Homasote is (surprisingly) very resistant to water damage. In fact it was o riginally made (100 years ago) for outdoor use, if you can believe it. Go t o the Homasote website to read the history. You can even find a letter writ ten by the arctic explorers who made their houses out of it, and one who di scovered some of the remnants 75 years later outside having soaked in water for years. Fascinating! After reading this I did my own test and soaked a piece in water for one week, then dried it and it is still fully intact. Al most no swelling. And as a bonus it has no formaldehyde and is made from 100% post consumer recycled content. One of the first "Green" commercialy pro duced materials.Hope that helps.