As a REALTOR I get the chance to show many homes to potential buyers. When I talk to Sellers about preparing their home, I often talk about the importance of first impressions. Nothing kills the positive impression of a new home quicker than the smell of mold and mildew in a vacant house. While air movement and a dehumidifier can control this condition if is difficult to correct after the fact. Realty Times has a good article about this subject. CLICK HERE to read the article.
Another issue, not covered in the article, has to do with humidity in the air and cool basements with no air movement. Condensation, dark rooms and no air movement often will create that moldy, musty smell in unoccupied homes.