Dark Colored Insulation is some areas. What would be the cause?

2009 build upper scale home both interior & exterior. Partially finished basement, No smells, No signs of moisture, exterior grading good, downspouts extended away, No evidence of any kind of moisture intrusion or leaks within the basement. I did not pull or cut the plastic covering to visually see the insulation. What would cause the dark ares? See Photos

Sorry start a new post before I upload the photos.
DSCN4499 DSCN4303

Air inflation from the condition space…I guess!

The PLASTIC is your problem!! No airflow to allow any condensation to dry naturally. YES… this will lead to mold if it hasn’t already!!


Jeff is right, that air barrier has to go, it is causing the moisture from the foundation to be trapped.
The moisture from the foundation has to be able to dry to the interior space.



Is this on all the walls or just one side of the house. Typically the foundation wall that faces the sun always has moisture in the insulation due to the temperature difference. I wouldn’t worry too much about mold as mold only grows on organic materials. I see this in new homes all the time. I would suggest to run a dehumidifier.

Agreeing with every other comment here I agree its the plastic holding moisture to the insulation.

Thanks for all the feed back. Here is a photo taken by the new owner. Not sure which side the photo was taken. The photos I uploaded were of the North side and NW corner. If it was from the plastic holding in moisture I would think it would show on the plastic, again NOT a hint of any type of moisture related smells. Heading there today for a buck sample to send in! Thanks again

It may or may not be mold, it could be bacterial or god knows what, feeding on trace amounts of, food for them, material used in the manufacturing process?

Trapped humidity.

I’ve seen this often and have had it tested for mold of which there was none. Lack of acoustic sealant around the insulated area and holes in the vapour barrier causes air flow which contains dust. There should be no moisture coming from the concrete if dampproofing is properly installed on the outside. Vapoiur barrier is essential for keeping moisture away from the cold concrete. Living in Canada I know that vapour barrier must be in place.

“Dampproofing” is typically only applied to a few inches above grade, not the entire height of the foundation wall. Lots of area for moisture to wick into the masonry walls and create moisture intrusion concerns.

That may be true, but here in Alberta 6 Mil vapour barrier is not optional. It must be in place and properly sealed.

Thanks for the first post to put a smile on my face this morning, Roy.
You’re The best!.
See, I always thought the measurement of inflation was referring to, inflation rate. An annualized percentile change of a pricing index over a prescribed time. Damn. Was I wrong!

Roy, if you saw that pattern in a thermogram. What would you consider?
Myself, home science. Suspect: Trapped moisture. Condensate trapped on the midwall barrier, likely due to the wrong type of barrier and wrong barrier application, but condensate nonetheless, on the warm side, but that’s just me.

Shay. No worries for adding another post to include the images. You did a good thing by bringing this ‘great question’ to the MB and visiting members.
Home Sciences should be your next education model. It will help you develop a good overall understanding about the buildings you will be inspecting daily, hopefully. With/over time, and patience, you will develop into a very capable home inspector gaining a greater market share in your area.

Air barriers. A systems of materials designed, and constructed, to control ‘airflow’ between a conditioned space and an unconditioned space. Try not to lose the link.

Best of luck with all your endeavors, Shay.

## Why are basements moldy? Because we build walls so they can’t dry. Simple changes in wall design will result in mould-free durable basements.

Fiberglass would be my last choice for insulating a basement wall assembly.
First choice, rigid or spray foam.
Second. Mineral wool.
If I had to use fiberglass Batt Insulation, it would be installed snug in stud bay cavities, an even blanket, up 24" from the slab or void stud bay for the first 24", again IF I only had only had fiberglass batts to work with.
Barrier. Paint the drywall. No manufactured barrier needed.
Hope that helps.


“Building science,” like psychoanalysis, is the disease it purports to cure.

In this case it is put forward by a Doctor, BSD-103: Understanding Basements

Joseph Lstiburek, based on Engineered facts.
Anytime you install a Poly on concrete and it touches it like the OP’s picture, it will turn dark from the moisture of the concrete alone. Poly does not belong on concrete unless you want to retain moisture from coming through. That is not fiction, it is a reality. It needs to dry out to the interior.

Joe Lstiburik is a great read, but in general most of so-called building science might better be described as locking the barn door after the horse has bolted.

What do you mean by that? Don’t believe in Joe’s material?

UPDATE: Good news, NO Stachybotrys! Thanks to all those who replied and provided their insight, links, and knowledge. The test results indicated Myxomycetes @ rara. Rara meaning lower then low in spore counts totaling 1. Myxomycetes listed as a normal fungal. I asked the lab tech about the dark discolor and he said that the insulation has been acting like a filter over the years and thats just where the dust and dirt has been captured. He also said that the (1) spore that was identified most likely floated in and just landed there. Your thoughts?

New question: How would of you handled this situation? Doing the inspection of the basement, No signs of moisture anywhere, No moisture related smells, yet you could see the dark discoloration in the insulation behind the plastic covering. Would you remove a section and take a look? Would you leave it and call it out in the report as something to further evaluate? Or would you do nothing? Thanks.