If insulation of wall surfaces is necessary after all other options have been explored, materials which provide the greatest R-value with the least impact should be used. In projects where plaster or sheet rock must be removed, it is possible to install rolled insulation. It is critical that the relationship between wall surfaces and historic wood work not be altered.

Introduction of Insulation

Owners should be aware that the introduction of insulation into wall cavities of historic frame buildings have the potential to cause short-and long-term damage to historic fabric. Heated air inside a building supports more moisture than cold, outside winter air. This warm air passes through un-insulated wall cavities and the moisture vapor then reaches dew point on the back side of exterior sheathing. Air movement within an un-insulated cavity causes this condensation to evaporate, and prevents dry rot. When wall cavities are insulated, moisture can become trapped within the insulation as it travels from interior to exterior.

Without air movement, this moisture causes wet insulation, which yields no insulation value, and begins to rot framing members. This long term damage is unseen until it causes serious deterioration. In buildings where synthetic siding has been added the problem is exacerbated because the symptoms are hidden. To avoid moisture damage and insure maximum thermal efficiency, a proper vapor barrier must be provided on the warm side of all insulation materials, whether they are applied under flooring, in the attic or in the walls.This barrier prevents the passage of moisture through a wall and prevents its accumulation in the insulation.

Creating a Vapor Barrier

There are several ways to achieve a vapor barrier: foil facing material on fiberglass insulation; Kraft paper facing only if it is backed with a bituminous or tar-like coating (Kraft paper alone is not a vapor barrier); polyethylene sheeting placed between the insulation and new plaster or sheet rock; or vapor barrier paints or other primers which provide a perm rating of 1.0 or less, applied to plaster or sheet rock surfaces.