Back to the Future: Heritage Masonry Retrofits & Freeze Thaw Risk
The popularity of brick masonry in building construction dates back hundreds of years, and it is due to a variety of reasons: clay brick is relatively easy to manufacture, it’s durable, and it’s easy to work with. Masonry buildings can last for decades or longer and have a pleasing aesthetic.
The advent of modern construction methods and materials, including fibre and foam insulation, and the desire to maintain historic brick buildings, have resulted in a movement into hybrid building types, with masonry walls from the 1900’s married to futuristic shining glass and metal façades soaring into the sky.
However, adding insulation and finishes to the interior of historic brick masonry walls changes the moisture behaviour of the brick, which can increase freeze thaw risk and cause damage. Fortunately, this risk can be ascertained with two pieces of information.
First, we can use hygrothermal modelling (i.e. computer-aided simulation of heat and moisture flow) to see how these walls will behave with interior insulation retrofits. The material properties of the brick are a crucial input for these models and inaccurate material properties will return an inaccurate model.
Second, we need a threshold value to compare the model results to – that is, what is an acceptable amount of moisture in the brick when the temperature drops below freezing? The critical saturation level (or Scrit) is the second part of the equation in determining retrofit freeze thaw risk.
With these two pieces of information, we can determine freeze thaw risk with great confidence and ensure a heritage masonry retrofit project goes smoothly.
Although buildings are still being constructed with brick masonry, there is an important distinction between historic and modern brick. Historic brick was created from a mixture of crushed clay, sand, and water. Often, the clay was local to the brick manufacturing location. As such, buildings in different geographic locations of the same time period can have variable brick properties. Bricks were formed by hand and kiln dried, which also contributed to the variability of its properties.
In contrast, modern brick (i.e. brick produced in the latter part of the 19th century) comprises ground shale, clay, and water and is formed and fired with machines. The chemical composition and quality control of the manufacturing process is superior to that of historic bricks, resulting in a higher risk when retrofitting historic brick buildings.
Figure 1: St. Leo’s School, Toronto [From https://upload.wikimedia.org/wikipedia/commons/b/b3/StLeosSchoolMimico.jpg]
Moisture and Critical Saturation in Bricks
Brick is considered a porous material. The structure of the material results in a large, mostly interconnected series of empty spaces. Historic bricks have maximum porosity values of around 30%, meaning that up to a third of the volume of brick is air space. Generally, we have observed porosity values from tested brick around 10-15%. When exposed to water, the brick acts like a sponge, adsorbing water into the pore space. In freezing temperatures, this water can turn to ice and the subsequent expansion can damage the brick.
Liquid water changing into ice results in a volumetric expansion of around 9%. If the pore space in the brick is sufficient to handle this expansion, then no damage will occur – i.e. there is enough extra room in the pores so that even if there is water, and it turns to ice, the brick is undamaged. This threshold level is the above-mentioned critical saturation level (Scrit). Scrit is measured using frost dilatometry, which is a laboratory process that subjects brick samples to varying levels of moisture and then allows the samples to undergo freeze-thaw cycles. The expansion of the brick is measured after a number of cycles.
In the same way brick will adsorb water, it can store it and dry out as well. As long as the drying cycles are sufficient to remove moisture in the brick, there would be no issues with freeze thaw.
The addition of interior insulation, however, changes the drying behaviour. Without interior insulation, the heat supplied in the building will warm the brick up. This is inefficient from an energy use perspective but allows for the brick to dry. Adding insulation reduces this drying potential, which forms the basis of the increased freeze thaw risk.
Figure 2: Typical brick slices for frost dilatometry measurements [From Ryerson Building Science Laboratory].
Figure 3: Typical brick testing setup [From Ryerson Building Science Laboratory].
Heritage Masonry Retrofit Functional Requirements
When starting a historic masonry retrofit project, it is important to outline the requirements and prioritize what criteria the design must achieve. At times, not all criteria can be met. Major factors include:
Generally, the primary goal with retrofit projects is to improve the thermal performance. It is important to consider the ratio of the historic façade to the rest of the façade. If the historic portion is very little, it may be prudent to avoid an interior retrofit at all if there is any risk of freeze thaw. If larger portions of the façade are historic brick masonry, a full analysis with laboratory testing and modelling would be a better choice.
Preservation of the Historic Façade
As above, if the durability of the brick is poor (either through visual observation or laboratory analysis), the addition of insulation at the interior would not be recommended. The sacrifice to thermal performance is sometimes a worthwhile trade-off to retain the longevity of the brick.
Although an interior insulation retrofit introduces new layers (including gypsum) to the interior, effort can be made to maintain some level of inward drying capability through the selection of semi-permeable vapour barriers, insulation, and paint. This is particularly important in colder climates.
Air leakage is the most significant contributor to energy loss and moisture movement through a wall assembly. An air barrier system that is continuous will prevent warm moist air from the interior conditioned space from reaching the interior side of the brick masonry. If this air meets the cold masonry, condensation can occur.
Entuitive’s Integrated Approach
Generally, there are two options for the hygrothermal analysis needed to decrease freeze thaw risk before retrofitting a historic brick building. The first option is to conduct hygrothermal modelling without any laboratory testing. No material properties are measured, and critical saturation is not determined. This option is faster and more cost-effective. However, the results are an estimate. Using the default material properties in the software can be a poor representation of the actual brick. Without the critical saturation level, there is no threshold level for comparison. Instead, an estimate of the occurrence of freeze thaw is developed and discussed. Option one is conducted in-house with industry-leading software.
Figure 5: Manulife Global Headquarters, Toronto; hygrothermal modelling was completed for this building.
The second option is more comprehensive and involves laboratory testing to find material properties and critical saturation. Entuitive has a close relationship with a Canadian testing laboratory that specializes in masonry testing, which further streamlines this process. Before the analysis, our team can assist in choosing and removing the bricks from the building. Our site reviews can help determine which areas are best for brick removals, and we can arrange for the removal and temporary patching of these areas. Once the brick is sent to the laboratory and testing is complete, we can begin modelling.
The material properties are used in the hygrothermal simulation to replicate actual behaviour, and the results are compared to the measured critical saturation level. This direct comparison allows us to accurately determine freeze thaw risk. We develop multiple models, varying things such as insulation type, thickness, and presence of vapour and air barriers. Finally, a summary report is produced that shows the risks of the various retrofit options.
Advanced Performance Analysis
Beyond hygrothermal modelling, our team can work directly with the design team to analyze proposed retrofit designs, conducting laboratory analyses and modelling as required. Collaborating with our Advanced Performance Analysis (APA) group for whole-building energy modelling, we can present a holistic durability and energy performance solution.
Additionally, clients can benefit from our Construction Engineering support for the procedure of retaining and building around an existing façade. The preservation of a historic brick façade while building a multi-storey tower (complete with parking garage, if desired) can be a challenge as there are multiple structural elements that are affected. Our Construction Engineering team can provide the support and expertise to help navigate the retaining of the façade while construction is ongoing.
Figure 4: Façade retention at 42 Hubbard, Toronto.
Heritage masonry retrofits can do wonders to preserve a historic building, ensuring its smooth transition from the past and present to the future, while also preserving the unique architectural fabric of a community. However, the introduction of interior insulation and finishes can increase the risk of freeze thaw, damaging the very bricks the project sought to preserve.
Our integrated team can provide comprehensive design assistance for performance, durability, and constructability of these retrofits in diverse locales and climates. Our relationship with testing laboratories and extensive experience in masonry materials allows us to deliver an optimal solution to any brick masonry retrofit project – as long as you don’t mind losing a few bricks here and there.
To learn more about how Entuitive can support heritage masonry retrofits, reach out to Kevin Zhang.
For more information regarding our Advanced Performance Analysis service, contact Tristan Truyens.
For more information about our Construction Engineering service, contact Jason Jelinek.
Feature Image: Spadina Sussex Student Residence, Toronto; courtesy Diamond Schmitt Architects.