Internal Insulation of Masonry Walls

A version of this article appears in the July/August 2014 issue of Home Energy Magazine.

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Load-bearing masonry buildings are a significant portion of the existing building stock. DOE’s Building America program has set a goal of reducing home energy use by 30–50%, compared to 2009 energy codes for new homes and preretrofit energy use for existing homes. To achieve this goal, it will be necessary to improve the insulation and air sealing of mass masonry walls, if mass masonry residential buildings are to be addressed. To help the industry address the energy and moisture performance of masonry walls, Building Science Corporation (BSC), one of ten DOE Building America teams, has been conducting research and development that has led to the publication of a recent guide, “Measure Guideline: Internal Insulation of Masonry Walls.” This guideline is summarized in the present article. The full text of the guideline can be downloaded from the Building America web site (see “learn more”).

Overview

Exterior insulation provides the ideal conditions for building durability. However, many buildings cannot be retrofitted with insulation on the exterior for reasons having to do with historic preservation, cost, zoning or space restrictions, or aesthetics. Adding insulation to the interior sides of the exterior walls of such masonry buildings in cold—and particularly in cold and wet—climates may reduce performance and durability. Specific moisture control principles must be followed to ensure a successful insulated retrofit of a solid load-bearing masonry wall.

Uninsulated masonry (even thick multiwythe construction) would have an average R-value of roughly R-5, which is far below current energy code requirements. In cold climates insulation provides substantial benefits (see “Insulating Masonry Buildings in Cold Climates,” HE March/April 2010, p. 29). The wintertime thermal-mass benefits of leaving masonry uninsulated are negligible in heating-dominated climates, compared to locations with high diurnal swings around the interior set point (as found in milder climates).

Adding insulation to the interior sides of the exterior walls will increase building airtightness. This in turn can cause indoor air quality (IAQ) problems. Mechanical ventilation, pollution source control, and combustion safety measures must be implemented to address these problems. When examining the moisture problem, the fundamental premise is that mass masonry walls manage moisture in a different way than modern, drained assemblies. Therefore, the balance of moisture (into and out of the wall) is strongly affected by interior insulation. For one, the masonry wall becomes colder when it is insulated on the interior. The inside face of the masonry wall changes from seeing moderate temperatures to regularly experiencing freezing temperatures. In addition, adding interior insulation reduces drying to the interior by cooling the masonry, by adding vapor-impermeable layers on the interior, and by minimizing energy flow through the wall. Also, moisture flow caused by air leakage into the interface between the masonry and the insulation can lead to condensation. Finally, moisture can rot or corrode embedded wood timbers, reducing durability. Excellent airtightness is required to prevent these problems.

There are various ways to insulate masonry walls on the interior. Installing drywall on a steel stud wall filled with batt insulation is not recommended. This approach will promote wintertime condensation and mold growth in the wall, caused by leakage of interior air into the cold interface between the insulation and the masonry. This would be exacerbated in a pressurized building.

A more successful approach is to spray an airtight insulating foam directly on the interior side of the existing masonry. With this approach, all air leakage condensation is strictly controlled. This is the most practical way to achieve high levels of airtightness in existing buildings. The spray foam also acts as a moisture barrier, and any incidental rain penetration will be localized and controlled. High-density closed-cell polyurethane foam is generally a good choice for thinner applications (for example, 2 inches of closed-cell SPF). Open-cell semipermeable foams (for example, 5 inches of open-cell SPF) can be a good choice for greater thickness if the interior is kept at a low humidity during winter and the outdoor temperature is not too cold.

Rigid foam board insulation of various types has been used in interior retrofits, but it is far more difficult to install, since great care must be taken to ensure that the board is firmly in contact with the masonry, and that it forms a complete air barrier. Note that bare masonry was found to be a significant source of air leakage; this indicates the need for an air barrier layer, which is typically applied to the interior face of the masonry.

Another assembly option is to combine spray or rigid foam board with fibrous, air-permeable fiberglass or cellulose insulation to create a less expensive high-R wall assembly. Climate conditions, and thus the need to control condensation, determine the relative thickness of the foam layer.

Thermal bridging through wood framing will have a minimal impact on thermal performance if wood stud framing allows for at least 1 inch, and preferably 2 inches, of insulation. However, thermal bridging through light-gauge steel framing has a significant impact. In this case, the gap for insulation between the framing and the masonry should be maximized; preferably, little or no insulation should be placed within the steel stud bay. Steel stud clips on the back of the masonry also increase thermal bridging; they should be replaced with a thermally nonconductive material.

Controlling Moisture

It is vitally important to control bulk water entry into the wall when doing interior masonry retrofits. This is especially important because water leakage will no longer be visible from inside until it makes stains on the walls. If rain control cannot be addressed and upgraded, interior insulation should not be installed.

Windows and doors are nonabsorbent and hence shed all the rainwater that strikes them. To prevent rainwater from degrading the masonry, rainwater surface drainage must not be concentrated on the wall below the door or window, and this water should be shed from the face of the building. Drainage and shedding are accomplished by installing a sloped sill detail with end dams, and a sufficient drip edge beyond the wall below. Rowlock windowsills are especially vulnerable, as they are composed of individual bricks with mortar joints, which will be a source of water leakage. One possible solution to reduce water loading into the wall below is to overclad the rowlock course with metal flashing.

Leakage through the wall-to-window joint or the window unit itself can contribute to masonry moisture loading. A subsill pan flashing should be installed that directs this water out onto the sill to the exterior. Copings and parapet caps can suffer from problems such as inadequate slope, incorrect slope, inadequate overhangs, and inadequate drip edges, all of which can cause bulk water to accumulate on the masonry below. Projecting drip edges and waterproofing under the cap should be installed to address these problems.

Details such as stonework and band courses can collect water and deposit it on the face of the building. Solutions include overclad caps and drip edges below these features.

Roof-wall interfaces can also collect water. Kickout flashings will prevent this problem.

Downspouts, rainwater leaders, and scuppers, when they are improperly designed or when they fail to function, can concentrate a tremendous amount of water, making freeze-thaw (FT) damage very likely.

When brick is buried below grade, severe subfluorescence and spalling may result from capillary water uptake (moisture wicking) through the brick. The recommended solution is to eliminate capillary contact between the soil and the brick. A risk close to grade is splashback; this is reduced with softer landscaping (not pavement), or by keeping roof and wall drainage away from the adjacent ground.

Another durability risk is the hygrothermal behavior of moisture-sensitive wood beams embedded in the load-bearing masonry. Researchers ran simulations to examine the thermal and moisture behaviors of embedded beams before and after insulation. Overall, these simulations indicate substantial uncertainty as to how embedded wood members in masonry actually behave in service after insulation retrofits. Further research is warranted, including the use of two-dimensional hygrothermal simulations and in situ measurements in both insulated and uninsulated configurations.

Insulation of a Masonry Building

When considering the interior insulation of a masonry building, we recommend that builders and contractors should take the following steps to assess the risks associated with this retrofit.

1. Conduct a site visit assessment. Assess rain leakage, poor detailing, and existing FT damage.
2. Conduct simple tests and modeling. These tests address dry density, liquid water uptake, saturation moisture content, and basic hygrothermal/WUFI modeling.
3. Conduct detailed tests and modeling. These tests address thermal conductivity and Fagerlund’s critical degree of saturation, or Scrit.
4. Conduct a site load assessment. Assess driving rain load and run-down patterns; monitor rain deposition with driving-rain gauges.
5. Conduct prototype monitoring. Retrofit a small area of the building, and monitor temperature and moisture content, including comparisons to models.
6. Create a program of maintenance and repair, perhaps in the form of a building owner’s manual.

Although many of these interior insulation retrofits are being implemented throughout North America, more research on the topic is still needed. Research should address comparisons between models and in-service behavior, increasing the database of interior insulated mass masonry buildings, an improved understanding of rain loadings on walls, and the use of clear sealants, such as silanes and siloxanes.

Exterior Insulation of Mass Masonry Structures

Retrofitting existing buildings on the exterior is the best possible technical solution: exterior insulation provides the highest level of durability, energy efficiency, and comfort with the least technical risk. Specifically, externally applied insulation and air/water control layers have the following advantages:

• The insulation and air/water control layers can easily be made continuous and thus protect the existing structure from rain, condensation, and temperature swings.
• Thermal bridging at floors and partitions is eliminated.
• Thermal mass benefits are enhanced.
• Access to conduct the work is often easier.

Given the option, exterior insulation should always be the preferred, least-risk retrofit. This insulation can take the form of an overclad, such as an exterior insulation and finish system or a drained-panel system over insulation and membrane.

Interior Insulation of Mass Masonry Structures

Despite the advantages of exterior insulation, many buildings must be retrofitted on the interior, for reasons such as historic preservation, zoning or space restrictions, or aesthetics (see Figure 1). Load-bearing masonry buildings often (not always) have historic significance and highly valued aesthetics that preclude exterior retrofits.

Interior retrofits of load-bearing masonry are often chosen to preserve the exterior appearance. There are many possible interior insulation approaches that are, by and large, reasonably well understood. Adding insulation, increasing airtightness, replacing windows, and improving rain control constitute a normal retrofit package. Adding insulation to the walls of such masonry buildings in cold (and particularly cold and wet) climates may cause performance and durability problems, particularly rot and FT damage.

There are specific moisture control principles that must be followed to achieve a successful interior retrofit of a solid masonry wall. The goal of this measure guideline is to present the current state of the art for guidance in terms of moisture-safe retrofits of solid masonry walls, with some discussion of specific details that have higher durability risks. This guideline provides engineering, architectural, and contractor guidance for assessing and minimizing the risk of FT damage arising from the interior insulation of mass wall assemblies. It also provides similar guidance for assessing and managing the decay risk of embedded wood structural members.

Critical Takeaways

Retrofitting existing buildings on the exterior is the best possible technical solution. Exterior insulation provides the highest level of durability, energy efficiency, and comfort with the lowest technical risk.

In cases where an exterior retrofit is not possible, adding interior insulation to the walls of masonry buildings in cold (and particularly cold and wet) climates may cause performance and durability problems, particularly rot and FT. Specific moisture control procedures must be followed to ensure a successful interior retrofit of a masonry wall.

This article was adapted from a DOE Building America program report.

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