What should I do if I find mold during a renovation in New Brunswick?

Stop work in the affected area immediately and assess the extent before proceeding — mould discovered during a renovation is a serious health and building science issue that requires proper remediation before any finishing work continues.

Finding mould mid-renovation is more common than most NB homeowners expect, particularly in older homes where decades of Maritime humidity, inadequate ventilation, and seasonal moisture swings have had time to do their damage. The good news is that catching it during a renovation — when walls are open and the source is visible — is far better than finishing over it and discovering it again in five years, worse and more expensive.

Assess the Scale First

Small surface mould patches (under roughly 1 square metre) on a single material like drywall or wood framing are generally manageable with proper precautions. Anything larger, anything that has penetrated deeply into structural lumber, anything involving black or greenish-black growth on multiple surfaces, or any mould accompanied by a strong musty odour throughout the space warrants a professional mould assessment before you touch anything. A qualified indoor air quality professional or mould remediation contractor can assess the extent, identify the species involved, and recommend a remediation scope. Do not attempt to diagnose the severity yourself — mould hides inside wall cavities, under subfloors, and behind vapour barriers where it is not visible from the surface.

Find and Fix the Source — This Is Non-Negotiable

Mould is a symptom, not the problem itself. The problem is moisture. In NB homes, the usual culprits are plumbing leaks (supply lines, drain connections, or wax ring failures under toilets), roof or flashing leaks that have tracked down into wall cavities, basement moisture infiltration from hydrostatic pressure during spring snowmelt, inadequate bathroom or kitchen ventilation exhausting into the attic or wall cavity instead of outside, and vapour barrier failures in exterior walls that allow warm interior air to condense on cold sheathing. Until the moisture source is identified and corrected, any remediation is temporary. The mould will return.

Remediation Approach by Scale

For small, contained areas on non-structural materials like drywall, the affected material should be cut out and bagged for disposal — do not try to clean mouldy drywall, it cannot be fully remediated and must be removed. Wear an N95 respirator, nitrile gloves, and eye protection. Seal the work area with poly sheeting to prevent spores from spreading through your home's HVAC system. Structural lumber (studs, joists, subfloor) with surface mould can sometimes be treated with a borate-based antimicrobial solution and encapsulated after drying, but only after the moisture source is corrected and the wood has dried to below 19% moisture content — verify this with a moisture metre.

For larger infestations or any mould involving your HVAC system, insulation, or significant structural lumber, hire a professional remediation contractor. In NB, this is not optional — improper remediation of a large mould problem can spread spores throughout your home and create a much larger health and financial problem than the original discovery.

NB-Specific Context

NB's housing stock skews older — a significant portion of homes in Saint John, Fredericton, Moncton, and smaller communities were built before modern vapour barrier and ventilation standards existed. Pre-1980 homes frequently have no vapour barrier in exterior walls, inadequate attic ventilation, and original bathroom fans that exhaust into the attic. These conditions, combined with NB's 30-50% annual indoor humidity swing, create ideal mould conditions inside wall cavities that can go undetected for decades. Also be aware that homes built before 1990 may contain asbestos in insulation, floor tiles, ceiling texture, or pipe wrap — if you encounter suspicious materials alongside mould, stop work and have the materials tested before proceeding. Asbestos abatement requires licensed professionals and is legally required in NB.

Practical Steps to Take Right Now

Photograph everything before disturbing it further. Run a dehumidifier in the affected space to begin drying the area. Do not paint over, encapsulate, or cover mouldy materials without proper remediation. If the affected area is in a basement, check your sump pump is operational heading into spring snowmelt season. Once remediation is complete and the moisture source is corrected, ensure your renovation includes proper vapour barrier installation, sealed penetrations, and adequate ventilation — an HRV system is one of the best investments an NB homeowner can make to prevent future moisture and mould issues.

Mould found during a renovation is a delay, not a disaster — but only if you handle it properly before closing up the walls. Need help finding an experienced renovation or remediation contractor in your area? New Brunswick Renovations can connect you with local professionals for free.