It would be helpful to know which kind of ferns you are trying to over winter. If you don't have id - a picture might help, although obtaining an identification of a fern through a picture is quite difficult as there are so many.

Hi, welcome to G o y, as Scottish says there are lots of ferns, some are completely hardy, whilst others are frost tender, and anywhere in between, so we would need to know which ferns you have before giving any advice.
As mentioned above, if you don't know which ferns you have, try putting a photo on here, you never know someone may be able to identiy them for you, Derek.