Dormant Mold, Real Risks, Smart Fixes

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.

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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?

Chapters

• 0:00 Welcome And Topic Setup
• 0:41 The Listener’s 1927 Crawl Space Dilemma
• 1:18 What Dormant Mold Really Means
• 2:40 Fruiting Bodies And Moisture Reality
• 4:00 Why Spray Foam Over Mold Backfires
• 5:26 Better Strategy: Air Seal And Wall Insulate
• 7:12 Keep Wood Exposed For Leaks And Drying
• 9:19 Final Advice And Resources

Transcript

SPEAKER_00: 0:00

You're listening to the CrawlSpace Ninja Show where healthy homes begin from the ground up. I am Michael Church, your host and founder of Crawl Space Ninja. Today we got a great episode for you. I had an opportunity to speak with AJ. He is our DIY specialist, and we talk about 50 to 55% humidity. What exactly does that do to mold? Is mold okay if it goes dormant? And I'm gonna give you my three steps to addressing mold in the correct order. Let's get into this episode with AJ talking about dormant mold and how that affects your house.

SPEAKER_01: 0:38

Okay, so I have a Ask a Ninja question for you. Um, this person has a 1927 home with a very little crawl space, it's about two feet tall, including a joist. Um, their concern is the subfloor. The wood's old, there's visible mold on it, and their true questions are if the humidity is kept below 50 to 55, sorry, 50 to 55%, should they still be concerned about the dormant mold? And then their follow-up question to that is can spray foam be applied directly over that mold if it's not active?

SPEAKER_00: 1:13

That is a fantastic question. And let me let me answer the first part. All right, so what does dormant mold mean? Okay, so um, you know, mold is everywhere. Let's just put that out there, right? So it's impossible for you to have a mold-free environment unless you had some kind of like clean room or something, you know, like where they make, you know, microchips or something like that, where they can't allow dust and all that kind of stuff in there because there's always something for mold to grow on. So, but dormant mold, in my opinion, is a little misleading. To say, to say something is dormant, you may instantly think, oh, dormant means dead, right? I mean, that may be what some people think, but it doesn't. It means it's alive, it's just not active. All right. And and you know, I always use the groundhog scenario, okay? So a groundhog, we have groundhogs here in East Tennessee, and they go dormant when they hibernate, right? But what happens? Let's say that I found a groundhog hole and I grabbed a stick and I dug it out and I found where they were nesting, and it's you know, January and it's cold, and and I start poking that groundhog with that stick. What do you think that groundhog's gonna do to me and that stick? Right? He's no longer gonna be dormant. Okay, so what I'm trying to say is is that the best scenario is to remove the mold, remove what caused the mold, and also apply something that kills its root. Okay, because mold, the food source, what you see with your eyeballs is the fruiting body. So, in other words, if you look outside in your yard and you know it rains a lot for three or four days, and then all of a sudden a mushroom pops up in the middle of your pristine grass, okay, there was mold under the grass before you saw the mushroom, right? So the what we see on wood is the fruiting body of the mold. That doesn't mean that the mold wasn't there, it could have been in the wood growing until it had high enough humidity or grew fast enough for it to start fruit. Now, why does mold fruit or why does it become visible? It's so it can spread. Okay, that's usually what happens. So if you take the word dormant mold, and then now you take closed cell or open cell spray foam, 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 else did it lock in with the mold? It locked in the moisture. Because there wouldn't be mold there if there wasn't some kind of moisture to start it in the first place. And yeah, they've been running the dehumidifier, it's under 55%, but they didn't tell me what the wood moisture level is. Okay. Is the wood moisture level low? Do they have some kind of hidden leak they're not they didn't know about? Like the toilet flange is leaking, and then they they air seal all of that subfloor and joist with spray foam, and now they got wet wood because they didn't know that they had a toilet leak. What happens now? That mold just grows and grows and grows, but now it's not visible anymore because they sealed it up inside of spray foam, right? I hate whenever we as contractors cover up the subfloor. I don't like covering up with spray foam, and I don't like it, I don't like covering up with insulation. I'm not saying there's not a reason to do both, but there's there's you know, if you have a house 20 years old, there's a really good chance at some point that house is going to have a water leak. Whether it's the toilet flange, whether it's the refrigerator ice maker, whether it's the washing machine, and if you've sealed up or covered up that subfloor and that water has nowhere to go except sit against that subfloor insulation, whether it's fiberglass, spray foam, or whatever, you're gonna have a big problem because you can't even dry it from underneath anymore. So, AJ, you know, what do we do? What's the first thing we do if there's subfloor insulation and we need to dry out a crawl space? We take it out. We take it out. How am I gonna take out spray foam, AJ?

SPEAKER_01: 5:54

I mean, you can't really. I mean, you could, but you can't really. It's it's it's it's a horrible mess.

SPEAKER_00: 6:00

Well, and a lot of people don't know this, but you know, spray foam is actually uh a form of plastic, right? It's it's an expandable plastic. So I have scraped off spray foam, and if you look really good, there's still a microscopic, almost waterproofing film still on the wood. So really the only way to get it off is not just to knock off the big pieces, but to actually, you know, sand off that that um adhesive part, you know, that's on the wood. So even if you take the spray foam off visibly, you still might be trapping moisture because because of that. So I'm not a big fan of spray foaming subfloors. I really I I just don't see a huge I am a big fan of air sealing penetrations, you know. So if someone had uh to to go back to this question, I know it's a low crawl space, but to me, I think you would be better off air sealing all the major penetrations. The other thing, too, is if you got a 1927 house, it might have like some kind of tongue and groove with gaps in it. So what happens when you spray foam? Where are the where let's say I've been in houses where I could see daylight or I could see the the room above, what happens if I spray foam underneath? Where's that spray foam going to go? It's gonna rise up into the room above, right? Um, so to me, I think you know, min uh I think minimizing the stack effect by air sealing the big gaps that you can seal in a in a subfloor and then insulating the walls of the crawl space to create that thermal break at the wall would be a better choice. And even, I mean, if you're in a really cold environment and the soil is super cold and you want to put like some kind of insulation on the soil itself, I wouldn't be against that either. But I would rather do those two places of insulation versus a spray foam of the subfloor. So hopefully that helps them out and uh keeps them from making a big mistake because once that spray foam goes on, you can't get it off.

SPEAKER_01: 8:01

Oh yeah. A big thing I tell a big thing I tell everyone is you never want to cover wood.

SPEAKER_00: 8:06

You don't? You don't. I mean, they're they're not in a crawl space. I mean, it's just a naturally wet environment, you know. Oh yeah. I I'd rather be able to see it. I and you know, the other advantage of not covering it, which this may seem very dumb, but I've actually had homeowners tell me that they noticed a a water leak because they could hear the water drip from the subfloor and hit the plastic, and you don't think about that, but you can actually hear that.

SPEAKER_01: 8:37

Yeah.

SPEAKER_00: 8:38

You know, you can hear it, it's it's kind of echoey when it happens, you know, especially like a drip, drip, you know. I mean, if it's falling three or four feet, so if I it's gonna make a loud noise. Yeah, it's gonna make a decent noise. So if I had put insulation up there and trap the moisture, even with fiberglass or rock wool, which rock wool doesn't really hold moisture, but it still can create a some kind of barrier. If I did that and then the water wasn't able to hit that plastic, then I would never know that it would that it had a mold or a water problem. So I'm not a big fan. I'm like you, I don't like covering this upload. Hopefully, you found that information very, very helpful. And again, thank you so much for watching. Make sure you check out the description of this show. There's all kinds of great information about if you need help, you're a DIYer, you want to do a consultation with myself or AJ. I've also got some really cool DIY guides that are now available if you want to check that out. So I am Michael Church with Crawlspace Ninja. Thanks again for listening to the Crawl Space Ninja show, and we hope you make it a happy and blessed day, and we'll see you later.