Cold sheathing: condensation & mold?
I have read most of the other threads on this topic–and would like to complement Martin and others who have provided great, detailed answers with patience!
My situation is similar but slightly different and I would appreciate some assistance:
- Home in the hills of Seattle, WA (elevation 800 ft, so colder winters than typical for Seattle metro area)
- Built in 1998. From outside-in the building envelope is natural wood siding (cedar/doug fir?), tar paper, 5/8″ OSB, 2×6 wood studs, R19 fiberglass. Solid slab foundation.
- The photo attached is from an approx 400 sqft room on the first floor, a half-basement by the technical definition. In this room, approx 1/3 of the wall space is a retaining wall, 1/3 interior walls, and 1/3 exterior walls.
- The builder left the room unfinished, so on the exterior walls and the retaining wall sides, R19 bats with the reflective Kraft paper were installed (although sloppily stapled). The ceiling is left unfinished as well, which has been useful over the years for access to wiring, running a new A/C line, etc..
- About 5 years ago, we installed a Honeywell humidifier to the main furnace (it was also replaced with a 95% efficient unit that vents to the outside) to increase ambient air during the winter when the blown air would cause dryness….to reduce colds/flu during winter months with small kids at home. We have traditionally set it to 50%, with a measured R.H. of around 46-47% in any given room. I have since turned the unit down to 35%, although we’d like to operate it around 40% if possible going-forward.
- In this HVAC room is installed this furnace and two gas hot water heaters, and the usual two 8-10% diameter conduits to the outside to provide fresh air venting for the hot water heaters (and former furnace)
We planned to finish this space to add value to the house by installing drywall and painting it, leaving the ceiling to be installed later likely using a drop-tile system so we can still access the overhead wires, etc.
Preparing the room, i removed the silver kraft paper so the drywall could attach to the studs with a better seal and also started to install a new power outlet in one of the bays when i noticed the dampness behind the R19 batts. I checked with a moisture meter, and 80% of the bay was 66% or higher. It seemed the moisture and mold were heaviest towards the bottom of each bay, with less moisture and mold at the top of the bays and near the edges where the insulation was less dense. I removed all insulation and every bay had moisture and mold (about 350 sqft).
Interestingly, 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. Played with a Dew Point Calculator and when I re-measured interior surface temperature of the OSB behind a (reinstalled) R19 batt, was around 43 to 46-degrees F, below the dew point for our 68 degree, 47% R.H. interior space. Things i’ve never known and learned!
So, my questions are these:
- Why dont i see moisture levels or mold in the 2×10 dimensional lumber blocks between the floor joists (see in the photo) since they too were also behind the same R19 batts?
- Should i really believe that the other exterior walls in the house that are finished dont have moisture or mold in them? Yes, we have at least two coats of high-quality latex paint of them (Devine is the brand), but i’m sure they must be somewhat leaky given what i have seen with some of the other construction over the years at our home.
Our remediation plan underway is to kill the mold (done, Microban Disinfectant Spray Plus), dry out all materials with heat and dehumidifers (underway), then re-install the R19 batts, add drywall (probably will use the mold-resistant version), mud&tape, then apply a PVA primer followed by two coats of latex paint. One contractor suggested we also put down a layer of 4mm plastic on the studs before the drywall to minimize re-introducing moisture from the mud/tape and paint to the bays given its December–but that seems suboptimal from what i read online.
And, our plan is to keep the ceiling as-is, meaning those 2×10 blocks between floor joists will still have R19 but be otherwise exposed to the warm interior air. Seems like these spaces should have problems exhibiting the same symptoms as the other bays, but i’m not sure why not…
Appreciate your detailed reply–this is a great, fun educational exercise for me and for the contractors i’m working with, although with a moderation of concern given the mold and moisture accumulation.
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