Waterproofing Concrete Block Foundations

Hollow-Core Cement BlockConcrete 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).

Concrete Blocks Are Semi-Permeable - Concrete blocks (CMUs) and cinder blocks have larger pores 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." 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.

Cinder blocks, lightweight CMUs, splitface blocks, and "popcorn blocks" are considered permeable (>10 perms) and make waterproofing even more difficult.

Block foundations provide only a weak barrier against water or vapor and they usually start leaking as soon as the exterior waterproofing coating deteriorates. A tall water column builds up inside hollow blocks and the hydrostatic pressure pushes water indoors through the thin wall of the lower blocks. Over time, water gradually leaches out the concrete and makes it still more porous.

How to Waterproof Your Concrete Blocks

Example Of A Concrete Block FoundationDIY Waterproofing Solution – If the concrete blocks are continuously wet or damp, drain the water accumulated inside the cores by drilling weeping holes in the bottom blocks (3/8" masonry drill) and let them dry out for a couple of days.

You will often see efflorescence (white minerals) or mold growth on damp concrete. Remove light efflorescence with a wire-brush (or wire-wheel). For heavier efflorescence, avoid dangerous acids and use our easy, spray-on Efflorescence Cleaner. Instead of inhaling fumes from chlorine bleach, remove molds and mildew the right way with BioZap All-Natural Mold & Mildew Cleaner.

Once the concrete is clean and dry, spray on RadonSeal® Plus Penetrating Concrete Sealer in three back-to-back applications half an hour apart. Cinder blocks and very porous blocks need four applications. RadonSeal penetrates deep into concrete (up to 4 inches), reacts with alkalis, expands, and seals the pores in concrete PERMANENTLY!

Unlike waterproofing paints or coatings, RadonSeal cannot peel and cannot be pushed off by hydrostatic water pressure.

IMPORTANT! Do not apply RadonSeal on actively leaking blocks or when a heavy rainstorm is expected within a couple days. Heavy water seepage can push RadonSeal out of the pores before it had a chance to properly react. Avoid applying RadonSeal on wet or internally water-saturated blocks because water dilutes the sealer and prevents it from reacting (drain them first with weeping holes).

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.

After the concrete has dried and RadonSeal has cured, fill the weeping holes with caulk or a patching compound.

Cinder blocks are more porous and harder to seal – check out "Waterproofing Cinder Blocks."

Lightweight blocks, split face blocks, and Haydite blocks are dry-pressed blocks made with little or no cement. Since RadonSeal works be reacting with cement, you need another waterproofing sealer – our LastiSeal Brick & Masonry Sealer. It is also a deep-penetrating sealer that fills the pores permanently (like filling the pores with epoxy).

Apply a Second Concrete Sealer on More Porous Blocks

Using A Combination of Waterproofing Sealers – While RadonSeal Plus successfully seals most concrete blocks, it may not suffice for very porous or already leached out blocks. Such blocks may still appear wet or leak. If your concrete blocks are still wet after applying RadonSeal, or if you want to guarantee the result to your customers, use our Ion-Bond Armor Subsurface Concrete Sealer in combination with RadonSeal Plus. The combination of the two sealers is also recommended for properly sealing cinder block walls.

Applying Ion-Bond Armor – Allow RadonSeal to cure for at least 10 days. Clean off any purged efflorescence or minerals and make sure the concrete is dry. Then spray the blocks with the Ion-Bond Armor. Ion-Bond is a penetrating sealer which bonds with RadonSeal and forms a waterproof membrane below the surface of concrete.

Prevention Pays Off

Prevention is always better than cure – it is much easier to seal blocks before they start leaking. Long-term water seepage dissolves lime and salts, which shows up as efflorescence ("white powder"). This enlarges the pores in concrete and further speeds up the seepage, leaving hard-to-seal leached out blocks.