Understanding Water Repellent Primer


February 2009

Moisture Management

Side Story: Understanding Water Repellent Primer

First of all, let me address the difference between water repellents and waterproofers. Many people think the two terms are interchangeable, but they really aren’t. Typically, waterproofers are thick, film-forming coatings. These types of coatings would often be used below grade, such as on a basement wall. As one example, we all have seen a new house under construction and the black coatings being applied to the exterior of basement walls. These coatings are meant to prevent water from entering the basement wall. They are high build (thick) applications that are meant to stop all movement of water and are not breathable (do not allow the movement of moisture vapor or moisture in a gaseous form). They are designed to stand up to soil and water’s pressure against the outside of a basement wall. Another example is water’s pressure in pools and fountains against their containment wall. This is referred to as hydrostatic pressure. Water repellents are not intended for use where they would be exposed to hydrostatic pressure. They simply cannot stand up to or resist that constant pressure being exerted on the surface. Water repellents are meant to repel rain and water over relatively short periods in comparison to below grade and pool or fountain situations.

The amount of water repellent in a product is measured by the products’ solids content and is noted as a percentage. For example, a siloxane water repellent may be designated as 7 percent, 10 percent, or 15 percent solids. Silanes are usually 20 percent or 40 percent because of the siliane’s volatility. Acrylics can range from 7 percent to as much as 30 percent to 35 percent. The amount of solids required will be dictated by the porosity of the surface. A dense, hard, burnt-face brick will only need 7 percent solids water repellent, but an extremely porous split-face block would require 20 percent solids. When choosing a solids content, it is best to remember that more is not always better. Too high of a solids content on a dense brick can cause a significant reduction in the brick’s moisture vapor transmission rate. This can lead to a build-up of moisture in the brick and mortar, which is undesirable.

Water repellents are generally divided into two groups: penetrating and film forming. Penetrating water repellents work by penetrating into the pore structure of the substrate and deposit their repellent component in and on the masonry’s pores. They do not, as a rule, alter the appearance of the surface to which they are applied. Some of the most common penetrating types would be the silane, siloxane, silicates and silicones, or a blend of these repellents. They are well suited for application to both vertical and horizontal surfaces. These types of repellents are well suited for use on older, historically significant buildings, where altering of the surface appearance would be undesirable.

The film formers, which include acrylics, mineral gum waxes, silicone resins, stearates and urethanes, are the most common. They work by forming a film on the surface to which they are applied. Their moisture vapor transmission rates will not be as good as the penetrating types and, in some cases, such as the urethanes, will have virtually none. Some people like a glossy or wet look to the surface, so the acrylic type repellents would be the type of choice.

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