I have heard and read a lot about vapor barriers in crawl spaces.
My personal opinion is that if there is plenty of ventilation under a house, a vapor barrier is not needed.
But during the winter, the vents are usually closed to save heat, so if that is the case, I can't see any reason not to add one.
I know of lots of real old homes that have never had vapor barriers under them, and if this one is 50 years old and has no structural problems, why is your sister worriied about it?
Castle, We recently did a job stripping the walls (wood lath and stucco)on an 80 year old residence in Florida. A very large tree fell on it during hurricane Charley, breaking it's back and knocking much of the stucco off the house. We provided 1 square foot of ventilation for every 100 square feet of the house and positioned it at the corners and the centers of the long walls. This house had no vapor barrier and the ground beneath the house was lower than the surrounding grade (The pool deck actually had drains that emptied under the house). With the rains and moisture in Florida, I think of this as the acid test. After all these years, the floor joists and flooring were in great shape.
I'm not opposed to a vapor barrier, but I wouldn't spend a lot of money just to get it.
Vapor barrier is a good thing, Just the ground moisture is cold and keeps things pretty moist underneath, A few open close vents help keep things aired out as well!!! I dont imagine the floor framing is PT lumber either, being 50 years old, Here code is anything within 16" of the ground has to be PT lumber!!
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