Internal Protection

Integral water repellents are an important part of the moisture control strategy in a single wythe wall. Repellency characteristics manufactured and locked in the design mix reduce the concrete's absorption properties and ensure permanent performance. Consequently, the entire unit is treated so as to provide a back-up layer of protection that lasts the service life of the unit and protects it from moisture during construction. Architects will want to note whether the manufacturer includes an integral water repellent as a standard feature of the CMU.

The figure below shows CMU blocks with and without a water repellent additive. On the block at the left, water poured on top of the block penetrates into the block. The block on the right wicks the water. For best moisture prevention, the integrated water repellent additive should be specified in the block and in the mortar. In fact, the NCMA recommends that the same manufacturer's water repellent for mortar and concrete block be incorporated for compatibility and the same reduced capillary action characteristics. These water repellent admixtures both serve as a vapor barrier and reduce the ability of moisture to travel via capillary action within the CMU. However, these admixtures do not stop moisture that enters through cracks in the wall, and they prevent any water that does penetrate from exiting easily. Consequently, other moisture reduction methods such as flashing and control joints are critical to achieve full mitigation.

Images courtesy of the Trenwyth team

Surface Protection

Applying a clear treatment, paint, or opaque elastomeric coating can enhance the water tightness of a wall. It is important to verify with the manufacturer that post-applied surface coatings are compatible with the block's integral water repellents. However, post-applied water repellents are less successful in moisture prevention than integral repellents. They have a limited surface life of approximately two to seven years depending on the manufacturer. While post-applied coatings can be a good surface repellent, they do nothing to prevent an untreated unit from getting wet prior to sealing. If using surface protection, most manufacturers recommend a two-coat system.

Flashing

While integrated and surface-applied water repellents can enhance the watertightness of a wall, they are not a substitute for proper design that incorporates flashing. When flashing is used, the importance of proper detailing cannot be over-emphasized. Water mostly moves downward—a principle that informs the locations for flashing on single wythe walls. Flashing is best located at the top of the wall, the window head or bond beams, the window sill, and the wall base. In the accompanying figure (see the next page), note the proper detailing of flashing at the window sill.