Windows and doors

Condensation on window causing mold on drywall below

Dave 18/11/2025 - 9:38 AM

I recently had a contractor put in a Silestone window ledge in my condo. I specifically called out that the windows get a lot of condensation and was picking a material that could withstand this. The contractor used a thin layer of drywall to create a flat (but not level) surface to mount the Silestone. Now that the weather is cooler, there is lots of condensation on the windows and I can see green mold forming on the surface plaster he used over the drywall facing. I'm concerned about it getting worse over the winter. How can I prevent this from getting worse over the winter? I'll probably need the work completely redone. How can it be done differently to create a better mounting surface and properly seal the ledge so this doesn't happen again?

My Pro Renos

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Shelburne, Ontario

You’re right to be concerned — what you’re seeing is a classic case of condensation meeting an unsuitable substrate. Drywall and plaster are porous, absorb moisture, and once trapped behind a hard surface like Silestone, they become a breeding ground for mold. Short-Term Prevention - Control humidity: You can run a dehumidifier. This should reduce condensation on the glass. - Improve airflow: Keep your blinds/curtains open during the day and ensure warm air can circulate to the window surface. - Temporary seal: Apply a mold-resistant caulk or sealant along the exposed plaster edge to reduce moisture penetration until you redo the work. - Surface cleaning: Wipe mold with a mild detergent solution (avoid bleach on porous plaster — it doesn’t penetrate). Dry thoroughly after cleaning. Long-Term Correction - Redo the work properly.

Answered18 November 2025

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