U.S. General Services Administration
Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
Rewrite
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
The application of water repellents to masonry is generally not necessary and not recommended. However, this standard includes general information on water repellents and descriptions of several different types that are used.
Characteristics of Water Repellents:
- Usually transparent
- Do not seal the surface
- Resist liquid water penetration, but allow water vapor to enter and leave the wall through the masonry pores.
Water repellents are often applied to masonry surfaces unnecessarily due to misconceptions that they afford protection resulting in the eventual low maintenance of the masonry.
Misconception #1:
Water repellent coatings prevent the collection of dirt and pollutants on the masonry surface.
Reality:
This is sometimes true; however, water repellent coatings often attract dirt to the surface and retain it.
Misconception #2:
Water repellent coatings prevent the development of efflorescence on the masonry surface.
Reality:
Water repellent coatings can prevent salts from migrating to the surface of the masonry; however, this can lead to a build-up of salts just behind the masonry surface and can eventually cause the masonry to spall.
Misconception #3:
Water repellent coatings will prevent water penetration to the interior of the building.
Reality:
Water repellent coatings offer some resistance to liquid water penetration, but will retard evaporation of water already present in the masonry, which can lead to more serious problems.
For example, the presence of a water repellent coating will cause rising damp to travel higher in the wall as it searches for a surface from which to evaporate.
Types
There are three different types of water repellents used for treating masonry surfaces. They are:
- Silicones,
- Silanes, and
- Siloxanes.
Silicone Oils, Organic Sealers and Surface Coatings:
Characteristics:
- Silicone oils, organic sealants and surface coatings deposit waterproof or water repellent solids on or in the masonry surface, blocking the natural porosity and permeability of the masonry.
Disadvantages:
- Effectiveness of water repellency depends on the alkyl group used (which directly influences its resistance to alkaline conditions), the amount of exposure to ultraviolet light and the level of moisture in the masonry when the silicone is applied.
- Low resistance to alkaline building materials. Alkaline surfaces attack most resins and oils, greatly reducing their life-expectancy.
Silanes:
Characteristics:
- React with moisture to form its water repellent characteristics
- Composed of smaller molecules and, therefore, can penetrate deeper into the masonry
- Compatible with a wide range of solvents
- Alkali resistant
- Can be applied to slightly damp substrates
- Exhibit good vapor permeability
Disadvantages:
- Highly Volatile:
- High concentrations (40%) of the material must be used for effective treatment and can, therefore, be expensive.
- Effectiveness may depend on environmental conditions; Adverse effects may result from high temperatures, extremely low humidity, heavy winds or drought.
- Naturally and especially in high humidity, silane monomers are lost to rapid evaporation. Therefore, the material must be high enough in solids to compensate for the loss.
Reliance on Alkaline Catalyst:
- Effectiveness depends on the presence of alkaline materials.
- Water repellent effect is reduced on neutral surfaces such as brick, terra cotta and some natural stones.
- Limited Surface Repellency:
- Surface repellency develops slowly and in some cases not at all.
Siloxanes:
Characteristics:
- Contains a built-in catalyst, reducing dependency on alkaline materials for effectiveness.
- Provides excellent surface repellency on both neutral and alkaline substrates.
- Can penetrate deeply into the masonry substrate.
- Can be applied to slightly damp substrates
- Alkali resistant
- Durable
- Evaporates slower than silanes
- Has a high flash point
- Exhibits good vapor permeability
Disadvantages:
- NOT effective on some types of pure limestone materials.