Found mold on the back of drywall during a Tuscany reno, what now?

Start by measuring the patch, because Health Canada's size bands determine who should do the cleaning.

If the patch is small (one to three areas, each under one square metre), a healthy person can clean it with water and dish detergent, wearing safety glasses, disposable household gloves and a disposable N95 respirator. No bleach is needed. Because drywall is porous, Alberta Health Services advises that porous materials like drywall typically require removal rather than surface cleaning, so the affected section is normally cut out rather than wiped down. Before anyone cuts, check whether the drywall or its joint compound could contain asbestos (see below).

If there are more than three patches, or a patch between one and three square metres, Health Canada calls that medium, and the CMHC guide says it should be cleaned up by qualified and trained maintenance staff. Anything larger than three square metres is extensive, and CMHC says that belongs with qualified and trained mould remediation contractors.

What drives the decision and the cost

A few things shape the scope before anyone prices the job: how far the moisture has spread behind the wall (probe adjacent sections before closing up), whether the source of the water has been fixed, and whether there is any stipple, drywall joint compound or other material in the area that could contain asbestos. If any part of the house was built before 1990, Alberta OHS bulletin ASB004 lists drywall mud among the materials that may contain asbestos, and the Alberta Asbestos Abatement Manual adds stippled finishes. If you are unsure of the build date or the materials, have bulk samples tested before disturbing anything further. Published Calgary-area lab costs run $65 to $145 per sample.

For contractor-led mould remediation, published Calgary-area prices read in September 2026 run $1,800 to $6,000 for a medium job (roughly a 30 to 50 square foot basement wall) and $3,000 to $15,000 or more for large areas like a whole basement. A mould inspection or air-quality test, if you want one before work starts, runs $200 to $800, though Alberta Health Services notes that air testing is usually not recommended when you can already see mould.

What a fair quote should include.

Ask any contractor you speak with to confirm these items in writing before you sign:

• A description of the containment method to stop spores spreading to the rest of the house during removal
• Removal and bagging of all affected porous materials, not just surface treatment
• Whether an independent clearance test is included, and which lab will count the samples (clearance done by the same crew that did the work is worth questioning)
• Disposal method for the debris
• Proof that workers hold a current Alberta asbestos worker card if any suspect materials are in scope

There is no Alberta licence or notification requirement specific to mould remediation, so the written contract and the WCB clearance letter are your main protections. Any contractor taking a deposit before finishing the work must hold a Service Alberta prepaid contracting licence; check that before handing over any money.

Two quotes for the same wall can differ most in the containment plan, the disposal method, the post-remediation check and the warranty, so line those four up before you choose.