Q: We are converting a masonry office building into condominiums. The building is constructed with solid masonry walls that are three and four wythes thick. We would like to insulate the walls to make them more energy-efficient, but we are concerned about moisture problems. How should the walls be insulated? Should we use a vapor retarder? If so, where should we place it?
A: Older multi-wythe masonry walls often managed water penetration by absorbing significant amounts of rain during storms. The walls were thick enough that very little water ever reached areas where it would cause problems. The water evaporated slowly over the days or weeks following heavy rains. This water evaporated both to the interior or the exterior air.
Since older buildings were typically not humidified in the winter, any remaining water that was stored within the walls before the onset of cold weather evaporated to the interior since the heated air was very dry. Since buildings typically were not air conditioned in the summer, there were fewer opportunities for condensation to develop. These wall systems did not contain insulation or vapor retarders. The lack of insulation and vapor retarders also helped the walls to dry quickly. Because the window openings were small in most older masonry buildings, they actually performed moderately well as far as energy efficiency.
When converting an older, multi-wythe masonry building into a humidified/air-conditioned insulated building, the moisture control dynamics of the wall system are significantly changed. Installing insulation on the interior side of the masonry greatly affects the temperature changes and moisture dissipation of the wall system. With insulation on the interior face of the masonry, the masonry will be insulated from the interior air and, as a result, will be much colder in the winter and much warmer in the summer. These larger temperature swings can create cracking problems from thermal expansion.
The potential for moisture problems also increases. In the summer when moist masonry walls are heated by the sun, the hot humid air within the wall can condense on any portions of the wall system on the interior side of the insulation that are cooled by air conditioning. During cold weather, as interior humid air moves through the walls, the cool temperature of the masonry walls increases the chances that moisture from this interior air will condense within the masonry. Airflow through the insulation may condense on the interior surface of the masonry, resulting in frost. Many moisture problems associated with insulating older multi-wythe walls result from air movement.
Changes to interior air conditioning and humidification and changes to the insulation of the exterior walls will generally have some negative impact on the performance of the wall system. The goal is to minimize these negative effects.
In my experience, the approach that minimizes the negative impact on the exterior walls is to insulate using spray foam insulation that is fully adhered to the interior face of the masonry. If properly applied, the foam insulation will also create an air barrier and a vapor retarder. The foam air barrier will greatly restrict moist air from the masonry walls from moving to the interior during the summer.
This same air barrier prevents the moist interior air from moving through the wall system and condensing on the interior surface of the masonry in the winter. Since the spray foam insulation works both as a vapor retarder and as insulation, no other vapor retarder should be used. Being fully adhered to the masonry, it does not allow for air space between the insulation of the masonry, which can allow frost.
Although properly applied foam insulation will greatly reduce the risk of moisture problems within the building, there is a greater potential for efflorescence and freeze-thaw problems on the exterior. This is because the foam insulation prevents drying to the interior. The cool temperature of the masonry during the winter will increase the potential for freeze-thaw problems.
To address these problems, it is essential to perform some work on the exterior of masonry walls when the interior climate is modified and the walls are insulated.
The exterior work should include repointing and other repairs to reduce water penetration, adding gutters on roofs and covers on masonry ledges and sills to restrict water penetration, and critically examining and modifying areas as necessary to prevent water from being concentrated on the masonry walls.