December 21, 2011

Abstract: 

Load-bearing masonry buildings are a significant portion of the existing building stock. Given the Building America goals of reducing home energy use by 30%-50% (compared to 2009 energy codes for new homes and pre-retrofit energy use for existing homes), insulation and air sealing of mass masonry walls will need to be a component of this work if mass masonry residential buildings are to be addressed.

Executive Summary

Load-bearing masonry buildings are a significant portion of the existing building stock. Given the Building America goals of reducing home energy use by 30%-50% (compared to 2009 energy codes for new homes and pre-retrofit energy use for existing homes), insulation and air sealing of mass masonry walls will need to be a component of this work if mass masonry residential buildings are to be addressed.

Exterior insulation provides the ideal conditions for building durability; however, many buildings cannot be retrofitted with insulation on the exterior for reasons such as historic preservation, cost, zoning or space restrictions, or aesthetics. Adding insulation to the interior side of walls of such masonry buildings in cold, and particularly cold and wet, climates may cause performance and durability problems. There are specific moisture control principles that must be followed for a successful insulated retrofit of a solid load-bearing masonry wall.

In terms of cost-effectiveness, uninsulated masonry (even "thick" multi-wythe construction) would have an average R value of roughly R-5, which is far below current energy code requirements; application of insulation has substantial benefits. 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 setpoint (as found in milder climates).

Increasing the building airtightness (which would result from this interior retrofit) can cause indoor air quality problems: mechanical ventilation, pollution source control, and combustion safety measures must be implemented to manage this risk.

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 the application of interior insulation. For one, the masonry wall becomes colder: the inside face of the masonry wall changes from seeing moderate temperatures to regularly experiencing freezing temperatures. In addition, the wall has reduced drying to the interior (by cooling the masonry, and by adding vapor impermeable layers on the interior), and the amount of energy flow through the wall (and thus drying potential) has been minimized. In addition, moisture flow due to air leakage into the interface between the masonry and insulation can result in condensation problems; excellent airtightness is desirable to prevent this. Another issue is rot/corrosion of embedded elements: embedded wood timbers are a common embedded element with durability concerns.

In terms of interior insulation assembly options, drywall on a steel stud wall filled with batt insulation is not recommended. It has a high likelihood of wintertime condensation and mold growth in the wall, due to 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 involves spraying an airtight insulating foam directly to the interior side of the existing masonry: all air leakage condensation is strictly controlled, and it is the most practical approach to achieving high levels of airtightness in existing buildings. The spray foam also acts as a moisture barrier, and any small amount of incidental rain penetration will be localized and controlled. High-density closed cell polyurethane foam is generally a good solution for thinner applications (e.g., 2” of ccPCF), and open-celled semi-permeable foams (e.g., 5” ocSPF) 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 as the interior retrofit, but is far more difficult to build as it requires great care in ensuring that the board is firmly in contact with the masonry (no gaps), and that a complete air barrier is formed. Note that bare masonry has been measured as a noticeable source of air leakage, thus showing the need for an air barrier layer, typically applied to the interior face of the masonry.

Another assembly option is to combine spray or rigid board foam with fibrous, air permeable insulation (fiberglass or cellulose) to create a lower cost high-R wall assembly. The relative thickness of the foam layer is a function of condensation control, and thus climate conditions.

Thermal bridging through wood framing will have a minimal impact on thermal performance if wood stud framing allows for at least 1” of insulation (preferably 2”). However, thermal bridging through light-gauge steel framing has a significant impact: the gap for insulation between the framing and the masonry should be maximized; preferably, there should be minimal to no insulation placed within the steel stud bay. Steel stud clips back to the masonry also have a significant thermal bridging effect; they should be replaced with a non-thermally conductive material.

Controlling bulk water entry into the wall when doing interior masonry retrofits is of vital importance, especially as water leakage will no longer be visible from inside until damage occurs to interior finishes. If rain control cannot be addressed and upgraded, interior insulation should not be implemented. A variety of critical details were examined, showing problems and solutions.

Windows and doors are non-absorbent, and hence shed all the rainwater that strikes them. To prevent masonry durability problems, rainwater surface drainage must not be concentrated on the wall below, and this water should be shed from the face of the building. This drainage and shedding is accomplished by a sloped sill detail with end dams, and a sufficient drip edge beyond the wall below. Rowlock window sills 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. To prevent these issues, a subsill pan flashing should be installed, which directs any of 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, or inadequate drip edges: they can all cause accumulation of bulk water on masonry below. Projecting drip edges and waterproofing under the cap are vital details to implement at these assemblies.

Details such as stonework and band courses can result in water concentration and deposition on the face of the building. Solutions include overclad caps and drip edges below these features.

Roof-wall interfaces can be another source of water concentrations; details such as kick-out flashings are critical to prevent these issues.

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

When brick is buried below grade, severe subfluorescence and spalling may result, due to capillary water uptake (i.e., moisture “wicking”) through the brick. The recommended solution to these buried brick issues is to eliminate capillary contact between the soil and the brick. Another risk close to grade is splashback; these issues are reduced with ‘softer’ landscaping (i.e., not pavement), or 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. Simulations were run to examine the thermal and moisture behavior of embedded beams before and after insulation. Overall, these simulations indicate that there is substantial uncertainty in 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.

When considering the interior insulation of a masonry building, a series of steps are recommended to assess the risks associated with this retrofit, with greater certainty with added steps, as follows:

  1. Site Visit Assessment (assessment of rain leakage, poor detailing, existing freeze-thaw damage)
  2. Simple Tests & Modeling (dry density, liquid water uptake, saturation moisture content, and basic hygrothermal/WUFI modeling)
  3. Detailed Tests & Modeling (thermal conductivity, Fagerlund’s Critical Degree of Saturation or Scrit)
  4. Site Load Assessment (assessment of driving rain load, run down patterns; monitoring of rain deposition with driving rain gauges)
  5. Prototype Monitoring (retrofit of a small area of the building, and monitoring of temperature and moisture content, including comparisons to models)
  6. Maintenance and Repair (creating a recommended program of inspection/repair, perhaps in the form of a building owner’s manual)

Although many of these retrofits are being implemented in locations throughout North America, there are still major needs for continued work and research on this topic, including 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 research on clear sealants such as silanes and siloxanes.

1 Introduction

Load-bearing brick masonry buildings are a significant portion of the existing building stock in the East Coast and Midwest regions of the United States. However, adding insulation to the interior side of walls of such masonry buildings in cold, and particularly cold and wet, climates may cause performance and durability problems in some cases. Exterior insulation provides the ideal conditions for building durability; however, many buildings cannot be retrofitted with insulation on the exterior for reasons such as historic preservation, cost, zoning or space restrictions, or aesthetics.

Figure 1: Historic mass masonry buildings

Interior insulation of existing mass masonry structures is a measure that is being implemented in current Building America projects, including the Habitat for Humanity of Merrimack Valley project in Lawrence, MA (Figure 2), and the Byggmeister brick row house renovation in Roxbury, MA (Figure 3).

Figure 2: Habitat Merrimack Valley building and site evaluation (current Building America project)

Figure 3: Byggmeister project building and site evaluation (current Building America project)

There are specific moisture control principles that must be followed for a successful insulation retrofit of a solid load-bearing masonry wall. This measure guideline presents the current state of the art of guidance in terms of moisture-safe retrofits of mass masonry walls. This document is divided into the following sections:

  • Background places the problem of masonry insulation in context, including the advantages of exterior insulation, the relative risks of interior insulation, and citations of previous research.
  • Decision-making and Tradeoffs covers further material which includes costeffectiveness, performance tradeoffs (including quantification of energy benefits of mass vs. insulation), and system interactions.
  • Retrofitting for Durability describes the balance of wetting and drying that is occurring in building enclosure elements, and how this balance changes when mass masonry walls are retrofitted with interior insulation. It points out specific durability issues, such as freeze-thaw damage, interstitial condensation, and damage to embedded wood timbers.
  • Interior Retrofit Assemblies and Solutions then covers a variety of interior insulation details that have shown good performance in service, as well as describing problematic assemblies, and why they are prone to failure. A further discussion of thermal performance (overall insulation value) is included in this section.
  • Interior Retrofit Problematic Details and Solutions examines exterior water control detailing of masonry structures, which is key to a successful interior insulation retrofit; this could be considered a guide to conducting field reviews of these buildings. Some key details include windows, band courses, roof-wall interfaces, parapets, downspouts, and areas at or close to grade. A case study of a building that showed problems postretrofit is also included, to show what can go wrong if water control details are not addressed.
  • Embedded Wood Member Research is a summary of a conference paper that examined the effect of insulation on masonry-embedded wood structural members in detail, using three-dimensional thermal simulations, and one-dimensional hygrothermal simulations. Overall, these simulations indicate that there is substantial uncertainty in how embedded wood members in masonry actually behave in service after insulation retrofits. Further research is recommended, including more advanced hygrothermal simulations, and field monitoring.
  • The final section, Assessment, Analysis and Risk Management, presents a series of six recommended steps to assess the risks associated with this retrofit, and to determine what can be done to mitigate this risk. Each incremental step reduces uncertainties in the evaluation (and thus reduces risk). These steps included diagnostic tests and techniques to manage risk of freeze thaw damage to the masonry.

2 Background

2.1 Context

Reducing the energy consumption of buildings has become increasingly imperative because of the combined demands of energy security, rising energy costs, and the need to reduce the environmental damage of energy consumption. In addition to new buildings, a vast stock of existing buildings—the great majority of which have poorly insulated enclosures—exists.

Upgrading, renovating and converting buildings to new uses and higher performance levels involve numerous challenges. A socially, culturally, and economically important class of buildings is solid, usually load-bearing, masonry buildings, typically built before the Second World War. These buildings have value because they often have good reserves of structural capacity, are located near urban centers, are beautiful or historically significant, and often offer flexible, low-cost space that can be easily converted to housing.

2.2 Exterior Insulation of Mass Masonry Structures

Retrofitting existing buildings on the exterior (Figure 4 through Figure 7), 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 (masonry) 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

Figure 4: Exterior insulation retrofit overclad

Figure 5: Exterior insulation retrofit approaches (EIFS left; insulated metal panel cladding right)

Figure 6: Drained panel spray foam exterior insulation (and airtightening) retrofit of a solid masonry wall;
window detail

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