Skip to main content

Ready to hire?

Post your job in minutes, browse real reviews and choose who to speak to.Post a job

Need some tips or advice?

Ask a question
Windows and doors

Condensation on window causing mold on drywall below

Dave 31/12/2025 - 9:20 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?

Are you a pro and able to answer this question?

Log in

3 Answers

My Pro Renos

No reviews yet

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.
Answered1 January 2026
2

RDI

No reviews yet

Pickering
The interior vapour barrier (most likely poly) will need to be sealed with acoustic sealant to or past the thermal break of the window. If there are any gaps, they need to be sealed with spray foam afterwards.
Answered30 December 2025
1

Topclasshomeimprovement

Rating: 5 out of 5
North York
Silicone around the trims properly
Answered31 December 2025
0