The grout darkens in certain areas of the shower — is that mold or just moisture retention?

Dark grout in shower areas is typically caused by moisture retention, soap residue buildup, or early-stage mold/mildew growth - all of which are common in high-humidity environments like showers.

Moisture retention is the most likely culprit, especially if the darkening appears consistently in certain areas like corners, along the tub line, or where water tends to pool. Grout is naturally porous and absorbs water, which can cause it to appear darker when wet and may not fully dry between uses. Poor ventilation exacerbates this issue, keeping grout damp for extended periods.

Soap scum and mineral deposits from hard water (common in Ottawa) can also cause grout discoloration. These residues build up over time and create dark, dingy areas that trap more moisture and dirt. If the darkening has a filmy or chalky appearance, this is likely the cause.

Early mold or mildew growth appears as dark spots or patches, often black or dark green, and may have a musty smell. This typically develops when grout stays damp for too long due to inadequate ventilation or damaged grout that's lost its water resistance. In Ottawa's climate, bathroom humidity control is especially important during winter months when ventilation may be reduced.

To determine the cause, clean the area thoroughly with a grout cleaner or bleach solution. If the darkening disappears, it was likely soap buildup or surface mildew. If dark spots persist or return quickly, you may have deeper mold penetration or grout that needs resealing or replacement.

Prevention involves improving bathroom ventilation with exhaust fans, sealing grout annually, and regular cleaning with mold-preventing products. If the grout is old or damaged, professional regrouting may be necessary to prevent water damage to the substrate behind your tiles.