I am a first-time homeowner and I started seeing the color of the material between window blocks of my bathroom turn green over the 6 month time period of moving in. I am wondering:

  • what is the cause
  • what type of material is that
  • how to possibly fix it

P.S, I live in an upper midwest of the U.S. so it gets very cold during the winter.

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  • The bottom photo shows moisture collecting on the glass. I would suspect it is mold from inadequate ventilation. – mikes Feb 14 at 9:40

The mortar /grout between the blocks needs to be cleaned and sealed, I use a 3% hydrogen peroxide and water solution to take care of mold. Once cleaned I would recommend a sealer be applied any quality tile/grout sealer will keep this from happening or reduce it for several years.

What’s happening, the rough surface catches dust, soap and fine debris, being in a moist or damp area mold starts growing. There are many products on the market at your grocery store that will help with mold removal but cleaning and sealing is the first step.

The next step is to make sure during/ after showering the exhaust fan is running this will reduce the moisture in the bathroom. In my area a 15 minute timer for the extractor fan in the bathroom has been required for a few years. When the lights are turned on the fan starts and keep running for 15 minutes after the lights go out. With new tightly built homes this significantly reduces the possibility of mold growth in the Pacific Northwest and will help anywhere.

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