The use of “sealers” on exterior historic masonry is a questionable intervention. Since all historic masonry walls, as well as the individual historic masonry units, need to “breathe”, (i.e. allow moisture vapor to escape), there should be a specific purpose in using some form of coating. Only after physical waterproofing via repointing and physical repair or damaged units should chemical “waterproofing” be considered. Those considered must have properties which maintain a high vapor transmission which will allow moisture to escape. The specific purposes to use a water repellent are:
- Inhibiting deterioration of the masonry units by not allowing the wetting/drying cycles, (the very cause of historic masonry failure over time), to occur through the unit or the joints. Although water repellents are not traditional historic material they can act as a “sacrificial barrier” to weather away from the action of the elements before more of the historic fabric weathers away.
- As a grain strengthener, (surface consolidant/water repellent).
- As an inhibitor to capillary action and the absorption of water into the building if the absorption of the brick is greater than the masonry’s ability to release the water back into the atmosphere before entering the building’s interior.
Overall great concern should be employed and testing should be carried out before simply spraying a building with a water repellent. In the turn of the last century masonry buildings were waxed for protection and sure enough the wax yellowed and picked up atmospheric pollutants which greatly discolored the building. Silicon sealers soon followed which yellowed if not simply breaking down from ultra-violet rays of the sun. Now silane and siloxane breathable water repellents are in vogue. These materials seem to be effective for some applications. The question remains as to whether the breathability of the historic fabric will in time be compromised. Our advice is to remain conservative and do not introduce chemicals to historic masonry just to do it as a final measure.