/The reasons for this situation are:
1, probably because the location of the house is not so sunny, and it is not ventilated often. In particular, some houses that have been renovated but no one lives all the year round are often airtight, and it is easy to have moldy walls.
2. It is also possible that the wall insulation of the house is not done well, which will lead to catching cold.
3. It may also be because the indoor pipes are aging or damaged. Because the pipeline surrounds the whole house, once the pipeline ages, it will drip or leak. Then the whole house will be wet and the walls will be moldy easily.