Some modern curtain wall systems include open joints in their projecting metal components to allow for drainage and evaporation of intruded moisture, or to facilitate air movement for pressure equalization, as with rainscreens. Closed joints in these systems include a variety of foam or synthetic rubber gaskets to seal various metal-to-metal and metal-to-glass connections and, once again, allow for movement. Semi-rigid preformed gaskets are typically used because they more firmly hold glazing units in place than sealant caulking alone yet provide enough flexibility to avoid cracking. To seal glazing units to the gaskets, butyl caulking is often used, but it can be difficult to handle and provides poor abrasion resistance, which is why it tends to be reserved for this type of concealed condition.
Historic restoration considerations
When correcting deterioration in existing and historic enclosures, joint replacement is often a key part of a repair program. Joint materials such as sealant become dry and brittle as they age and are exposed to sunlight, and mortar is subject to freeze/thaw damage as it absorbs moisture and can crack from movement. Given the important responsibilities of joints, maintaining them helps prevent other issues from developing. If failed joints are left unattended, they can accelerate deterioration in adjacent materials and decrease energy performance. However, it is not always clear if old joints should be replaced in kind, or if a contemporary alternative material should be introduced.
Windows
For many historic steel and wood windows, joints between glass and sash frames and muntins were secured and sealed with glazing putty, a mixture of chalk, calcium carbonate, and oil. When these joints deteriorate and begin to leak, there is often an inclination to seal over or replace putty joints with caulking. However, even when sealant is properly installed with a bond-breaker, it typically will not outlast glazing putty if the latter is regularly maintained with linseed oil coating and painting. Similar inclinations to caulk leaking joints often occur with stained-glass windows, which were historically joined with lead cames (soldered metal supports) and sealed with a mixture of linseed oil and calcium carbonate. This provided a durable glazing joint system with a balance of rigidity and flexibility. Although expensive, the most preservation-minded and longest-lasting approach involves removing the windows and reassembling them in a specialized glass restoration shop, so all cames can be replaced and resealed, and the delicate single-pane stained glass can be handled carefully.
Masonry
While most masonry construction since the early 20th century has used cement mortar, many older structures used lime mortar. Since cement is several times stronger, harder, and less vapor permeable than lime, using cement mortar for repointing deteriorated joints in these structures could damage old and weathered masonry units, which have become softer over time. Once again, moisture can become trapped in the masonry and cause it to spall. Although some manufacturers provide pre-engineered restoration mortars for historic applications, where feasible, it is best to have the original mortar tested to confirm its composition and have a matching mortar custom reproduced. It is also important to use appropriate hand tools when performing repointing work at historic thin joints, as saw-cutting can unnecessarily enlarge original joints, degrading performance and character. Further, for historic buildings designated as landmarks, repair projects are often subject to review from municipal preservation boards, and it is commonly required that repointing mortar match the original in color, texture, and profile (tooling).
Sealant caulking was not typically used in historic masonry assemblies, but there are scenarios where introducing such joints may be beneficial. An example is the lack of expansion joints in existing walls, often revealed by cracking near corners or at transitions between components, such as windows. In these situations, large sections of masonry may require rebuilding with the introduction of sealant and backer materials between new and existing sections, often in conjunction with steel relieving angles supporting the masonry. However, such installations can present aesthetic concerns, especially when the structure is landmarked. Such modifications not only generate material differences, as sealant cannot replicate mortar texture, but introducing expansion joints can visually interrupt the existing brick or stone layout. Masonry reconstruction can also break bonding patterns and requires the use of new steel reinforcement to achieve integrity. While such alterations may be more easily justified to preservation boards at less visible facades, when introduced along walls that face public spaces, approvals can become challenging, and details may require special sensitivity to historic configuration. Moreover, verifying sealant adhesion may require performing field tests and/or applying a chemical primer before sealant installation.
Roofing
Although many steep-slope roofs have open joints, some historic terra cotta roofing, such as Mission Style and Spanish Style tiles, can be mortar set in lieu of nailed. When these joints fail, it likely means the tiles have begun to de-bond from the substrate, and while selective tile repairs are possible, replacement of the system may eventually be required. While the system could potentially be replaced with a mechanically fastened tile roof, which may provide cost savings, there are considerations to weigh before adopting such a retrofit strategy. Landmark status may require mortar setting unless there is a structural justification to prevent replacing the assembly in-kind. Also, mechanical attachment requires a suitable substrate for fasteners to resist pull-out loads, so the decking and support structure may require retrofitting. Further, while not watertight, mortar does play a role in weatherproofing, and therefore different underlayments may be required to mitigate air and water infiltration.
