Overly Heavy Modern Pieces
It should not be a struggle to lift a Mid-Century side chair. If you can barely pick it up, this may be a sign the piece has been exposed to the elements. Foam was the first choice for stuffing seats and backs post-1920’s, but it can swell and retain water if exposed to a lot of moisture (think flooding, rain or extreme, long-term humidity). If this is the case, the foam will not be usable and will crumble to pieces when you remove the existing fabric. New foam will be required, and you should probably check the wood frame for signs of rot, as well.