Dryline it on treated battens. Use insulation boards with vapour barrier if you can or minimum foil backed boards. Dont put rockwell inbetween battens against wall. You need cavity. Personally I wouldn't render both sides of a solid wall. If you dryline the the inside you could put a waterproof render on the outside, this will protect against penetrating damp. If your sure this is what it is...this time of year condensation is rife. You need to inform your tenants of steps to avoid condensation.