Drafts, cold floors and uneven temperatures — not uncommon problems for a house built 100 years ago.

Evan Clements and Precia Harthcock of Jaffrey love their 1926 home, but they acknowledge the drawbacks of houses built in that era. The room that is now their sitting room, for example, was once a porch that had been enclosed and is noticeably colder than other parts of the house. The second floor was …