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Don’t Forget Dry-In

A big, fancy lake home was being built for a wealthy client. Due to a scheduling mix-up, the wall cavities were insulated and the drywall was installed on the main floor before the roofers arrived. After several days of rain, the place was pretty much in ruins. The customers paid a visit to the site and found their million-dollar dream home had become a nightmare. They wanted everything torn out.

The builder hired me to come out and assess the situation. He hoped that I could explain to his customers that no harm had been done. I’m afraid to say he was disappointed: All the drywall was damp around the edges, and the insulation and the sheathing behind it were damp. The wall cavities smelled musty and the studs were developing telltale coin-sized green spots. At this stage, there’s no miracle fix. Flooded property can be successfully restored as long as drying begins immediately. In this case, the partially completed building had been allowed to sit wet for over a week. The weather had been damp and overcast, so there was little natural drying. Powerful dehumidification could have removed the moisture, but not until the building was sealed up. Yet when I got there, they were still waiting for the roofer.

I often hear builders say that time is money, but sometimes you just have to say no. Getting things done right is important, but sometimes getting things done in the right order is most critical.

A Conspiracy of Sun and Rain

It was a 12-year-old home in a suburban development. The kids were shooting pool in the family room in the walkout basement when one of the boys noticed a mushroom growing from under the carpet against the wall. So Dad tore out the paneling and drywall and found that floor-to-ceiling rot had destroyed the framing and sheathing. His first fear was that he had caused the problem himself; he suspected that he had improperly flashed the ledger board when installing a new deck off the kitchen directly above the rotten wall. But when he pulled off the siding, he found that the framing and sheathing were rotten above the deck as well. The trail led upward, directly to the sill of the kitchen casement window.

His wife was concerned that tearing out all the moldy, rotten wall material had contaminated the house, so I was asked to come out and test the home. She also was curious why the window was leaking, so I asked a builder friend, who told me that the weatherstripping on that brand of window tended to break down with UV exposure.

I invited him out to the home and, sure enough, we found a gap in the seal that ran around the inside of the window frame. The window was on the south elevation, where it got lots of direct sunlight, and the weatherstripping had simply disintegrated, leaving a 1/2-inch by 1/2-inch hole in the bottom corner of the frame. Because of the way the window frame was assembled, this hole allowed water to run directly into the wall cavity whenever it rained. Studies have shown that such a seemingly small hole can leak gallons of water during a driving storm. Trapped between the sheathing on the outside and the vapor barrier on the inside, the water had nowhere to go. The framing and sheathing stayed soaked, rotted, and eventually produced the fruiting bodies that the kids discovered.

The home was out of warranty, so the builder had no legal obligation to fix the problem. When contacted by the homeowner, the window manufacturer suggested that the homeowner pursue the manufacturer of the weatherstripping. Needless to say, the owner’s blood began to boil. Before things got ugly, however, everyone saw the light of reason — with the help of a persuasive attorney. The homeowner repaired the water damage, the window maker replaced the south and west windows, and the builder stepped up and replaced the metal siding that had been torn off. In the end, the home was repaired and its value restored.

Spots on the Poly

The owners moved into their new home in the spring. By fall, they called the builder to complain about mold in their walkout basement. I was asked to take a look. The walkout walls were unfinished, and the poly vapor barrier and face of the fiberglass insulation were speckled with small dark spots. Microscopic examination of tape lifts from the back of the plastic showed dense growth of Cladosporium mold.

Having seen this problem many times, it was easy to identify the cause. Water vapor will condense into a liquid on a surface that is cooled below the dewpoint. Hot, humid summer air may have a dewpoint above 70°F. On such a day, a can of pop taken from a cooler will almost instantly be covered with water droplets. That’s because the surface of the cold can may be more than 30°F below the dewpoint.

Basements are often the coolest part of a home. On a hot, humid day, outdoor air with a dewpoint of 70°F that finds its way through the siding, sheathing, and insulation will reach the back of the vapor barrier. If the temperature of the basement is 68°F — 2°F below the dewpoint — the plastic sheeting will become the first condensing surface. Although the amount of condensation per square inch is much less than would form on a cold pop can, a hot spell of some duration may still provide the necessary moisture conditions for mold to grow. The extent of mold growth is a question of how wet for how long.

There are a variety of fixes for this problem. Although the risk of occupant exposure to the mold in a sealed wall cavity is debatable, once the mold is discovered it becomes a customer-service headache. The owner usually requests that all the moldy areas be stripped of poly and insulation, then cleaned up, and everything properly replaced. Hanging drywall on the studs may warm up the plastic enough to keep it above the dewpoint. Applying inch-thick foil-faced insulating board under the drywall will do a better job of warming up the poly and will provide a more comfortable conditioned space as well. Another option is to replace the conventional poly with a “smart” vapor retarder like the MemBrain from CertainTeed, which will allow warm, humid summer air to pass through without accumulating on the back.

Those Pesky Party Walls

Townhomes are a popular option for home buyers with tight budgets or limited space needs. In most areas, fire codes require that side-by-side units be separated by party walls detailed to slow flame spread and prevent smoke penetration. The obvious best practice is to install masonry walls of poured concrete or block. A cheaper and quicker choice is parallel studwalls with facing layers of gypsum-board sheathing — typically paper-faced drywall, which is the cheapest.

Drywall may have a fire rating, but it doesn’t perform well when it gets wet. Imagine trying to preserve a wet newspaper by sticking it in a plastic bag. Similarly, when it comes to drywall, the rate of drying must exceed the rate of wetting. Damp conditions and still air are the best accelerants for mold growth.

I was called out to a job site by a local code official to inspect a mold problem on a townhome development under construction. The scenario was the same one I had seen on other projects: The gypsum-board party walls of several of the units had a luxuriant growth of black mold.

The source of the problem was obvious. During construction, the gypsum-board party walls were built on footings surrounded by bare dirt. When hot, rainy weather turned the ground to mud, a fungal garden grew, with visible mold extending in a horizontal band from the mudsill up as high as 4 feet. I cut a 6-by-6-inch-square piece out of the moldy drywall on one wall and found that it had mold on both sides. I could also plainly see mold on the back of the adjacent wall’s gypsum board.

An argument could be made that the mold on the inside of the party wall presented no risk, as the wall is supposed to be air-sealed to prevent smoke migration. It would be interesting to hear a potential buyer’s reaction if disclosure rules required the seller to explain that even though the inside of the wall was covered with mold, it was harmless. I suspect that most customers would opt for a mold-free unit.

But how does one correct such a problem without tearing out one side of the party wall? Surface scrubbing might work, but how does one scrub the inside?

In the end, the builder took the bull by the horns: He supported the upper stories with temporary bracing, tore out the offending paper-covered party wall, and replaced it with concrete block. It was a costly repair, but it worked, and he was left with a property he could sell with pride instead of excuses. He also learned a lesson that will prevent such a problem on future projects.

Mac Pearce is an environmental health consultant in St. Paul, Minn.