Q: On a basement remodel, the existing CMU foundation had a lot of efflorescence. We added foundation waterproofing and corrected the drainage to deal with the moisture issues. Do we need to remove the efflorescence before we finish the walls with rigid foam, 2x4 framing, and drywall?
A: Foster Lyons, an engineer and building-science consultant, responds: One of the saving graces of concrete block (CMU) is that it is extremely porous. CMU is more like no-fines concrete than brick or normal-weight concrete. In fact, this material is so porous that it puts up almost no resistance to the movement of water. That’s one of the reasons you typically see basement interior efflorescence on the CMU and not on the mortar between the blocks. Water is leaving the assembly through the CMU rather than through the mortar (see photo, right). Mother Nature always takes the path of least resistance, and when the water exits, it leaves behind dissolved salts (efflorescence) on the surface of the block.
Do those salt deposits pose a problem? Not really. CMU is so porous, internal osmotic pressures don’t build up the same way they do in brick and normal-weight concrete. In brick and normal-weight concrete, the pore structure is so small that osmotic pressure caused by the salts on the exterior surface can break the material, causing structural damage. But the extremely porous structure of CMU means no internal pressure and therefore no structural damage.
With the new waterproofing and drainage measures for the foundation, the amount of available water has been greatly reduced. The salt in the CMU is not unlike mold—without water, the problem doesn’t exist. And speaking of mold, the salt that leaches out of CMU in the efflorescence process does a pretty good job of dehydrating and killing mold spores. So in that regard, the salt deposited on the wall acts as a good anti-fungal agent.
However, that salt will attract water in both liquid and vapor form. Mother Nature doesn’t like this type of salt concentration and will try to dilute it. The salt acts to pull water from whatever source it can—the CMU, the ground, the footing, the interior air—to self-dilute. For this reason, I suggest removing this salt. Don’t worry about being too thorough. Just stiff-brush the salt from the wall, sweep it up, and throw it in the trash. That treatment won’t remove every last speck of salt, but it will take care of the bulk of it. I don’t recommend acid washing the wall or wet washing it in any way. Using water to remove the salt only adds water to an assembly that you would prefer to keep perfectly dry.
More critical than removing the salt crystals from the CMU is air-sealing the interior rigid insulation. Tape the rigid insulation at all joints, and seal along the top and bottom edges with caulk. This air-sealing will reduce the potential for condensation behind the new rigid insulation by preventing warm, moist interior air from coming in contact with the cool surface of the CMU.
Some moisture may still get through the CMU, and it needs a way to dry. About the only way for water vapor to exit is through the interior insulation, so don’t prevent it. If the home is in climate zone 3 or below, any rigid insulation with a perm rating above 0.1 will work. If the home is in climate zone 4 or above, the insulation should be “vapor semi-impermeable” (greater than 0.1 perm but less than 1.0) or “vapor semi-permeable” (greater than 1.0 but less than 10). Do not rely on “vapor permeable” (greater than 10 perms) insulation—such as fiberglass only—in the 2x4 interior wall in climate zone 4 or above where there is a risk of condensation on the above-grade section of the foundation wall.