Mold on Concrete Block Walls in South Florida Homes
Most homes in Miami-Dade and Broward are built with concrete block and stucco, usually called CBS construction. It stands up to hurricanes and termites, and many owners assume it is also immune to mold. Block does not rot, but it is porous, it soaks up water like a hard sponge, and it releases that water slowly toward whichever side is drier. With the AC running most of the year, the drier side is usually the inside of your home.
How a block wall is put together
Understanding the layers explains where mold shows up. From outside in, a typical wall has paint, stucco, the block itself, then thin wood or metal furring strips fastened to the block, sometimes a thin layer of insulation, and finally drywall. The gap behind the drywall is narrow and unventilated. Mold rarely feeds on the block. It feeds on what touches the block: furring strips, the paper back of the drywall, baseboards, dust and paint.
In garages, utility rooms and some older homes the block is left exposed or simply painted. There the growth is visible on the surface.
Where the water comes from
- Stucco cracks. Hairline cracks are normal as a building ages, but they let wind-driven rain into the block. Cracks at window corners and where different materials meet are the usual entry points.
- Failed paint and sealant. Exterior paint is part of the waterproofing. Chalky, faded paint and dried-out caulk around windows and penetrations let the wall absorb water during every storm.
- Window and door openings. Water gets in around the frame and runs down inside the wall below the sill.
- Ground contact. Soil, mulch or planters piled against the wall, sprinklers hitting the stucco, and poor drainage keep the bottom course of block wet. Moisture then wicks upward.
- Condensation. Humid outdoor air working through a wall meets a cold, air-conditioned interior surface. Closets and rooms kept very cold on exterior walls are prone to this.
- Plumbing. A slow leak in a line run through or along the wall keeps the block saturated from inside.
Mold or efflorescence?
A white, fluffy or crusty deposit on block is frequently mistaken for mold. Often it is efflorescence, mineral salts left behind when water moves through masonry and evaporates.
- Efflorescence is white or grayish, crystalline or powdery, and crumbles to a dry dust between your fingers. It tends to dissolve when sprayed with water.
- Mold can be black, green, gray or white, looks fuzzy, spotty or slimy, smears when wiped, and usually comes with a musty smell.
Efflorescence is not a health concern by itself, but treat it as evidence. It proves water is traveling through that wall, and the same moisture will support mold on whatever organic material is nearby.
Signs of a problem behind the drywall
- Baseboards on exterior walls that are stained, swollen or pulling away.
- Paint that bubbles or peels on an exterior wall, particularly low on the wall or under windows.
- A musty smell in one room that is stronger after several days of rain.
- Dark spotting along the bottom of the wall or behind furniture and headboards.
- Rust stains bleeding through paint where drywall screws or metal furring are corroding.
Cleaning exposed block
A small area of growth on bare or painted block, such as a garage wall, is something many owners can handle. Block is durable enough to scrub.
- Wear gloves, eye protection and an N95 respirator, and ventilate the space.
- Scrub with a stiff brush and a detergent solution. Avoid dry brushing, which puts spores into the air.
- Rinse lightly and dry the wall quickly with fans or a dehumidifier.
- Watch the area for the next few weeks, especially after rain.
Do not coat a damp wall with waterproofing paint from the inside and call it finished. If water is still entering from outside, an interior coating traps it in the block, and the coating eventually blisters and peels.
Fixing the cause
The lasting repair is nearly always on the exterior. Have stucco cracks repaired, recaulk windows and penetrations, and repaint with a quality exterior coating when the old paint is chalking. Keep soil and mulch below the level of the interior floor, aim sprinklers away from the walls, and make sure gutters and grading move water away from the foundation. Inside, keep relative humidity below 60 percent, ideally between 30 and 50, and leave a few inches between furniture and exterior walls.
When professional remediation is needed
If growth is behind finished drywall, the affected drywall, furring and insulation generally have to be removed under containment, the block cleaned and dried, and the water entry corrected before the wall is rebuilt. Painting or patching the interior side alone leads to the same stain returning in the next rainy season. If you want testing or a written report, an independent licensed mold assessor provides that separately. For walls that are stained, musty or soft, request an on-site remediation estimate at drymiami.com or call (954) 778-7999. The visit is $79, credited in full toward the job if you hire DryMiami.
Mold problem at home?
We come out, find the moisture source and give you a written remediation estimate. The visit is $79, credited in full toward your job if you hire us.
