Mildew vs. Mold
Mildew and mold are both fungi, but they differ in how they grow, where they appear on exterior surfaces, and how they respond to cleaning chemistry. Mildew is the surface-level, flat, gray-to-white growth that forms on damp surfaces — siding, concrete, outdoor furniture, shower walls. It grows on the surface of the material and is relatively easy to kill and remove with sodium hypochlorite at standard soft-wash concentrations. Mold, on the other hand, is a deeper-rooted growth that penetrates porous materials like wood, drywall, insulation, and unsealed concrete. Black mold (Stachybotrys) visible on interior walls signals a structural moisture problem, not a cleaning problem.
In depth
On exterior surfaces in Madison, the growth most commonly called "mold" by homeowners is technically mildew — it is surface-level, responds to soft washing, and does not indicate structural damage unless the underlying material is constantly saturated. True interior mold requires remediation, not cleaning. The distinction matters because one is a cleaning service and the other is a construction service, and confusing them leads to either over-billing for remediation or under-treatment that leaves active mold behind a fresh coat of paint.
How this shows up on our jobs
Almost everything homeowners call 'mold' on their siding is mildew — surface growth that our soft wash kills in a single visit. True interior mold requires a different approach entirely, and we'll always be honest about which situation we're looking at.
Services where this matters
Related terms
Mildew vs. Mold
Mildew and mold are both fungi, but they behave differently. Mildew is a flat, surface-level growth — usually white, gray, or yellowish — that stays on top of a substrate and can typically be cleaned away with soft-wash chemistry. Mold is three-dimensional and penetrates into porous materials (wood, drywall, paper), spreading its hyphae deep into the structure. On exterior surfaces, what most homeowners call "mold" is often mildew or algae; true structural mold on the outside of a house is uncommon without an underlying moisture problem.
Biocide
A biocide is any chemical agent that kills living organisms — bacteria, fungi, algae, mold, mildew, lichen. In exterior cleaning, the most common biocides are sodium hypochlorite (bleach), quaternary ammonium compounds ("quats"), and hydrogen peroxide. Each has a place: sodium hypochlorite is the fastest and most cost-effective broad-spectrum kill; quats provide residual antimicrobial action that slows regrowth on treated surfaces; peroxide is the mildest and is used where chlorine would damage the substrate.
Soft Wash
Soft washing is a low-pressure exterior cleaning method that uses biodegradable detergents and a controlled bleach solution to kill the algae, mold, mildew, and bacteria that cause staining — rather than scouring them off with brute force. A soft wash rig delivers cleaning fluid at roughly 60-200 PSI, comparable to a strong garden hose, which is safe on siding, shingles, painted surfaces, screens, and caulking. The chemistry does the work: the solution dwells on the surface, breaks down the organic growth at the cell level, and is then rinsed clean with fresh water.
Algae vs. Lichen
Algae (most commonly Gloeocapsa magma on roofs) is the photosynthetic organism responsible for the black streaks running down asphalt shingles, the green haze on north-facing siding, and the slippery film on concrete walkways. Lichen is a symbiotic partnership between a fungus and an alga — a completely different, far more persistent organism that looks like a flat crusty patch with a raised perimeter. Lichens physically anchor into the substrate with root-like structures, which is why they do not wash off like algae does.
Sodium Hypochlorite
Sodium hypochlorite (NaOCl) is the active ingredient in household bleach and the primary disinfectant used in professional soft washing. It is sold to exterior cleaners in 12.5% concentration (commonly called "SH" or "pool shock"), roughly twice the strength of consumer bleach. When applied in a properly mixed soft-wash solution, it kills algae, mold, mildew, lichen, and bacteria at the cellular level within minutes and breaks down into salt water and oxygen.