Mold on inside of plywood sheathing
I recently purchased a super insulated house in the Concord, NH area. The house is 3 years old. From my investigation and discussions with the previous owner the wall system is double stud insulated with 3 layers of 3.5″ Roxul Comfortbatt. The sheathing is Huber Zipwall with taped joints. The siding is cement board clapboards nailed directly to the sheathing. The inside is drywall and I’m told gasketed at joints and electrical outlets. there is no interior vapor barrier and the paint appears to just be regular latex. We use the HRV and we keep it pretty dry in the winter, around 35-30% humidity, sometimes 25%.
based on the reading I’ve done on this site I have learned that the double stud wall system can cause condensation problems on the inside of the Plywood sheathing. I did some investigating this August and sure enough I noticed a few black mold spots on the inside of the plywood. The plywood and the insulation felt dry, but there were definitely a few sporatic black spots of mold.
After a long winded background, my question is: do I have a minor or major problem here? Do I need to rip the siding and sheathing off and put a rain screen on? or is a little bit of mold okay if the sheathing is drying out each summer and it’s sealed in by the drywall? No one in my family has complained about mold allergy reactions but it still makes me nervous. Thoughts?
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Sounds like construction moisture that is now gone. Post pictures, have someone check it out. Have the builder come by and see what he has to say.
Where is the mold, north side a botom of walls?
Tell us more and more pictures...
We had a moldy log home near me... a mold removal company came... did great job and the home is fine now... it got moldy from no heat over 5 years of being for sale mostly in the finished cellar walls and carpets. Carpets tossed, walls cleaned. Done.
Rain screens are great but that retrofit will be expensive as all get out. I would think tens of thousands of dollars.
You do not have plywood, it is OSB a better version of OSB... Actual plywood is a better product IMO than OSB which is the least expensive sheathing and why used. Zipwall is a great improvement of OSB. I still have concerns if it gets hit with moisture from and on the back side. The new idea is to place the sheathing in between insulation so it does not condense water out of moist air.
Post pics... you may have no problem and it may be something that happened during the drying out of the new frame. Post pics.
Cement board siding might not be the best choice to apply over a build like yours. Expensive choice too. I wouldn't build what you bought.
I see many holes in the story. For starters there's no mention of humidity measurements during the spring, summer and fall month's. Only winter.
Mold is good, right? It helps you identify a problem.
Your answer for a "properly" constructed "tested" double wall system may be right here....
I agree with the other comments. We need more information.
However, if no one in your family is sick, and the sheathing is dry, old mold spots are harmless in that location, in my opinion.
Thanks everyone - I'll make get back into the wall and take some photos. Hopefully the mold is from the moist construction environment as I was told the house was dry-walled in late fall and not finished and moved into until March which means that it went through at least one winter with high indoor humidity.
To answer the question about spring summer humidity, we dont run a lot of AC so the humidity is typically whatever it is outside. I also only ran the HRV for showers. The readings averaged in the 60%'s and 70%'s all summer.
Another piece of info is that the previous owner told me the blower door came in around 1.25 ACH
I guess I'll keep an eye on it and see if the mold increases and starts to rot the sheathing. Obviously, I really don't want to retrofit a rain screen. If the issues gets worse, do I have another alternative? Unfortunately, I can't retrofit what Building Science suggest at this point as the house is complete.
Maybe this can help you understand mold inside of your home and why humidity needs to be controlled. Martin you may also want to take a look at this for a side note.
60% to 70%rh is high and "I think" Martin will agree.
Surface Mold and Other Biological Growth
The following conditions are necessary and sufficient for mold and other biological growth to occur on surfaces:
mold spores must be present
a nutrient base must be available (most surfaces contain nutrients)
temperature range between 40 (4.4°C) and 100°F (37.7°C)
relative humidity adjacent to surface above 70% 
Richard - thank you for the information. I understand why mold grows and I know why its on my sheathing. My concern is whether or not I have a BIG problem or a moderate problem. ie, should I plan on ripping out siding soon or simply keep an eye on things over the next couple of years. The responses regarding construction moisture make the most sense to me and hopefully that was the culprit. Especially given that the zipboard is dry right now at the end of the summer. Knowing that this is an issue, next summer I will make a point to run the AC or a dehumidifier a little more often (which pains me to waste the energy on)
Also - FYI my estimate of 60% to 70% was for the dead of summer. I would estimate that the typical humidity through spring and early summer and then fall averaged 50%. Currently, its 30-40% in my home and like I said, its 25% to 30% in the winter.
You need to remove bath moisture for at least 20 minutes after showers and baths.
Your humidity is the same as any one like me who also does not use AC.
Your humidity is fine.
Run your bath fans longer. Buy digital timer switches and get all to always use them. You have an HRV.... a bit different... use it on high for and after shower use.... Use it.
My bet is all is fine.
You didn't specify if you opened up the north side and if it was just at the bottom of the wall. Please post more info!
you need to be running your HRV continuously, year round, 24/7, with bath switches that boost run mode from low to high for a few minutes. That alone will reduce your humidity levels, and reducing interior humidity levels are critical in your house for the reasons you have observed.
The siding should have been installed over a rain screen. There are several reasons for this, but one is to reduce moisture accumulation on the backside of the siding, which can under the right conditions lead to sheathing rot. A second reason is to promote the drying of the sheathing from interior moisture as you are seeing. I'd put this on a list of things to work on over time.
I'm in the area & could look at the house if you'd like.
You wrote, "You need to be running your HRV continuously, year round, 24/7, with bath switches that boost run mode from low to high for a few minutes. That alone will reduce your humidity levels."
Your advice is not quite right. While operating an HRV during the winter months will lower indoor humidity levels, it won't help during the summer.
Concord, NH can get hot and humid during the summer months. If you operate your HRV for 24/7 during hot, humid weather, you will be raising your indoor humidity levels, not lowering them. The more you operate your ventilation fan, the more humid your home will become.
High humidity is less common in central NH than near the coast, although we do get high humidity on occasion. If the house is conditioned by air source heat pumps (mini splits), they will reduce the humidity as part of either the cooling or drying modes.
My concern is homeowners who decide to minimize their electrical use by using their ventilation equipment sporadically, rather than as an integral part of the building system. I stopped using timed bath fan exhaust only ventilation systems in favor of HRVs when I realized that they were typically turned off and rarely used.