Hello! 

I’m renovating a bathroom in my 125 year old home in Northern Michigan. The bathroom has lathe and plaster walls with old wood panel wainscoting on the bottom 1/3rd of the walls. The plaster above the panelling is in rough shape so I’m planning to remove the plaster, add vapor barrier, then drywall over it. However, I’m hoping to keep the panelling (and the plaster under it) intact. Two walls are exterior with cellulose insulation behind the lathe. 

Are there any obvious issues with adding the plastic vapor barrier just on the top half of the wall – or is it even necessary since it won’t be fully sealed? 

Any suggestions for improving the insulation behind the lath on the exterior walls without removing the lath or siding? 

Thanks for any ideas! 
-Chad