would argue this problem alone can cause irreversible damage if not caught in time and seems to be a very common issue among older Volvo XC90s. The sunroof drainage hoses can cause the vehicle to have water build up at the bottom of the vehicle that can fry modules and damage wiring. A very costly repair. If you hear water at the bottom of your vehicle take it in to be repaired immediately or risk water rising high enough to damage modules and wiring.

If you aren't able to get it taken in for repairs you may be able to stop the flooding by replacing the L shaped rubber hose. There are a total of four for the sunroof drainage system. The first two are under the front A pillars the ones with the hand grips on it. Take the pillars off there should be two screws under the tabs holding the hand grips on then pry the panel off and there should be a L shaped hose at the bottom of the pillar. The other two are at the rear C pillar under the lower C pillar panel and foot step panel. Pry it back a bit and you should see the rubber L shaped hose. If it's decayed and not attached to the body then water is pouring out. If all of the hoses are connected then it could be clogged at the sunroof area. I haven't been able to see where the hoses connect so can't give any insight on those areas.