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Historic Preservation - Technical Procedures

Spectitle:

Types Of Masonry Water Repellents

Procedure code:

0718001S

Source:

Prosoco, Inc. Product Literature - Kansas City, Ks

Division:

Thermal And Moisture Protection

Section:

Water Repellents

Last Modified:

02/24/2012

Details:

Types Of Masonry Water Repellents



TYPES OF MASONRY WATER REPELLENTS


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:

    1.   Usually transparent

    2.   Do not seal the surface

    3.   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.

There are three different types of water repellents used for
treating masonry surfaces.  They are:

    1.   Silicones,

    2.   Silanes, and

    3.   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.

                         END OF SECTION