Is this renovation doomed from the start?
Cocteau3 wonders whether he’s witnessing a structural problem in the making.
“Sounds crazy, but I recently saw an old, restored house that has its original brick foundation,” Cocteau3 writes in a post at Fine Homebuilding’s Breaktime forum. “The foundation was in good shape structurally, but the renovators built a new single course concrete block wall around the perimeter, in front of the brick on the inside of the cellar walls.
“The attempt is to reduce the transmission of water vapor and reduce any mold issues,” Cocteau3 continues. “It looks good right now, but I am concerned the vapor will continue to pass through the brick and build up either behind the concrete block, or pass through the concrete block. It seems either scenario will cause the mortar to fail between the two, and possible cause the concrete block to give way.”
Cocteau3 says the non-for-profit renovation agency that’s doing the work is mainly concerned with mold, an issue best addressed by installing a waterproofing membrane on the outside of the foundation. But that’s not going to happen.
“Any thoughts on the success or failure of this concrete block veneering method?” Cocteau3 asks.
Moisture probably won’t be a problem
To Perry525, mold is unlikely to be a problem, and any condensation that collects on the wall surfaces will not pose a structural threat to the inner block wall.
“Mold needs both food and water to grow,” he writes. “The world is covered in mold spores, but if you look around mold doesn’t grow in most places, because either there is no food or no water. Mold doesn’t grow on clean brick or concrete–no food.”
Condensation should be minor, nothing that a ventilation system can’t handle, Perry525 says, and water problems collectively won’t bring down the inner walls.
“However, if the water table outside is high and there is a high rate of water ingress at some time, then the wall could come down,” Perry525 adds. “It is more likely that provision has been made for the water to run down the original brick wall, with it being lead to a sump to be pumped out safely. This type of provision is quite common.”
DanH agrees with Perry’s assessment. “So long as provision has been made to drain the space between, there should be no problem,” he writes. “Mold doesn’t grow on concrete (unless it’s ‘fiber-cement’ or some such).”
The only way that moisture could cause a problem, Perry525 adds, is if seeped into cracks and then froze, causing spalling. “This [is] unlikely to happen in a basement where the temperature basically remains setady around 40F,” he says. “Condensation on/in a concrete wall only happens when the wall is below dew point and the basement temperature is steady.”
When ‘mold’ is really something else
By rights, mold probably shouldn’t be growing on concrete or brick surfaces that don’t provide food for mold spores. Yet dust and organic material that collects over the years may be enough to give mold a toehold.
“Yeah, I’ve seen people using Chlorox solution to try to scrub “mold” off of [concrete masonry units],” DanH writes. “It doesn’t work very well, but hit it with weak acid and it will wash right off. You will occasionally see dark-colored mold on paint on CMU, but it’s very superficial.”
And the reason that acid works when bleach will not? The problem isn’t mold at all, but efflorescence, DanH writes, a buildup of “lime-like minerals.”
“The color will generally clue you–generally efflorescence deposits will be white or grayish. Sometimes bluish, and on rare occasions a darker color, depending on the types of dissolved minerals,” DanH continues. “Mold will occasionally be blue-gray, but most often a darker gray tending towards black.
“Efflorescence will have a rough, spiky appearance/texture, while mold will be smooth and slightly fuzzy. Efflorescence will appear near cracks in the masonry where water comes through the fastest. Mold will appear where the surface gets damp, but where fresh water isn’t constantly coming through. It’s really quite hard to confuse them once you learn how to recognize them.”
Acid, he says, will loosen or dissolve efflorescence while making only a slight effect on mold. A diluted bleach solution will turn mold a much lighter color instantly, but will have little effect on efflorescence.
And the real winner is … the mason
To Andy Engel, the source of the moisture is a mystery. “Where is it coming from?” he asks. “Is it from soil moisture, or is it from humid air condensing on the cold foundation? The solution depends on the answer to this question.”
Regardless, Engel doesn’t see the point of adding an inner block wall.
“In neither case can I imagine the inner block wall making any contribution except to the pocketbook of the mason,” he writes.
This approach to fixing a leaky block or stone foundation goes a few steps farther than adding an interior block wall. It provides drainage, insulation, and the option for finished space.