By David W. Boyer
Editor’s note: See main story, An Epic Tale: The Monumental Effort to Restore NY Landmark.
Many restoration architects and conservators discourage the use of protective treatments of any kind on historic masonry. They argue that buildings which have survived for generations without benefit of protective treatments have demonstrated the ability to withstand weathering.
Others cite horror stories related to failed attempts at using protective treatments only to suffer accelerated decay of the historic fabrics.
|A film-forming sealer is being stripped from the masonry of 111-year-old Folly Theater, Kansas City, Mo., after moisture, trapped beneath it, caused the coating to blush and delaminate. After cleaning, the theater was re-treated with a breathable, penetrating water-repellent.|
Though I would never recommend application of a protective treatment to historic masonry in place of necessary repairs to the building façade, protective treatments do perform an important function on historic buildings.
Many people involved with older buildings fail to realize that in adapting older buildings to modern uses, we alter the way the entire wall assembly responds to weathering influences.
Relatively massive load-bearing masonry walls used to have the capacity to absorb and gradually release large volumes of water over time. That was before the introduction of modern air-handling systems, energy-efficient doors and windows, watertight building seals, and replacement mortars which boast better “weathering” characteristics than soft historic mortars they are designed to replace.
|Treated vs. untreated: Half of this block (photo on left) was treated with a room-temperature vulcanized (RTV) silicone water repellent, then hit with a bucket of water. The untreated side has let water penetrate and “wetted out.” Unable to penetrate the treated side, the water has beaded up. This type of water repellent is used for highly porous surfaces. In photo at right, tiny water-repellent molecules line the concrete pore, chemically bonding with the minerals in the substrate. They repel liquid water trying to enter the pore, but let water vapor evaporate out of the pore, allowing the concrete to “breathe.”|
Many of the drying mechanisms that older buildings used to benefit from have been systematically cut off by well-meaning interventions aimed at improving the energy efficiency of the overall building and/or the comfort levels of building occupants.
When water vapor condenses, accumulates or penetrates into the historic masonry of recently “restored” buildings, the only avenues remaining for natural drying are to the buildings’ interiors (avenues that are often cut off by contemporary wall coverings), or through the face of the masonry itself.
Though the capacity of the historic masonry to absorb and retain moisture is unchanged, its ability to dry is dramatically reduced. Under these circumstances, people who apply protective treatments that significantly reduce the ability of underlying surfaces to dry, such as film-forming treatments, may indeed do damage to historic masonry.
It is for these reasons that we have always stressed the importance of high vapor permeability when developing and recommending protective treatments for historic masonry. The role of any protective treatments applied to historic brickwork should be to substantially reduce the tendency of that brick to absorb bulk rain water, while having little or no impact on its ability to dry.
Five characteristics to look for in protective treatments for masonry and concrete
1. Breathability. The treatment should let water vapor out without letting liquid water in. This helps keep bulk water from the weather out of walls, while permitting walls to dry naturally should water find its way in through other means, such as condensation of water vapor from humid interior spaces.
2. Penetrability. The treatment should soak in, lining the concrete pores with water-repellent molecules. Penetrability contributes to the following characteristics 3 and 4.
3. Long service life. The molecules should chemically bond with the substrate for a protective service life measured in years. Since the molecules line the pores, rather than sitting on the surface, they are not subject to being abraded away by traffic.
4. Invisibility. The treatment should NOT create a surface film or darken the concrete, masonry or stone. Because penetrating treatments don’t sit on the surface, they generally have no impact on surface appearance or texture. This is particularly important for historic building fabrics.
5. Supportability. The product should come from a reputable manufacturer who backs it up with field and technical support, and toll-free telephone customer service.
About the author
| David W. Boyer|
After earning his B.A. in Architecture and Urban Design at the University of Kansas, David Boyer began his career with PROSOCO in 1980, and in 1992 was appointed president.
He has written numerous articles, monographs and white papers and lectured extensively on the proper ways to clean and protect masonry exteriors, both historic and newly constructed. Most recently he has focused on the importance of incorporating air barriers into contemporary masonry-veneer, cavity-wall designs. He has also helped pioneer new products and procedures for the safe, effective and efficient installation of finished concrete flooring. Boyer is a member of the board of directors of the Sealant, Waterproofing and Restoration Institute.