Keeping Masonry Walls Warm and Dry

Recently, this author was reviewing proposed details for the renovation of an historic brick masonry building located in downtown Seattle. During this review, questions were raised regarding how much insulation to add to what has been until now, a massive but uninsulated solid masonry wall system, and whether the addition of a vapor retarder membrane to the wall system would be beneficial. After some research on the topic, our conclusions were not as obvious as one might expect.

Normally, building assemblies are designed with the understanding that insulating buildings is good practice, and that higher levels of insulation are implicitly better than lower levels. Building designers are taught this in architecture and engineering school; and this understanding is reinforced by energy codes that set minimum insulation values for buildings and green building guidelines that provide incentives for even greater reductions in the energy required to condition the spaces within our buildings.

However in the case of an existing historic structure, "more insulation is better" may be too simple an answer. While it is generally true for all buildings that higher insulation values will reduce building energy consumption, in the case of older buildings, this benefit must be traded off with the potentially harmful effects that adding insulation to an existing wall system may produce.

In the case of our building under consideration, the proposal was to add rigid foam insulation on the interior side of the 12-inch thick masonry walls. (Adding insulation to the interior would preserve the historic appearance of the building exterior.) However the concern became that the more insulation added to the interior side of the wall, the lower would be the temperature of the brick during the colder months of the year. And as discussed in a previous article on this site, Air, Moisture, and the Building Envelope, keeping a wall warm can be an effective strategy for keeping a wall dry as well.

So our concern was that, up to this point in the life of the building, the exterior walls had been kept relatively warm--and dry--by the building heating system. By changing this, would we risk creating a colder wall system that could become more vulnerable to the effects of greater temperature extremes and added moisture? After some research on the subject, our decision was to apply just one inch of rigid foam insulation (with an insulation value of R-5) to the interior side of the walls. Essentially this strategy aims to balance the greatest possible reduction in energy use with the least reduction in exterior wall temperatures.

Additionally, we decided to add a polyethylene sheet membrane vapor retarder to the interior side of the wall assembly. The intent of adding this component was to reduce moisture movement into the wall system from the building interior, both by water vapor difusion and by the direct passage of moisture-laden air. While the benefits of vapor retarder membranes in a relatively mild climate such as Seattle can be debated (such membranes can reduce wall drying in the warmer months of year), our judgement was that the overall effect would be a net positive.

More Info
Loadbearing masonry construction is discussed in Chapter 10 Masonry Loadbearing Wall Construction of the textbook.
In researching this topic, this author found Rehabilitation of Solid Masonry Walls (National Research Council of Canada) particularly informative. This Council's Rehabilitation of Masonry Assemblies page has a variety of additional articles and references.