Why Is This Sheathing Moldy?
A Seattle homeowner finds damp and moldy stud cavities and wants to correct the problem before re-insulating and adding drywall
Brian Lent has discovered something in his walls that no homeowner wants to see: mold.
Preparing a ground-floor room for drywall, Lent pulls some fiberglass batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. from a 2x6 stud cavity and notices the back side of the OSB sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. is damp. A moisture meter reveals that in 80% of the bay, the moisture content is 66% or higher. Moisture and mold are heaviest at the bottom of each bay.
“Interestingly,” he writes in a Q&A post at GreenBuildingAdvisor, “after a panicked weekend a few days ago thinking we had an exterior leak, and much research on the internet and from GreenBuildingAdvisor, came to the consensus that it's a cold sheathing condensation problem.”
His house is in the hills of Seattle, where winters are slightly colder than they would be downtown. The 400-square-foot room in which Lent is working is really a half-basement. It had been left unfinished and was serving as a mechanical room for HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment, including a furnace with an attached humidifier, and two gas hot-water heaters along with two 8-inch duct supplying outside air. He also discloses that he's been running a humidifier during the winter to make indoor air more comfortable.
His remediation plan, already underway, includes killing the mold with a disinfectant, drying out the materials with heat and dehumidification, and then reinstalling the R-19 batts. He’ll finish the walls with mold-resistant drywall, a primer, and two coats of latex paint.
Is he on the right track? Or is there something missing from this equation? That’s the focus of this Q&A Spotlight.
The humidifier is a big mistake
Indoor relative humidity in his area should average about 30% at 70 degrees F during the winter, writes Dana Dorsett. “The only way you would ever need to add humidity is if you were ventilating at ridiculous rates during the middle of a cold snap,” he says.
If the relative humidity is 50% at 70 degrees, the dew point is about 51 degrees, Dorsett adds, but the mean temperature at Lent’s elevation of 800 feet is about 39 degrees. The result? “The cold sheathing is going to take on significant amounts of moisture.”
He suggests limiting indoor relative humidity to no more than 35%.
Running a humidifier, says GBAGreenBuildingAdvisor.com senior editor Martin Holladay, “is always a mistake.” Should the air inside a house get too dry in winter, it’s usually a sign that the house has a lot of air leaks. The solution, he says, is to plug them — a task that is made easier with the help of a blower door.
“Once you have lowered the humidity of the room, and once you have convinced all of the members of your family of the importance of destroying your humidifier with a sledgehammer, you should probably consider insulating these problematic stud bays with closed-cell spray polyurethane foam,” Holladay says. “The foam will provide an effective air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. to prevent the interior air from contacting the cold OSB.”
Other insulation options
While effective, spray polyurethane foam is an expensive insulation option, and Lent wonders whether he’ll have to pay a premium to interest an insulation contractor in such a small job.
There are a couple of other options, as Dorsett points out:
- Blown-in cellulose. Cellulose has the capacity to pick up some of the moisture to become a buffer, and the borateBoron-containing chemical that provides fire resistance to materials such as cellulose insulation and provides decay and termite resistance to wood products. Borate is derived from the mineral borax and is benign, compared with most other wood treatments. fire retardants used in cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. are effective against mold and fungus. Further, Doresett says, “The air-retardancy of cellulose even at 2-lb. density is about 90% better than that of low-density fiberglass, (at 3-lbs+ density it's about 99% more air-retardant), nearly eliminating the convective transfer of moisture from the interior side to the exterior.”
- Rock wool batts. When installed perfectl, rock wool batts block air nearly as well as 2-lb. cellulose, Dorsett says, and have an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 23.
- Double up the batts. Compress both unfaced R-13 batts and the original R-19 batts into the stud cavities for an R-20 to R-21 wall.
- Flash-and-batt. Spray in an inch of foam as an air sealing layer and vapor retarder on the interior side the OSB sheathing, and then compress the old R-19 batts on top of the foam.
Now, about that ventilation system
Lent wrote that he planned to connect the two 8-inch-diamter passive outdoor air ducts to his furnace's forced-air distribution system. Holladay responded that this move would make the house “wildly over-ventilated (and dry).”
“If you want to install this type of ventilation system — called a central-fan-integrated supply ventilation system — you need to include a FanCycler control, and you need to include motorized dampers and a commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. process that verifies the air flow,” Holladay writes. He suggests Lent have a look at a blog on the topic.
Lent replies that he has already installed a FanCycler control, and that he’s programmed the control to stay open as much as the software will permit, 50% of a 24-hour period.
“However,” he adds, “the concern that is not addressed in that document is coordinated timing: there is no guarantee that when systems that evacuate internal air (i.e., bathroom fan during a shower, whole-house fan, cooktop) the external damper via the FanCycler will be open. Thus, we can’t ensure the house will be consistently positively pressurized. So, the proposal is to always be pulling some amount of outside air into the central system (at the expense of increased energy consumption, understood).”
Dorsett writes that ventilating a house with a furnace is a “common but pretty lousy way to go about it, since you only get the ventilation when it’s running and the ventilation rates go up when you least want it (when the outdoor air is colder and drier).”
Aaron Birkland says the ideal airflow rate for a FanCycler must be calculated to match the house, taking many factors into consideration. The goal should be to open the outside damper for a certain amount of time per hour while the air handler is running; and also to make the air handler run for a certain length of time each hour if it won’t do so naturally.
“As you can see, it's intended more as a means to meet a specified target level of ventilation, rather than provide constant positive pressure to the house,” Birkland writes. “The fact that it does induce a small degree of positive pressure is a side effect of how it happens to operate, rather than a design feature. So opening the supply vents or adjusting the programming to force lots of outside air is kind of a bad thing.
“It will lead to over-ventilation, and won't really do anything useful or consistent as pressurizing the house goes,” he adds. “I'd say it's best to just calculate the ‘right’ level of ventilation for your house, and program the FanCycler to aim for that.”
Our expert’s opinion
GBA Technical Director Peter Yost added these thoughts:
It’s great that Brian Lent found GBA in time to understand how his exterior walls got moldy. GBA emphasizes the importance of a continuous air barrier on those same exterior walls. That is the real way to keep these walls dry after his rework of the walls is done. As Martin Holladay noted in his first answer to Lent's questions, "Plug the air leaks. ... Consider insulating these problematic stud bays with closed-cell spray polyurethane foam."
While it’s true that cellulose is a lot more airtight than fiberglass batts, it’s not anywhere near tight enough to be the dedicated air barrier. Flash and batt or the airtight drywall approach is the way to go while Brian has the opportunity.
And that same continuous air barrier will improve or eliminate wintertime dryness issues. I never have found a home in which wintertime dryness was not the result of either air leakage or overventilation.
Building America research done quite some time ago at Building Science Corporation shows that the positive pressure associated with Central Fan Integrated Supply (CFIS) ventilation is very slight, in the range of no more than 3 Pa. And that positive pressure is intermittent as well. Sure, there are more sophisticated mechanical ventilation systems than CFIS, but it does deliver fresh air and distribution of that fresh air as well. Martin’s blog referenced above does a thorough and fair job of assessing the different approaches to residential mechanical ventilation, including CFIS.
- Brian Lent
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