Dormant Mold, Real Risks, Smart Fixes

Send us a text

ver wondered why “dormant mold” keeps coming back the moment the seasons change? We pull back the curtain on what dormant really means, why 50–55% humidity isn’t a cure by itself, and how a moldy subfloor turns into a hidden hazard when spray foam goes on too soon.

• 20% OFF DIY Products Code: "youtube" Use at https://supply.crawlspaceninja.com
• 10% OFF DIY Mold Testing: https://fas.st/t/8cfyT8P3

AJ joins me to tackle a real homeowner question from a 1927 house with a tight crawl space and visible mold. We break down the fruiting-body concept so you understand what you’re seeing versus what’s inside the wood, then dig into wood moisture content, leak pathways, and the blind spots created by covering the subfloor. You’ll hear why foam is great for targeted air sealing at penetrations and rim joists, but risky when used to blanket a subfloor that hasn’t been remediated and dried. We share simple checks for toilet flange leaks, ice-maker lines, laundry connections, and bath drains—plus why you should listen for drip sounds on plastic ground covers as an early warning.

From there we lay out the correct order of operations: remove visible growth, eliminate the moisture source, apply a root-killing treatment, verify dryness with a moisture meter, then insulate smart. Instead of burying problems, focus on sealing big gaps, insulating crawl space walls to reduce the stack effect, and keeping the subfloor visible for inspection. We talk practical targets for wood moisture, sizing dehumidification, and how to balance energy savings with long-term durability in older homes.

If you’ve ever been told to “just keep humidity under 55 and spray foam it,” this conversation will save you money and headaches. Hit play to get a clear plan for mold remediation, moisture control, and crawl space insulation that protects your home for the long haul. If this helped, subscribe, share it with a neighbor, and leave a quick review—what’s your biggest crawl space question right now?

Support the show

Check out Michael's book, "Crawl Space Repair Myths-Busted" now available on Amazon!

Schedule a consultation: https://buymeacoffee.com/csninja/extras

Need help: https://crawlspaceninja.com or https://supply.crawlspaceninja.com

Episode Transcript

SPEAKER_00 (00:00):
You're listening to the CrawlSpace Ninja Show where
healthy homes begin from theground up.
I am Michael Church, your hostand founder of Crawl Space
Ninja.
Today we got a great episode foryou.
I had an opportunity to speakwith AJ.
He is our DIY specialist, and wetalk about 50 to 55% humidity.

(00:21):
What exactly does that do tomold?
Is mold okay if it goes dormant?
And I'm gonna give you my threesteps to addressing mold in the
correct order.
Let's get into this episode withAJ talking about dormant mold
and how that affects your house.

SPEAKER_01 (00:38):
Okay, so I have a Ask a Ninja question for you.
Um, this person has a 1927 homewith a very little crawl space,
it's about two feet tall,including a joist.
Um, their concern is thesubfloor.
The wood's old, there's visiblemold on it, and their true
questions are if the humidity iskept below 50 to 55, sorry, 50

(01:00):
to 55%, should they still beconcerned about the dormant
mold?
And then their follow-upquestion to that is can spray
foam be applied directly overthat mold if it's not active?

SPEAKER_00 (01:13):
That is a fantastic question.
And let me let me answer thefirst part.
All right, so what does dormantmold mean?
Okay, so um, you know, mold iseverywhere.
Let's just put that out there,right?
So it's impossible for you tohave a mold-free environment
unless you had some kind of likeclean room or something, you

(01:37):
know, like where they make, youknow, microchips or something
like that, where they can'tallow dust and all that kind of
stuff in there because there'salways something for mold to
grow on.
So, but dormant mold, in myopinion, is a little misleading.
To say, to say something isdormant, you may instantly
think, oh, dormant means dead,right?

(01:58):
I mean, that may be what somepeople think, but it doesn't.
It means it's alive, it's justnot active.
All right.
And and you know, I always usethe groundhog scenario, okay?
So a groundhog, we havegroundhogs here in East
Tennessee, and they go dormantwhen they hibernate, right?

(02:19):
But what happens?
Let's say that I found agroundhog hole and I grabbed a
stick and I dug it out and Ifound where they were nesting,
and it's you know, January andit's cold, and and I start
poking that groundhog with thatstick.
What do you think thatgroundhog's gonna do to me and
that stick?
Right?
He's no longer gonna be dormant.

(02:40):
Okay, so what I'm trying to sayis is that the best scenario is
to remove the mold, remove whatcaused the mold, and also apply
something that kills its root.
Okay, because mold, the foodsource, what you see with your

(03:03):
eyeballs is the fruiting body.
So, in other words, if you lookoutside in your yard and you
know it rains a lot for three orfour days, and then all of a
sudden a mushroom pops up in themiddle of your pristine grass,
okay, there was mold under thegrass before you saw the
mushroom, right?
So the what we see on wood isthe fruiting body of the mold.

(03:28):
That doesn't mean that the moldwasn't there, it could have been
in the wood growing until it hadhigh enough humidity or grew
fast enough for it to startfruit.
Now, why does mold fruit or whydoes it become visible?
It's so it can spread.
Okay, that's usually whathappens.
So if you take the word dormantmold, and then now you take

(03:53):
closed cell or open cell sprayfoam, they didn't really say
which, and you spray over it.
Did that kill the mold?
No.
No, it didn't kill the mold.
It locked it in, but what elsedid it lock in with the mold?
It locked in the moisture.
Because there wouldn't be moldthere if there wasn't some kind

(04:14):
of moisture to start it in thefirst place.
And yeah, they've been runningthe dehumidifier, it's under
55%, but they didn't tell mewhat the wood moisture level is.
Okay.
Is the wood moisture level low?
Do they have some kind of hiddenleak they're not they didn't
know about?
Like the toilet flange isleaking, and then they they air
seal all of that subfloor andjoist with spray foam, and now

(04:35):
they got wet wood because theydidn't know that they had a
toilet leak.
What happens now?
That mold just grows and growsand grows, but now it's not
visible anymore because theysealed it up inside of spray
foam, right?
I hate whenever we ascontractors cover up the

(04:56):
subfloor.
I don't like covering up withspray foam, and I don't like it,
I don't like covering up withinsulation.
I'm not saying there's not areason to do both, but there's
there's you know, if you have ahouse 20 years old, there's a
really good chance at some pointthat house is going to have a
water leak.

(05:16):
Whether it's the toilet flange,whether it's the refrigerator
ice maker, whether it's thewashing machine, and if you've
sealed up or covered up thatsubfloor and that water has
nowhere to go except sit againstthat subfloor insulation,
whether it's fiberglass, sprayfoam, or whatever, you're gonna
have a big problem because youcan't even dry it from

(05:39):
underneath anymore.
So, AJ, you know, what do we do?
What's the first thing we do ifthere's subfloor insulation and
we need to dry out a crawlspace?
We take it out.
We take it out.
How am I gonna take out sprayfoam, AJ?

SPEAKER_01 (05:54):
I mean, you can't really.
I mean, you could, but you can'treally.
It's it's it's it's a horriblemess.

SPEAKER_00 (06:00):
Well, and a lot of people don't know this, but you
know, spray foam is actually uha form of plastic, right?
It's it's an expandable plastic.
So I have scraped off sprayfoam, and if you look really
good, there's still amicroscopic, almost
waterproofing film still on thewood.
So really the only way to get itoff is not just to knock off the

(06:22):
big pieces, but to actually, youknow, sand off that that um
adhesive part, you know, that'son the wood.
So even if you take the sprayfoam off visibly, you still
might be trapping moisturebecause because of that.
So I'm not a big fan of sprayfoaming subfloors.
I really I I just don't see ahuge I am a big fan of air

(06:43):
sealing penetrations, you know.
So if someone had uh to to goback to this question, I know
it's a low crawl space, but tome, I think you would be better
off air sealing all the majorpenetrations.
The other thing, too, is if yougot a 1927 house, it might have

(07:04):
like some kind of tongue andgroove with gaps in it.
So what happens when you sprayfoam?
Where are the where let's sayI've been in houses where I
could see daylight or I couldsee the the room above, what
happens if I spray foamunderneath?
Where's that spray foam going togo?
It's gonna rise up into the roomabove, right?
Um, so to me, I think you know,min uh I think minimizing the

(07:26):
stack effect by air sealing thebig gaps that you can seal in a
in a subfloor and theninsulating the walls of the
crawl space to create thatthermal break at the wall would
be a better choice.
And even, I mean, if you're in areally cold environment and the
soil is super cold and you wantto put like some kind of
insulation on the soil itself, Iwouldn't be against that either.

(07:48):
But I would rather do those twoplaces of insulation versus a
spray foam of the subfloor.
So hopefully that helps them outand uh keeps them from making a
big mistake because once thatspray foam goes on, you can't
get it off.

SPEAKER_01 (08:01):
Oh yeah.
A big thing I tell a big thing Itell everyone is you never want
to cover wood.

SPEAKER_00 (08:06):
You don't?
You don't.
I mean, they're they're not in acrawl space.
I mean, it's just a naturallywet environment, you know.
Oh yeah.
I I'd rather be able to see it.
I and you know, the otheradvantage of not covering it,
which this may seem very dumb,but I've actually had homeowners
tell me that they noticed a awater leak because they could

(08:27):
hear the water drip from thesubfloor and hit the plastic,
and you don't think about that,but you can actually hear that.

SPEAKER_01 (08:37):
Yeah.

SPEAKER_00 (08:38):
You know, you can hear it, it's it's kind of
echoey when it happens, youknow, especially like a drip,
drip, you know.
I mean, if it's falling three orfour feet, so if I it's gonna
make a loud noise.
Yeah, it's gonna make a decentnoise.
So if I had put insulation upthere and trap the moisture,
even with fiberglass or rockwool, which rock wool doesn't

(08:58):
really hold moisture, but itstill can create a some kind of
barrier.
If I did that and then the waterwasn't able to hit that plastic,
then I would never know that itwould that it had a mold or a
water problem.
So I'm not a big fan.
I'm like you, I don't likecovering this upload.
Hopefully, you found thatinformation very, very helpful.
And again, thank you so much forwatching.

(09:19):
Make sure you check out thedescription of this show.
There's all kinds of greatinformation about if you need
help, you're a DIYer, you wantto do a consultation with myself
or AJ.
I've also got some really coolDIY guides that are now
available if you want to checkthat out.
So I am Michael Church withCrawlspace Ninja.

(09:39):
Thanks again for listening tothe Crawl Space Ninja show, and
we hope you make it a happy andblessed day, and we'll see you
later.