Josh, a builder in Columbus, Ohio, has been hired to add a bathroom in the attic of an existing house. Although he had hoped to use cellulose insulation in exterior walls, the homeowner’s budget allowed fiberglass batts. Josh was counting on the kraft paper facing on the insulation to serve as a vapor retarder, but to his surprise the building inspector insists the paper be removed before the insulation is installed.
What gives? And will the inspector’s decision increase the risk of moisture problems in the bathroom, surely one of the most humid rooms in the house?
The Q&A discussion actually provided some support for the misunderstood inspector, as well as a look at materials and building techniques that will keep moisture problems at bay.
Will water vapor rot the walls?
Between tub and shower, sinks and toilet, a bathroom has a high potential for water damage, not only from leaks of liquid water but also from water vapor that can collect inside exterior walls and condense. Vapor retarders are there to slow the diffusion of moisture, so the building inspector is off base, argues senior editor Martin Holladay.
Even so, Holladay adds, vapor diffusion is rarely as big a problem as we might think. If the inspector’s ruling underscores an “incomplete understanding of building science,” it’s probably not going to have a serious practical effect. This is one battle that’s probably not worth fighting.
Robert Riversong isn’t so sure the inspector is wrong. Paper facings on conventional gypsum drywall provides food for mold. Riversong notes that the tiled portion of the wall will include a Schluter-Kerdi waterproofing membrane, so removing the kraft paper may have a double benefit: reducing the chances for trapped moisture by having one rather than two vapor retarders,…