After gutting my bathroom I discovered that the exterior wall has no insulation, which makes me think my house isn't insulated at all. I'm not sure what to call it exactly but the exterior planking, the layer beneath the siding, is actually drywall. Is this normal? I'm not sure if there is a vapor barrier. Will it be a problem if I install isulation and new drywall on the inside if there is no vapor barrier on the outside of my house?
You do not want a vapor barrier on the outside of the wall, you want it on the inside. Go ahead and insulate, then tack your vapor barrier (4 mil plastic sheeting) onto the studs and fasten the sheetrock over that. If you're thinking of housewraps (Tyvek) as vapor barriers, they are to block air infiltration and actually let moisture escape. You always want your vapor barrier on the warm side of the wall. Never put a vapor barrier on both sides as you'll end up trapping moisture in the wall leading to bigger problems.
I have always been told that Greenboard (sheetrock used in bathrooms) can not be installed over a vapor barrior. I know that it's kind of a grey area when you are talking about exterior walls but you probably don't want to use greenboard on the exterior wall if you put up a vapor barrier. Or am I way off... ?
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