Why is my house humidity at 80%?
If you have a crawlspace with an earth bottom, it's possible the humidity is coming up through that--check whether you have a vapor barrier below your house. You probably don't, given the age of the house.

Getting a dehumidifier is definitely a possibility. Or a 'swamp cooler' arrangement, where cold water is used to make condensate that you then pipe out of the house, but that really only works in higher temperature air.

(edit) You may find some of this writeup really helpful.