Does a CMU Block Foundation Need Waterproofing on the Sides?
If you are building a new home or repairing a basement, you may wonder if the exterior sides of your Concrete Masonry Unit (CMU)—commonly known as cinder block—foundation require a waterproofing treatment. The short answer is yes. In fact, CMU foundations are significantly more vulnerable to water intrusion than poured concrete walls due to their hollow cores and numerous mortar joints.
1. The Porous Nature of CMU Blocks
Unlike a monolithic poured concrete wall, a CMU foundation is an assembly of hundreds of individual blocks held together by mortar.
- Capillary Action: CMU blocks are inherently porous. They act like a sponge, "wicking" moisture from the damp soil through a process called capillary suction.
- Hollow Cores: If water breaches the outer shell of a block, it enters the hollow core. Once inside the "cells" of the wall, water can travel vertically and horizontally, eventually seeping into your basement or crawl space.
- Mortar Joints: Every horizontal and vertical line in your foundation is a potential "cold joint" where settlement cracks can allow bulk water to enter.
2. Waterproofing vs. Damp-proofing: Know the Difference
In many home improvement projects, these terms are used interchangeably, but they are technically very different.
- Damp-proofing: Usually a thin, asphalt-based spray or "tar" coating. It is designed to stop vapor transmission and incidental soil moisture. However, it cannot bridge cracks and will fail under hydrostatic pressure (standing water against the wall).
- Waterproofing: This involves a thick, elastomeric membrane (either liquid-applied or peel-and-stick sheet) that can stretch. True waterproofing is required to stop liquid water from entering under pressure.
3. The "Positive Side" Waterproofing System
The best time to waterproof a CMU foundation is during construction on the "positive side" (the exterior). A professional-grade system involves three layers:
- Parging: A thin coat of cement-based mortar applied over the blocks to create a smooth, flat surface. This fills the pores of the blocks and prepares them for the membrane.
- Primary Membrane: A rubberized asphalt or polyurethane coating that creates an impermeable skin over the parge coat.
- Drainage Board: A dimpled plastic mat (like DELTA-MS) that protects the membrane from rocks during backfilling and provides a "path of least resistance" for water to drop down to the footer tile.
4. Consequences of Neglecting Waterproofing
Skipping this step in a home improvement or build project can lead to several long-term structural and health issues:
- Efflorescence: That white, powdery substance on basement walls is actually salt left behind as water evaporates. It is a visual signal that your foundation is "breathing" water.
- Molded Drywall: If you finish a basement without waterproofing the CMU exterior, the trapped moisture will inevitably rot your wood studs and grow mold behind the insulation.
- Structural Degradation: Over decades, the constant cycle of water entering and freezing inside the hollow cores can cause the faces of the CMU blocks to "spall" or flake off, weakening the foundation.
5. Can You Waterproof from the Inside?
If the exterior is already backfilled and you have a leak, you can perform "Negative Side" waterproofing. This usually involves crystalline coatings (like Xypex) that penetrate the block to block the pores. While effective at stopping dampness, it does not prevent the block itself from being saturated. Whenever possible, exterior "Positive Side" treatment is the superior choice.
Conclusion
A CMU block foundation absolutely needs waterproofing on the sides. Because cinder blocks are highly porous and rely on mortar joints, they lack the natural water resistance of other materials. By investing in a parged exterior wall, a high-quality membrane, and proper drainage, you protect your home improvement investment and ensure a dry, healthy interior environment for the life of the structure.
