I do agree with with this as being the most likely cause of the moisture issues that caused the mold. Wraping the ducts will work if he wraps every last duct and plenum. I beleive the reson no one wants to say this for sure is because there are to many unknowns. The home owner is looking for answers and he came to you to find the issue and give your professional advise on the root cause. The problem I see is that even for your self you are not quite sure. Be carefull what you say in the report as it can come back to bite you in the ***.
I would say something like this:
The mold issue is a complicated one. The RH was at 50% at time of inspection and the Floor joist and surrounding area read 10-12% moisture when checked with a pin style moisture meter. The root cause could be many things but is suspected to be from the duct work and plenums sweating when the air conditioning was being used during the summer months. This could not be verified for the following reason(s):
Your on your one for these
Hope this helped.
I am a bit perplexed at all of the complicated causes which are being suggested. When faced with a large and somewhat uniform mold coverage such as you’re describing, 99% of the time it is high humidity caused by moisture intrusion through the foundation. Current condition of the gutters, drainage around the house or the lack of stains in the basement tell you nothing about the previous history. Looking at your photos I wouldn’t believe for a moment that the owner actually cleaned the entire basement. If he actually did clean the basement and used a solution of diluted bleach then he aggravated the problem. Bleach is 95% or more water and it does not work on wood.
Forget the heating system, forget localized leaks through the roof, the chimney or the plumbing system, forget anything the owner claims about where the mold was or where he cleaned and definitely forget about sweating pipes and ducts(those are a symptom not a cause). If the relative humidity is currently 50% in his basement, it is not far from being perfect for mold growth. 60 to 65% is all it needs to continue growing. Most homeowners rarely look at the subfloor or the joists in their basement. I doubt your homeowner knows how long that mold has been there.
When you have a uniform coverage of mold growth in a midwestern basement like you’re describing, start looking for large injections of humidity into the space:
- Evaporation of moisture through the foundation caused by porous foundations(is it brick?) and buildup of moisture at the exterior
- a sizable leak or backdrafting of exhaust from gas burning appliances
- a humidifier on the furnace return ducts which is actively being used
- ventless natural gas heaters in the space
- small plug-in humidifiers being used in the home
- a previous flood in the basement or seasonal moisture leakage through the walls
Condensation is a symptom and should never be confused with the real cause of mold growth which is always going to be trapped moisture/humidity. It looks like you’re dealing with a very old home which most likely has a basement which stays closed up and rarely used. The older the foundation the more likely it is that simple moisture evaporating into the space is causing the high humidity. Here’s the note I include after detailing where the mold growth was found and in what quantity:
Recommend testing for bio-growth and implementing all necessary remediation as determined by a qualified industrial hygienist. Correction of all moisture issues which encourage this type of growth must also be a priority to prevent potential future moisture and mold problems.
I will talk with the owner about the possible causes but I will not detail it on the report. Nine times out of ten potential causes are very easily discovered. If the basement is one where high levels of humidity are typical, then the only real answer is to control the moisture on the inside using a dehumidifier. If you suggested that you have already given him some of the best advice you could.
Jim
Sweating ducts is a symptom of high humidity not the cause. Humid air just condenses on any surface that is below the dew point. There are several possibilities but in my book there are two basic sources for the origin of the water; outside and inside. I then break it down, if possible, in to either liquid water or water vapor. Liquid water usually causes mold to grow at or near the wet area. The uniform coverage and location of your moldy subfloor points to water vapor not liquid water. If there was no apparent water leak in the basement or dryer vented into the basement, etc. to generate a significant amount of water vapor then its likely it came from the living space above the subfloor. Since there was no plywood the humidity generated above the floor migrated down through the subfloor. At first you might not think a significant amount of humidity is generated within the living space but it does. I have seen several cases where the subfloor in a basement was insulated with the paper or foil vapor barrier on the bottom, which trapped the humidity generated within the living space that migrated through the subfloor (with plywood) and created a significant amount of mold on the joists and subflooring. Here is a good article on the subject: http://www.homeenergyresourcemn.org/stream_document.aspx?rRID=2991&pRID=2990
Is there an echo in here? 
Is there an echo in here?
Aspergillus/Penicillium will grow with just high humidity. If it is 50% now, you can beat it gets way over 60% in the basement during the summer. That is mostly the cause. Fix whatever is causing the high humidity such as leaks. Have the mold removed to IICRC S520, which requires an Indoor Environmental Professional to be involved.
As far as taking two outdoor samples, it all depends on what testing standard you use. If you are going to take air samples within 20 feet of a building, (which I do not know why somebody would want to) you better take two outside.
I am unsure what you mean by the inside air looks the same as the outside air. Was there a lot of outside mold spores in the inside sample? You can email the lab results if you want to my email address jbraun@mo.nachi.org.
I wouldn’t put any speculation about causes in the report. All my “most likely” conversations would be orally.
So you are saying that you would tell a client “specifically” what is causing the mold, even though you were not hired in that capacity?
Jim are you thinking migratory air
Dennis,
I am not sure what you are asking. Please explain in better detail.
very informative…thank you for your input…
I did send you the lab results, but to keep the others abreast.
I’m following the guidelines within the InterNACHI mold certification course.
Inside and outside air samples being the same, meaning low concentrations of the same mold types. And the same mold types found outside were the same found inside.
No, I didn’t say “specifically”. But I often give oral *opinions *about various things which are not part of formal inspection record.
That is not what you said in post #27…
Typo. I think he meant “wouldn’t”. 
Whoops! I meant to say “wouldn’t”. Thanks.
I believe Cameron has hit the nail on the head with Post #22!
Definitely a high humidity issue attributable to causes other than a condensing pipe. Dehumidifier recommended.