Waterproofing Concrete Block Foundations

Concrete blocks, cinder blocks, and lightweight CMU's are much more porous compared to poured concrete. This makes concrete block walls more suseptible to water seepage and potentially harder to seal.

By RadonSeal | Updated | February, 12 2020


How to Waterproof Your Concrete Blocks

Concrete blocks and cinder blocks are much more permeable to water and water vapor than poured concrete. According to ASHRAE Handbook, the permeability of concrete (1:2:4 mix) is 3.2 perms per inch of thickness. A good quality newly poured concrete wall 8-10 inches thick has a permeance (=permeability/thickness) of 0.4-0.3 perms (=3.2/8 - 3.2/10).

Heavyweight Concrete Blocks Are Semi-Permeable - Concrete blocks (CMUs) have larger pores than poured concrete and only thin walls (about 1-1/4-inch) next to their hollow cores (3.2/1.25 = 2.6 perms). Materials with permeance of 1 to 10 perms are classified as "semi-permeable," although the permeance of concrete blocks varies widely. Some tests show 4.8 perms for hollow concrete blocks and 2.4 perms when the blocks are filled with a slurry mix of concrete.

Block foundation walls will provide only a weak barrier against capillary water seepage and water vapor. Concrete blocks commonly begin to show signs of dampness or worse, leak water, soon after exterior waterproofing coatings deteriorate, crack, or fail. Cracks in the concrete blocks and mortar joints are exposed where water easily makes its way through. A tall water column builds up inside the hollow blocks and the hydrostatic pressure pushes water indoors, seeping through the pores and thin walls of the lower blocks. Over time and consistent exposure, water gradually leaches through the concrete pushing out efflorescence and enlarging the pores, making the blocks that much more porous.

Do you know the difference between cinder blocks and concrete blocks? Cinder blocks, lightweight CMUs, splitface blocks, haydite, and "popcorn blocks" are considered permeable to water and vapor (>10 perms) which makes waterproofing them even more difficult to seal!

DIY Waterproofing Solutions

Above grade split face block wall

 If the interior side of the concrete blocks continuously dampens or wet, drain the water accumulated inside the hollow cores by drilling weeping holes in the bottom blocks (3/8" masonry drill) and allow them dry out for four or more days (dryer the better).

You will often see efflorescence (white minerals) or mold growth on damp concrete. A light amount of efflorescence can be removed with a wire brush or mechanically with a wire-wheel drill attachment. For stubborn or heavier deposits of efflorescence, use a combination of wire brush and an acid cleaner. You can avoid having to use dangerous acids with use of our easy, spray-on Efflorescence Cleaner which will quickly dissolve the salts and kill mold and mildew growth. To just remove mold and mildew spots, use BioZap All-Natural Mold & Mildew Cleaner instead of strong acidic cleaners and inhaling fumes from the use of chlorine bleach.

 Once the concrete is clean and thoroughly dry, apply RadonSeal Deep-Penetrating Concrete Sealer to the walls (read application instructions). RadonSeal works by penetrating below the surface of concrete, reacting with lime and alkalis internally, and curing as a hardened mineral within the microscopic pores and capillaries of concrete. RadonSeal will help to block and greatly minimize capillary water seepage and vapor through the matrix of concrete. Unlike waterproofing paints or coatings, RadonSeal cannot peel and cannot be pushed off by hydrostatic water pressure.

 After the concrete surface has dried and RadonSeal has initially cured, plug the weeping holes with caulk, epoxy, or a concrete patching compound.

Tip – Good ventilation after the application or using a fan, dehumidifier, or heater will help, particularly in damp areas, to evaporate water from the sealer so that it can react and cure before the next big rain. If you are dealing with excessive ground water due to heavy rains and snow melt, it may be in your best interest to hold off sealing the concrete till drier weather returns.

When To Use RadonSeal & Ion-Bond Armor Combination

Combining Waterproofing Sealers – The combination of using RadonSeal Plus followed by Ion-Bond Armor Elastomeric Concrete Sealer provides the tightest possible seal for poured concrete and concrete blocks. Ion-Bond Armor is also a penetrating sealer which chemically bonds with RadonSeal forming a a strong waterproof membrane below the surface of concrete.

It is strongly recommended for more challenging applications such as; cinder blocks foundation walls, concrete blocks that have consistently seeped water in the past (leached out blocks), thin concrete, sandstone basement walls, or before framing basement walls.

Not all basements and blocks are the same! There is a possibility that RadonSeal Plus alone, may not completely suffice for overly porous or leached out blocks (read about Lightweight CMU's below). Such blocks may show improvement after using RadonSeal, but can still dampen after heavy rainfalls. If the surface of the concrete blocks still get wet after applying RadonSeal, or if you want to guarantee the result to your customers, use Ion-Bond Armor Subsurface Concrete Sealer in combination with RadonSeal Plus.

Applying Ion-Bond Armor – Allow RadonSeal Plus to cure for at least 10 days after application. This will give RadonSeal enough time to initially cure inside the concrete. If RadonSeal purges efflorescence or salts from within the concrete, remove when needed (RadonSeal Instructions). Make sure the concrete is thoroughly dry (use a fan if needed). After 10 days, apply a wetting application of Ion-Bond Armor (Ion-Bond Armor Instruction).

Waterproofing Lightweight CMU's

Lightweight Split Face and Haydite Blocks are dry-pressed blocks made with little or no cement. Used commonly for above-grade construction as decorative blocks, facades, retaining walls, garden boxes, and hardscape. Since RadonSeal relies on hydroxide produced as a result of the cement curing process, as the cement content decreases, the ability for RadonSeal to properly work also decreases.

When sealing CMUs with low-cement content, the appropriate sealers of choice would be our LastiSeal Brick & Masonry Sealer or DryWay Water-Repellent Sealer. Chemically different, both sealers share similar characteristics. Both will penetrate and plug the pores of the blocks below the surface, waterproofing the substrate against rain water.

Prevention Pays Off

Prevention is Better Than Cure! It is much easier for waterproofing sealers, paints and membranes to work on blocks before they start leaking! Long-term water seepage dissolves lime and salts, which shows up as efflorescence. Leaching enlarges the pores in concrete and further speeds up the seepage, leaving hard-to-seal leached out blocks.

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