TOPIC: No sub floor

Anyone come across a house with no sub floor. This house just had hardwood flooring placed directly on the joists.  The house was built in the mid 1940's. Seemed solid enough. Any thoughts/ recommendations to tell my client?

I normally see T&G planks laid on a diagonal as the subfloor in homes of that age, plywood in newer or renovated homes. I have infrequently seen much older homes (1880s-1920s) where the wood flooring was laid directly over joists without a subfloor. I have seen a number of cabins and converted buildings with wood flooring laid directly over joists, usually pre 1920s or 1930s.
Was the entire home done in this manner or just a room or two?
Drafty home, floors that squeak and pop, dips or slopes in the floor, may need to add support under china cabinets, pianos and the like. Very important to have vapor barrier in crawlspace and to keep sump pumps in good order and to keep crawlspace dry.