High indoor humidity in winter shows up in ways many homeowners miss: water droplets or frost on the inside of windows, water pooling on sills, black spots around window frames or in corners, a musty smell in closets or the basement, frost or moisture stains in the attic. Left alone, these can lead to mould and costly damage. Sherwood Mechanical explains how to bring humidity back to a safe level for the outdoor temperature. Learn the signs of high indoor humidity in winter.