One of our most frequent old home losses here in the Northeast is water damage from ice dams. Historic homes did not typically feature vents in the ridge or soffits. Adding louvers can detract from the exterior appearance and are not authentic either. Robert Mueller Middlesex Mutual Dear Robert, Ice damming is an incredibly complicated phenomena that has produced great hysteria and building mythology equaled only by moisture condensation. Of course the devil is in the details.
Roof ice is a big business in Vermont too.
My company makes several hundred thousand dollars worth of thermal renovations every year that are driven by ice damage. You'd be interested to know that sound thermal envelope design works without regard to age and building style. Our clients properties range from museums to shacks, and from 3 to 300 years old. The trick to preserving historic buildings is to knit the existing fabric of the structure into a modern air barrier without affecting its appearance. The process has been developed for 30 years and is presently called Weatherization.