What to Do When You Find Mold During a Remodel

The Remodel Discovery Nobody Wants

Your contractor pulls off a section of drywall and there it is: a patch of dark, fuzzy growth spreading across the studs behind your bathroom wall. Maybe it smells musty. Maybe the drywall was soft and discolored. Either way, the discovery changes the trajectory of your remodel.

Finding mold during a renovation is unsettling, but it happens regularly. Bay Area homes are particularly susceptible because of the region’s coastal fog, seasonal rain, and older building practices that did not prioritize moisture management the way modern codes require. The good news is that mold is a solvable problem. The key is handling it correctly from the moment of discovery.

Why Mold Appears During Remodels

Mold needs three things to grow: moisture, a food source (wood, drywall, insulation), and time. In a closed wall cavity, even a small, slow leak can create the perfect environment.

Common Sources in Bay Area Homes

• Plumbing leaks. Even minor drips from supply lines, drain connections, or aging pipe joints can saturate wall cavities over months or years.
• Roof and flashing failures. Water intrusion around chimneys, skylights, and roof valleys often runs down inside walls where it is invisible until demolition.
• Condensation. Poorly insulated exterior walls can accumulate moisture when warm interior air meets the cooler wall surface, especially in San Francisco and Peninsula cities with heavy fog.
• Bathroom ventilation. Bathrooms without proper exhaust fans or with fans vented into the attic rather than to the outside allow moisture to accumulate in wall and ceiling cavities.
• Older construction. Homes built before modern vapor barrier and waterproofing standards are more susceptible to moisture intrusion at multiple points.

Why You Did Not Know It Was There

Mold grows where you cannot see it: behind walls, under flooring, above ceilings, and inside HVAC ducts. It can thrive for years without any visible sign on the room side of a wall. Many homeowners live with hidden mold for a decade or more without realizing it. The remodel simply reveals what was already there.

Health Risks You Should Understand

Not all mold is equally dangerous, but all mold should be taken seriously.

Common Mold Types Found in Homes

• Cladosporium: Olive-green to brown, commonly found on wood, textiles, and damp drywall. Generally causes allergic reactions rather than severe illness.
• Aspergillus: Varies in color (white, green, yellow, black). Found on walls, insulation, and paper products. Some species can cause respiratory infections, particularly in people with weakened immune systems.
• Penicillium: Blue or green, often found on water-damaged materials. Common allergen that can trigger asthma symptoms.
• Stachybotrys (black mold): Dark green to black, grows on materials with high cellulose content (drywall, wood, paper) that have been wet for extended periods. Produces mycotoxins that can cause serious health effects with prolonged exposure.

Who Is Most at Risk?

People with asthma, allergies, chronic respiratory conditions, or compromised immune systems are most vulnerable to mold exposure. Young children and elderly adults also face elevated risk. Even healthy adults can experience symptoms from significant mold exposure, including coughing, sneezing, eye irritation, skin rash, and headaches.

What to Do: Step by Step

Step 1: Stop Work Immediately

The moment mold is discovered, all work in the affected area should stop. Demolition, sawing, sanding, and even sweeping can disturb mold colonies and release millions of spores into the air. These spores can spread to other areas of the home through the HVAC system or simply drift through open rooms.

Step 2: Do Not Disturb the Mold

Do not attempt to clean, scrub, or remove the mold yourself. Do not spray it with bleach (which is ineffective on porous materials and can make the problem worse). Do not try to dry it out with fans, as this will spread spores throughout the house.

Step 3: Contain the Area

If possible, seal off the affected area from the rest of the home using plastic sheeting and tape. Close any HVAC vents in the area to prevent spore distribution. If the mold is in a small area and was discovered during controlled demolition, your contractor may already have the area partially contained.

Step 4: Call a Mold Professional

Contact a licensed mold remediation company for testing and assessment. In California, mold assessors and remediators should hold appropriate certifications and insurance. The assessment will typically include:

• Visual inspection of the affected area and surrounding spaces
• Air sampling to measure airborne spore concentrations
• Surface sampling to identify the mold species
• Moisture mapping to locate the source of water intrusion

The assessment results will determine the scope of remediation needed and provide a cost estimate.

Step 5: Address the Moisture Source

Remediating the mold without fixing the moisture source is pointless. The mold will return. Your contractor and remediation professional should work together to identify and resolve the underlying water intrusion, whether it is a plumbing leak, roof issue, condensation problem, or ventilation deficiency.

Step 6: Complete Professional Remediation

Professional mold remediation follows a standard process:

1. Containment: The affected area is sealed with plastic sheeting, and negative air pressure is established to prevent spore migration.
2. Air filtration: HEPA air scrubbers run continuously to capture airborne spores.
3. Removal: Contaminated materials (drywall, insulation, carpet, wood that cannot be saved) are carefully removed and disposed of in sealed bags.
4. Treatment: Remaining structural materials are treated with antimicrobial agents.
5. Drying: The area is thoroughly dried to appropriate moisture levels.
6. Verification: Post-remediation testing confirms that spore counts have returned to normal levels.

Step 7: Rebuild with Prevention in Mind

Once remediation is complete and verified, your remodel can resume. This is an opportunity to rebuild with better moisture protection:

• Install moisture-resistant drywall (green board or cement board) in wet areas
• Upgrade bathroom ventilation to properly sized exhaust fans vented to the exterior
• Add vapor barriers where appropriate
• Use mold-resistant paint in high-moisture areas
• Ensure proper insulation to prevent condensation

How Much Does Mold Remediation Cost?

Remediation costs vary widely based on the extent of contamination.

Scope	Typical Cost Range
Small area (less than 10 sq ft)	$500 - $2,000
Moderate (10-100 sq ft, single room)	$2,000 - $10,000
Large (multiple rooms, structural involvement)	$10,000 - $30,000+
Extensive (whole-home, HVAC contamination)	$20,000 - $50,000+

These costs are in addition to the repairs needed to fix the moisture source and rebuild the affected area. For remodeling projects, the remediation cost is added to the overall project budget, typically extending the timeline by 1-3 weeks.

When to Test vs. When to Just Remediate

If you can see visible mold, you already know it is there. Testing to identify the species is optional but recommended in certain situations:

• Test when the mold may be Stachybotrys (black mold) and you need to determine the health risk level
• Test when there are health complaints and you need to document the mold type for insurance or medical purposes
• Test when the extent is unclear and you need to determine whether hidden mold exists in areas beyond the visible growth
• Skip testing when the visible mold is clearly defined, the moisture source is obvious, and you plan to remediate regardless of species

Most experienced contractors recommend erring on the side of remediation. If mold is visible, it needs to go, regardless of what type it is.

How Experienced Design-Build Teams Handle Mold

At Custom Home Design and Build, we have encountered mold on many Bay Area projects. Our approach includes:

• Pre-construction moisture assessment. During our Phase 1 design process, we evaluate the home’s moisture risk areas and check for signs of past water damage. When warranted, we conduct moisture testing before demolition begins.
• Immediate containment protocol. If mold is discovered during construction, our crew stops work, contains the area, and contacts our remediation partners the same day.
• Coordinated remediation. We work directly with licensed remediation firms to integrate the remediation process into the project schedule, minimizing delays and keeping you informed at every step.
• Better rebuilding. After remediation, we rebuild with modern moisture-resistant materials and improved ventilation, reducing the chance of future mold problems.

Discovering mold does not have to derail your project. With the right team, it is a temporary setback that leads to a healthier, better-built home.

Protect Your Home and Your Health

Mold is a serious discovery, but it is a manageable one when handled correctly. Stop work, call a professional, address the moisture source, and remediate properly. Your remodel will continue, and the finished result will be better for it.

If you are planning a remodel and want a team that knows how to handle hidden conditions like mold safely and efficiently, we are here to help.

Contact Custom Home Design and Build to discuss your project and learn how our pre-construction assessment process identifies moisture risks before demolition day.