To Heat or Not to Heat

Is it a bad idea to leave our historic building unheated for the winter?

Pictured: The Dorchester Town Hall (which is heated through the winter).

The answer is: it depends.

Historically, many public buildings – and even rooms within heated houses – were left cold in the winter. This was possible because spaces were finished with products that could withstand swings in temperature and humidity, like oil-based paints, lime-based plaster, and calcimine ceiling finishes. As more modern trends in building and materials evolved, it became more and more common to develop interior spaces that would be heated year round. So, for the steward or a historic building in a modern society, the question remains, is it best to heat, or not to heat?

Temperature and Moisture are Key

Frost accumulation on the inside walls of a historic building.

According to former state architectural historian and author Jim Garvin, the main culprit when it comes to unheated spaces getting damaged is rapid changes in the humidity and/or temperature. When wood contracts and expands quickly, film finishes often develop permanent fine cracks. This is especially true when newer products (latex, water-based paints) have been applied atop the older and more stable finishes (oil-based or linseed oil-based paints). Similarly, where calcimine coatings on plaster surfaces have been covered with oil or water-based paint, you’re bound to see extensive peeling.

Mold growth can also be a problem, especially in the fall and spring when temperatures are above 40 degrees and humidity exceeds 65%. A dehumidifier that can handle the size room you have can really make a difference here. Ideally, you want interior spaces at or below 50% humidity.

Steve Bedard, of Bedard Preservation and Restoration, suggests the 50/50 rule. That is, the building could be heated to 50 degrees with a relative humidity of 50%. All interior doors, especially closet doors, should be left wide open to avoid microclimates for potential mold growth. All paintings and furniture along the exterior walls should be pulled away from the exterior walls whenever possible. A low-temperature alarm, which is very affordable, could also be used.

When to Take Action

If your town building has routinely gone through winters with no heating or humidity controls and no harm has come from the practice, then you’re probably OK to skip the oil bill. But if you’ve recently repainted surfaces, restored plaster features, or done other decorative finishes and the building is traditionally heated through winter, it’s best to play it safe and adopt the 50/50 rule.

If your town hall is very expensive to heat, then it might be time to consider energy efficiency projects like storm windows, attic insulation, and appropriate basement sealing. Interested in learning more? Check out our previously recorded session, Snug as a Bug in a Rug, from our Expo on the Road series. Scott T. Hansen, author of the award-winning book, Restoring Your Old House, discusses improving energy efficiency and comfort in a historic building.