There are many terrace properties which have been extended to the rear with a solid concrete floor where air vents have not been extended under the concrete plinth. These were extensions done prior to changes in building regulations that dictated otherwise. If a property has air vents on one or more sides, has no evidence of damp, but does not have air vents on opposite sides of the building why would it not be suitable for underfloor insulation? Regulations suggest that properties of this description should not be insulated.