Technical: Retrofitting Heritage Masonry Buildings

Adding thermal insulation to the interior of masonry walls must be approached with care. Here are some best practice dos and don’ts.

Toronto’s Waterworks development, designed by Diamond Schmitt, is transforming a 1930s masonry utility building into condos, a food hall, and a YMCA. Photo by Joy von Tiedemann Photography, courtesy Diamond Schmitt Architects

TEXT Eric A. Charron and Randy Van Straaten

Heritage masonry buildings make up a large portion of Canada’s urban structures—from the historic warehouses in the downtowns of thriving cities to the shops that line small-town main streets and squares. Late 19th- and early 20th-century construction provides warm, inviting, comfortably human-scaled settings that plunge us back into history and tell our collective stories. Not only are these structures significant assets to our physical environment and culture, but their robust assembly and appealing character make them adaptable to new uses.

While heritage masonry structures are typically energy inefficient, it is unrealistic and undesirable to replace these time-tested buildings with new net-zero ones. Hence, the deep energy retrofitting of heritage masonry buildings is a key part of achieving a zero-carbon, energy-neutral future while maintaining our cultural and architectural heritage.

The addition of interior thermal insulation to solid masonry walls is a common consideration for heritage retrofits. Best-practice solutions presented in this article are informed by research and feedback from the Ontario Association of Architects (OAA) Sustainable Built Environments Committee (SBEC), along with lessons learned from building science specialists and heritage consultants.

The Basics and Risks of Retrofitting Old Masonry Buildings

Reducing heat loss in older buildings is a key component of meeting energy efficiency targets such as Passive House and TEDI requirements. The most effective way to reduce heat loss is through upgrading the glazing systems by replacing or refurbishing windows with thermally broken framing, highly insulated glazing units, and weatherstripping repair. The second biggest item is to air-seal the building enclosure. Assuming glazing systems and air infiltration have been addressed, the next priority is to insulate the walls to reduce heating demand.

From a moisture management perspective, the best way to insulate a building is from the exterior, enclosing the structural walls within the thermal envelope. The structure will be heated as part of the interior conditioned space, where it will be kept warm and dry. But while wrapping the exterior is preferable from a building science perspective, it is not often viable from an aesthetic perspective or possibly due to access challenges. Additionally, conservation groups and associations tasked with advocating for heritage sites simply will not let masonry, which gives many buildings their heritage character, be covered up.

If we cannot wrap the exterior, the other logical place to insulate is from the inside. In doing so, one must consider how a significant reduction in heat loss can be achieved without creating decay or mould issues.

Moisture Control in Mass Masonry Walls

Excessive moisture is the main cause of decay and mould in wall systems. In older masonry buildings, wall thickness was commonly used to limit the risk of rainwater entry. Heavy masonry walls use hygric mass to control wetting by absorbing, storing, and drying rainwater. Thinner walls control rainwater entry through the use of less porous, denser masonry materials. The porosity of masonry, the mortar properties, and the presence of voids in the wall system all play a role in the control of rainwater. The balance of wetting and drying, as well as material vulnerability, dictates the susceptibility to decay in a particular exposure or environment.

Freeze-thaw is a major decay mechanism for masonry walls in cold climates. For decay to occur during freezing, masonry materials must reach a minimal critical degree of water saturation. Decay accelerates at higher moisture levels. Generally, masonry walls must be quite damp and exposed to multiple freeze-thaw cycles before damage occurs. So as long as the brick does not get damp, there is no problem. There are plenty of buildings with highly vulnerable masonry that have endured well, with limited moisture exposure.

Heavy masonry walls get colder when insulated on the interior. These walls also typically get damper, as there is less heat flowing outward through the wall for drying. This is why walls may be more vulnerable to freeze-thaw decay as a result of interior wall insulation retrofits.

The Three Ds

Many good measures for limiting risk of freeze-thaw decay and other moisture problems are derived from knowing the Dos and Don’ts of the 3 Ds: deflection, drainage, and drying.


1. DO minimize rainwater exposure. Use large overhanging cornices. Slope window sills with back and end dams. Repair masonry and repoint failing mortar joints. Design effective drip edges, parapets and flashings to drain/shed away from the wall.

2. DON’T allow grades and paving finishes to collect water near the base of a masonry wall. Slope grades away from the building. Where possible, do not extend masonry all the way to grade, as moisture and salts will work their way into the wall assembly.


3. DO drain walls and soils adjacent to the foundation. Install weeping tiles and drainage mats on foundation walls. Ensure internal wall drains are working. Include scuppers in roof designs.


4. DON’T use coatings or sealants that restrict drying. Masonry and mortar are porous materials intended to breathe. Painting a masonry wall will not inherently harm or damage the masonry, but the paint layer must be maintained to limit rainwater entry and to continue to allow for outward drying of the wall. When the paint layer begins to fail, the wall will be exposed to moisture, adding to the risk of deterioration.

5. DON’T allow vacated or mothballed heritage masonry buildings that are intended for future use to be left unheated. A cold, unheated wall will often accelerate the rate of deterioration of a mass masonry assembly and frost-heave often occurs.

Air Leakage and Water Vapour Diffusion

Further design considerations related to vapour and air wetting include the following:

6. DO limit cold-weather mechanical pressurization of the building with humid air. Pressurization of the building during the winter can drive humid indoor air through the exterior wall assembly, resulting in excessive wetting and the risk of freeze-thaw damage, frost accumulation, and mould growth. Mechanical systems should be designed to a neutral or slightly negative indoor pressure under all schedules of operation.

7. DO control interior humidity levels in winter. Although moisture flow through vapour diffusion tends to be minimal, poor vapour control can result in mould growth at high humidity surfaces within the wall assembly.  Retrofit designs should include adequate vapour control suited for their exterior and interior climates.

8. DON’T place a poorly installed air or vapour barrier on the inside of a masonry wall assembly. Polyethylene membranes (PE) have very low vapour and air permeance, but are difficult to air seal at the complex interfaces with the wall, including at floor joints and at the intersection with columns. Holes in the PE can carry moisture into the wall via air leakage. Ensure continuity of the air barrier in detail design and installation, and confirm through fog and/or whole building airtightness testing and inspection.

Risk Assessment Approach

Considering the general measures for moisture control in masonry walls, the following steps are recommended for assessing the risk of insulation retrofit approaches:

9. DON’T devise a solution without first looking at the condition of the building you are dealing with. Look for evidence of where water may be getting into the wall, such as areas of decay and staining. Check near the ground for spalled material and soft, sandy mortar. Verify the condition of walls not typically exposed to rainwater, such as the backside of exposed parapet walls—if they are in poor shape, water is getting in somewhere.

10. DON’T assume all wall exposures experience the same amount of wetting. Different wall exposures can be subjected to varying degrees of wetting. Examine the surrounding built environment and microclimate for possible effects of sun and wind drying, driving rain, and water shedding from other structures.

11. DON’T assume that all heritage masonry walls need to be exhaustively tested and analyzed. The value of the heritage asset should inform the decay risk assessment effort and expense—for instance, different levels of investment should be put into a monumental public landmark versus a single-family home. If hygrothermal analysis is being considered for the project, it will entail determining project-specific masonry properties along with creating invasive openings to confirm assemblies and conditions. Hygrothermal properties of the sampled materials are measured and used to predict conditions that may result from proposed interior insulation retrofit designs.  These conditions are compared to measured critical saturation levels during freezing condition to assess decay risk.

12. DON’T choose a repointing mortar that is incompatible with surrounding materials. Never use a high-strength mortar in a wall with original soft and flexible lime-based mortar or brick. Portland cement is strong and stiff rather than soft and flexible; mortar joints are supposed to accommodate movements and be sacrificial by design. Any replacement masonry or mortar should bond well with the adjacent materials and replicate similar performance under a range of weather conditions. Lab testing of mortars and masonry samples should be conducted to determine the most appropriate replacement materials.

13. DON’T devise an interior insulation strategy for a multi-wythe masonry building without first knowing where (and whether) the wall should be insulated. Every building is unique, and modifications over a structure’s lifecycle may have resulted in changes to the original construction. Retrofitting on the inside may create unforeseen expansion and contraction forces on a seasonal basis. If the masonry wall does not have control joints, it could create cracks and fissures on its own due to these forces.  It is important to analyze the potential for expansion and contraction that may result from an interior insulation strategy.   

14. DON’T insulate a mass masonry foundation below-grade from the interior without knowing the drainage capacity of the exterior soils. Conduct field inspections and geotechnical analysis to determine the soil drainage capacity. Check the basement walls for moisture; verify if the mortar is soft and sandy, or if the brick near the floor is crumbling. If these conditions are detected, insulate on the outside of the wall below-grade over a consolidated masonry render and waterproofing membrane. Sealing the interior of a damp foundation wall with insulation will make the wall colder and reduce its ability to dry from the inside. Leaving a cold wall exposed to exterior groundwater and freeze-thaw will gradually deteriorate the mortar and risk structural failure over time, or at a minimum may shift and crack the masonry assembly, thus allowing more moisture to enter the wall.

Contrary to popular belief, it is possible to insulate the interior of many heritage masonry buildings without an increased risk of decay. The catch is that it must be done very carefully and only after examining the existing structure and building assembly thoroughly.

With any interior retrofit approach, each building must be reviewed on a case-by-case basis and carefully considered for the correct retrofit. Heritage consultants, structural engineers, and building science specialists should always play a part in this process to ensure that each made-to-order solution will extend the useful life and reduce the energy dependency of our historical building stock.

Eric A. Charron (M.Arch., OAA) is an architect with Diamond Schmitt Architects and has been a member of the OAA’s Sustainable Built Environments Committee (SBEC). Dr. Randy Van Straaten (Ph.D., P.Eng.) is a sustainability and building science educator at Fanshawe College and provides building science consulting on a wide range of projects.