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Insulation Retrofit

Last year, we bought a 1850’s vintage house in Maine with no insulation. After paying our oil company too much we want to add some insulation. The plaster on the inside appears to be in good shape and the clapboards on the outside are in good shape. Despite post and beam walls, the wall cavities appear to be only 2 to 3 inches thick. We’d prefer to have the installation done from the inside, which I gather isn’t quite “standard” but I’ve never seen a “plug job” in clapboards that isn’t obvious from 100′ away. — East Boothbay, ME.

If the stud spaces within your walls is only 2″ you may have “back plastering” which is a lath and plaster sealing within the stud space. Ordinary insulation contractors may not recognize this complication and may install the insulation so that condensation occurs within the wall causing moisture damage.

Before doing any insulating at all be sure to do everything you can to limit air infiltration. Tighten up the exterior weather envelope by repairing woodwork and caulking open joints between woodwork elements. Add storm windows (interior or exterior) and refurbish the existing windows if they contribute to the historic character of the house instead of replacing them with plastic windows. In a 140 year old house this is bound to give you the greatest “bang for the buck.”

Then insulate the ceilings above heated spaces. This should only be done if the attic spaces above can be effectively ventilated to prevent moisture buildup. Painting the ceilings with a vapor retardant primer can help prevent moisture migrating through the ceilings, but the ventilation is a necessity.

One of my recent clients in Belgrade Lakes, Maine just had extensive ice damming and moisture damage due to a poorly designed insulation project. She had to remove most of the insulation, take off all the roofing, and reinstall it along with an effective ventilation scheme. It is worth getting the details right in the first place.

Last consider insulating the side walls. Insulation in the side walls of an older house can trap moisture within the wall during the winter causing superficial damage (exterior peeling paint) as well as significant structural damage (fungal decay). The source of the moisture is usually within the house due to a damp cellar and the moisture generated from showers, cooking, etc. This situation is frequently compounded by adding vinyl siding which traps even more moisture within the wall. In my experience over the past 25 years about half of the pre-1940 house with insulation and vinyl siding have excessive moisture within the walls, and about 10-15% have developed at least some superficial or more serious structural damage.

John Leeke is a preservation consultant who helps homeowners, contractors and architects understand and maintain their historic buildings. You can contact him at 26 Higgins St., Portland, Maine, 04103; or by E-mail:; or log onto his website at:

© John Leeke

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