Once again, contact with moisture is the culprit here, though water-resistant varieties of wallpaper are less likely to become wet and develop mold. Using a high-quality exhaust fan or dehumidifier and leaving the doors or windows cracked after bathing will keep this issue at bay. 

In most cases, it is not recommended to wallpaper over existing wallpaper, as the success depends on the type of paper already on the wall. While you might be able to cover non-coated wallpaper, you should never apply new wallpaper over a vinyl-coated one, which is more common. Doing so can lead to poor adhesion, resulting in bubbles, imperfections, and peeling. It can also trap moisture, potentially causing hazardous mold growth between the layers.

Even if the existing wallpaper is suitable, experts recommend applying only one new layer. Adding multiple layers (more than two total) can cause significant adhesion problems, leading to warping and bubbling. For the best results, the recommended approach is to remove the old wallpaper and all paste residue before hanging the new paper. If you are unsure about your wallpaper type, consider consulting a professional.

It can take a professional anywhere between several hours and a full day to remove wallpaper in one room. How long wallpaper removal takes depends on the type of adhesive, the wallpaper’s age, the condition of the walls, and more.

Removing a wallpaper border requires the same process to remove full walls of wallpaper. Clear the area, spray down the surface with your hot water and vinegar solution or store-bought spray, wait 10 to 15 minutes, then scrape it off with a putty knife. Sometimes, border wallpaper lifts easier than larger sheets, so you can just peel it right off the wall by hand.