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PODCAST: Where to find mold, Part 2

Today’s episode is part 2 about mold conversation. Vickie Swenson shares the top 10 areas where mold usually hides. 

According to Vickie, molds mostly grow in the base basement carpet, rim joist insulation, air exchanger intake ducts, attics, cement walls and basements, leaks under sinks, dark crawl spaces, insulation in newer homes, in-slab ductwork, and many splits in window air conditioners. She shares whether condensation and humidity or plumbing leaks cause more molds. Neil Saltzman also shares that mold grows in the dust. They also talk about how soil (moisture) affects the growth of mold in the basement and PVC being prone to mold.

Reuben, Tessa, and Bill ask about the sources of mold in the top areas, how to test them, and how to keep mold under control. Vickie highlights that mold control is about controlling moisture. Tessa also explains how ductwork in the attic grows mold due to standing water. She also asks about how mold spores in the air are filtered by the furnace. They also discuss how air purifiers help ensure the quality of air inside the house and the importance of cleaning air conditioners.

Reach Vickie Swenson through 612-508-2742 and



The following is a transcription from an audio recording. Although the transcription is largely accurate, in some cases it may be slightly incomplete or contain minor inaccuracies due to inaudible passages or transcription errors.


Bill Oelrich: We wanna start today’s episode by digging into basically the top 10 areas where Vickie tends to find the most mold.




BO: Welcome everyone. You’re listening to Structure Talk, a Structure Tech presentation. My name is Bill Oelrich alongside Tessa Murry and Reuben Saltzman. As always, your three-legged stool coming to you from the Northland, talking all things houses, home inspections, and anything else that’s rattling around in our head.


BO: Welcome to today’s episode, which is part two of our mold conversation. And back with us again today is Neil Saltzman, the inspection services, mold guy. One of the people on our team, our new team, who’s working with mold, and of course Vickie Swenson, who’s here from Minnesota Mold Inspections. Vickie is the godmother of mold in the State of Minnesota. [chuckle] Let’s just be honest. I mean, you set the standard, you write the rules, you write… [laughter] It’s true, it’s true. You’ve brought the vast majority of organization to this area, at least in our area. Would you agree to that? 


Vickie Swenson: Well, I’ll just say I’ve been doing it for a long time, so I appreciate those kind words.


BO: Yeah, you’re very modest. You’re very modest. But what we’re gonna do today is we started last time talking about mold, and Vickie brought to us 57 different places where she finds mold in houses, and we began to deconstruct this list and like what tends to happen when we together is we got completely off track and we started talking about this and that, it was still about mold, but today we’re gonna take a more focused approach. We wanna start today’s episode by digging into basically the top 10 areas where Vickie tends to find the most mold. So Vickie can you go ahead and just kinda run through your list, and then I wanna touch on something that you brought up briefly in our sort of pre-podcast conversation.


VS: So, like I said, when I made the list of the 57 or 59 places that mold hides, they weren’t in any real order. So I thought we might start this by actually I spent some time kinda putting them in order about what I find to be the top 10. And I’ve said this more than once, I think I said in the original podcast, basement carpet is the number one source. Rim joist insulation and unfinished basements, the top of the wall, the rim joist, the insulation on the rim joist is probably number two. Air exchanger intake ducts, attics, cement walls and basements, leaks under sinks, dark crawl spaces, insulation in newer homes that have unfinished walls that are above grade, usually see the insulation behind the plastic, in slab ductwork and many splits in window air conditioners. I think that’s a pretty good list of the top 10 places.


BO: Okay. And what is unique about this list, something that you’ve kind of observed over the years, but was confirmed when you went back and really began to look at this, you mentioned this before, and I want in your own words what you said.


VS: That when I look at… I’ve always felt like there is more mold caused by condensation and actual leaking. And when I made the list, it did confirm that really the only one of those places where leaks are responsible for the large majority is under sinks, all the other nine areas where mold grows is basically a condensation or humidity problem.


BO: And when you’re saying leak, you mean a plumbing leak.


Neil Saltzman: Yeah. Bog water.


VS: Bog water.


BO: And when you’re talking basement carpeting, is that from seepage or leakage from Mother Earth into the wonderful finished area of your basement? 


VS: Yes, and no. So basement carpet, even when people call, basement carpet would be about the fourth question I ask. So you start to figure it out by lifting the carpet and looking at the tack strips, most carpet is gonna have a pad and tack strips, like you would in an upper level. So it’s usually worse in the corners and it’s easier to lift in the corner, so I’ll usually start in the corner or maybe under an egress window or window. So when you lift the carpet, if you see rust on the nails, that’s usually just condensation, if you lift the carpet and the tack strips have disintegrated or they’re super black, you’ve had a leak there. So the older homes don’t have vapor barriers under the cement, and I think some people think cement is waterproof, but it’s not. So the main source of moisture is vapor that just comes up through your slab. And then also because basements are a hole in the ground, a lot of people don’t control their humidities, so you have two sources, the hydrostatic pressure vapor and the humidity, and that is enough to grow mold, even if you don’t have a leak, and that’s because the mold food in carpet is dust. Most carpet is synthetic, and they have put brand new carpet in a humid chamber, it won’t grow mold, it grows on the dust and there’s no way you can vacuum out all the dust. The older the carpet, the higher the risk. If it’s 20 years old and you haven’t controlled your humidity, I can almost guarantee that you’re gonna have mold.


Neil: I gotta chime in, ’cause it reminds me of fiberglass insulation. I can’t remember who recently put a paper out there and talking about how it doesn’t matter what fiberglass manufacturers say, I mean, they’ll say, “Oh, it won’t support mold growth.” It’s like it doesn’t matter what you say. Sure. Mold would literally grow on the fiberglass fibers in a hyperbaric chamber or whatever you just said, Vickie, but there’s gonna be dust in there and it’s gonna grow on that.


VS: So that’s true, when you see mold on carpet, on cement walls or insulation, it’s all growing on the dust, and weird there’s a lot of dust, that get some of these places, so that’s exactly right.


BO: You think sometimes you’ve seen inside of ductwork, there’s literally so much dust that’s accumulated over years and years and years. If somebody comes out and vacuums out your ductwork and you see what’s coming out of there, it’s unbelievable how much is there. I can’t imagine in under the right circumstances bad an HVAC system could actually be in terms of mold.


VS: And to interpret a HVAC now, ’cause…


Neil: Well, we can.


VS: That’s a big deal with carpet, it’s gonna come… Now leaks are there. [chuckle] I went to an arbitration once where the tack strips had disappeared and the woman said, “Nope, never a leak.” “Well, clearly it’s leaked, the mold has eaten your tack strips away.” [chuckle]


VS: Well, I don’t know if people just don’t notice it or they’re afraid to admit it in arbitration, but most of the time it’s humidity, but you can tell when it’s leaking, so I think that’s the main thing about carpet is people say so should I have it down there? I consider ceramic tile the best flooring for basements, I consider a laminate the worst… Some people use an indoor outdoor carpet with no tack strips, no pad. I don’t know what kind of chemicals they put in it to prevent mold, but that is an option. As long as you keep it really dry with the de-humidifier as bad as carpet molds, you still can have it if you’re super careful.


Neil: You said you consider laminate the worst for a basement.


VS: There are some people that think laminate is a good idea in basements, but the thing with laminate, it’s water-proof, if you had laminate in your kitchen and your dishwasher sprung a leak, the restoration companies would have to remove the laminate, to dry the sub floor. So when you put laminate on your basement or that dry core stuff, ’cause people put these dry core panels under their carpet, it really just traps any moisture that’s coming up from your slab, and if you do get a leak, it’s really trapped, so a lot of laminate in basements is gonna grow mold on the bottom side.


Neil: Alright.


Tessa Murry: And I’ve heard some building scientists talk about that, and there are those systems, dry core or whatever you mentioned before, where they are designed to basically, yes, trap moisture, but as long as they’re installed so that there’s no communication from that air space under that flooring into the air space in the room, then in theory, the building scientists say like, that’s okay, I guess the mold will grow there, it just won’t come up into the air that you breathe, you just have to make sure that it gets installed perfectly and it stays 100% sealed. How is that possible? 


VS: You are correct. If you have sandy soil, you’re on a hill, you have good gutters and everything’s perfect, but I had an arbitration a year ago, it was flooding so much, the water was coming up through the cracks, the scenes in the dry core and just molded the OSB. So I’m just still not a fan of it.


Tessa: I’m with you on that. Yeah, just the idea of having some sort of substance that is a vapor barrier and having a gap between that and the concrete floor, you’re going to get mold growing in there somehow. One way or another.


VS: Yep, yep I think the people with sandy soil are a little better off. I don’t know what you guys find in that area, but the Great Sand Basin of Coon Rapids, they have some pretty nice basements over there.


Tessa: Yeah.


Neil: Agreed. [laughter] We don’t in the city.


VS: Yeah, there’s so much clay around and that just contributes to problems.


BO: No kidding, that’s for sure. [chuckle] I’m starting to think that Vickie and Pat would have a lot of similar opinions about basements.


Tessa: Pat Holman? Yes.


BO: Yes.


VS: Yes.


BO: Being in the mixed up store things like mechanical equipment.


Tessa: Dehumidify them and… Yes.


VS: For me, when I do a mold inspection, I see an unfinished basement, it’s like my job’s half done because you’re not wondering what’s under that carpet, what’s under that dry core. What’s behind that, it’s just so easy to diagnose where the mold is, but it still happens. Or some old shelving or on your smit walls, but it’s just much easier to handle.


Tessa: Yeah, easier to clean up too.


BO: So we talked about sources of mold in the basement carpet, we got maybe two areas, it could be, bulk water from the exterior with negative grade disconnected downspouts or maybe a water event that happened, a water heater broke, leaked, flooded the basement or something similar that is allowing that mold to grow in the carpet, that’s what I see those two main sources.


VS: Exactly. And when people try to dry it themselves with a box fan and a shop vac it’s gonna grow mold.


Neil: What should happen if you have a leak event and your carpet gets wet, what should be done, is there a way that that can be dried out without having to replace the carpet? 


VS: If it’s really new carpet and hasn’t collected alot of dust, and you hire a good restoration company and they get it dried in time, that’s the way you can save it, but I’ve even had insurance companies have me test afterwards to see if it worked, and sometimes that will work if it’s new and you hire a good drying company.


Tessa: Define new.


VS: Six months. [chuckle]


Neil: Oh my, that’s really new.


Tessa: Okay. And how quickly would that carpet need to be dried out, do you think for it to be okay? 


VS: Within… Well, a drying company can dry anything within 72 hours, ’cause mold can grow in 24 to 48 hours, but they pretty much guarantee that they’ll dry your structure in 72 hours. They’ll take out the pad. The pad always goes. I’m not sure about the pad, I think I talked about this later, but someone told me that they’re… A lot of pads are made out of recycled car seats, and I did read about pads, it’s all recycled foams, so how many germs and mold spores come in that pad? So when your basement floods they don’t even try to save the pad.


Neil: Well then saving the carpet is, you gotta peel it all back, roll it up, put it to the side, then take all that pad out. There’s a lot of work, [chuckle] if labor means anything.


VS: I really like the polished concrete in basements, that’s another option that is really nice. I don’t know how you guys feel about epoxy in basements. Do you like the epoxy basement floors? 


Neil: I’ve never seen that done. It sounds wonderful, provided you have sub-slab heat. [chuckle] You gotta rupture the hot water pipes running underneath your slab then I’m all for it, but otherwise it’s like… That is darn cold.


Tessa: Yeah.


Neil: That’s like unusable.


Tessa: He’s talking about comfort-wise walking on a… Just a concrete slab on your socks is uncomfortable, so you want some sort of in-floor heat.


Neil: Yeah, exactly.


Tessa: Totally agree.


BO: How much do you know about hydrostatic pressure, Vickie and pushing this moisture over below the basement floor, and if you epoxy a basement floor, that’s basically having a vapor barrier at that point, right? 


VS: So that’s a question I can’t answer. I know it’s there, I don’t know if it’s stronger in some houses, I don’t know if it’s affected by stack effect, that is an area. I don’t know.


Tessa: I can’t answer that question either, but I was just… I would think epoxy would definitely reduce the vapor permeability of concrete for sure, but as long as that concrete doesn’t have some sort of vapor barrier on the other side, it could drive back to the soil potentially, or at least moisture can move through it in one way. In theory.


Neil: Yeah.


Tessa: I don’t know. I’m not qualified to answer that question.


VS: Yeah. Me neither. But when I talk about new houses being full of formaldehyde, and OSB, and spray foam, and everything, the EPA Indoor airPlus program, that is now here because new houses have so much formaldehyde and burn down quickly. The one thing I do like about the new houses are the basements. I do think the basements are better. They’re poured, they’re insulated and waterproofed better on the outside. So, as much as I’m cautious about the new houses, I do like their basements better.


Tessa: Yeah, can we take the framing from reed up on a 1950s house and take the basement from a 2010 house? 


VS: Again, this EPA Indoor airPlus apparently… The Department of Energy has declared that a lot of new homes in America are toxic. And, so there’s a program that some builders can follow to avoid some of these formaldehyde problems. I have somebody looking into that right now.


Neil: How do you end up with a formaldehyde problem in a brand new house, when, in theory, you’ve got agencies out there that are testing this material to make sure it’s safe? How do you end up creating a problem? 


VS: I am not sure. I just… I know that… So my house is 18 years old. I have OSB joists. I have plywood subfloor, Bildrite sheathing. I have tar paper, instead of Tyvek, and I have James Hardie siding. I have really good air quality. But…


BO: It seems like we shouldn’t be building toxic environments because these materials should have gone through a lab. They should have been tested. Are they safe? When you mix them with this and that, all in this enclosure that’s wrapped in plastic, you think somebody much smarter than me, would actually have figured out that, “Yeah, well okay. Well, okay. All we have to do is exchange this area every now and then.” And, even if you don’t, you’re still not gonna get hurt.


Neil: Yeah. So, what I was gonna say is, I was working for a fire fighter once, ’cause I’ve never trusted my OSB joists. And, he… This was probably five years ago, and he told me, “If a house… A new house has OSB joists, and the fire starts in the basement, we just let the house collapse into the basement, because it’s so… They’re gonna burn so fast.” So my understanding is that Weyerhaeuser tried to combat this with putting extra flame retardant on their joists. And it backfired to where they created more formaldehyde and had a big problem with that. So it’s my understanding now that in a new house, if they’re using OSB joists, and they’re not gonna finish the basement, you’ll see sheetrock on the ceiling to delay the burn, in case it starts on the basement.


Neil: But…


VS: I don’t know. I agree. This formaldehyde, it doesn’t make sense to me. But it’s very real.


Neil: Yeah, those are both options. The stuff that Weyerhaeuser had, they called it Flak Jacket. And, to the best of my knowledge, it’s still available today. They just reformulated it and they… It doesn’t off-gas all of this formaldehyde anymore. There was a big class action lawsuit against them. But, at the same time, the building code… What the building code had actually prescribed was that you cover the whole bottom of your joists with drywall. That’s what the building code said. And it said: Or, you could have some type of equivalent. And, that Flak Jacket was the equivalent level of protection. So, we got a few options here. We’re getting totally down a rabbit hole.


VS: Yeah. [laughter]


BO: This is what we do. This is why we like six last time.


Neil: Yeah.


BO: Alright, Vickie. We talked about basement carpet. We touched on rim joists last time. Pretty much in pretty good detail. Air exchange intakes. Why is that a thing? 


VS: So, an air exchanger is basically here because once we built the houses tighter, they’re not breathing like the old houses. So, I tell people all it is a mechanical ventilator. It brings in fresh air. Routes it through your furnace, then sends it back out. And I’m not an HVAC expert. I am a home inspector. So, I can’t even really talk about difference between HRV and ERV, but this is what I know. That air exchanger is pulling so much air in. And, the people that run them in the summer are pulling on dust and humidity and mold spores right into their air exchanger. And, so most of them are made with flex duct, that’s insulated. And, I don’t believe in… It’s really hard to clean flex duct. So, I open up the air exchanger. I put my hand up into that intake duct. I swab it. And, I know that almost every single one of them is full of mold.


VS: So, I’ve actually been thinking about proposing a code change because some people have a really nice situation. If your air exchanger is in a utility room, and your intake duct’s about four or five feet long, just replace it every three or four years, ’cause it is gonna grow mold. If your vent is on the other side of the house, and you’ve got a 30-foot run, and you’ve got a ceiling under that… These people are just having to live with mold and their intake duct because there’s really no way to clean it. So I think it should be a metal duct and insulated on the outside, because I do believe in the ability to clean metal ducts.


Neil: Well, you’re not coming to my house to test my duct [chuckle] because as long as I don’t know about it, I’m fine. [chuckle]


VS: Yeah, it’s one of those unavoidable sources.


Tessa: I’ve inspected a few houses where the flex duct has some sag in it. And there’s actually standing water in that. Have you guys come across that? Standing water…


Neil: No.


Tessa: In the intake? And, the only thing I can think of is just: If that’s running in the summer, pulling in hot humid air, and it’s in the basement, where it’s cool, and the house is air conditioned…


Neil: Oh, sure. What else is gonna happen? 


Tessa: You… Condensation, of course, and then it just sits in that duct. And, since it’s flex, if it’s not fully supported, it’ll just start to drip down to the lowest point and sag.


VS: Yeah.


Tessa: Just like flex bath and ducts in attics. That sort of thing. So, they can get real nasty.


Neil: Well, Tess you’re asking me if I’ve ever seen it? 


Tessa: Yeah.


Neil: And, my answer is: I’ve never seen it. But, I probably just haven’t known enough to look for it.


Tessa: Well I… So, after I found the one, then I started testing by kind of lifting up the flex duct intake on a few more…


VS: It’s a big challenge. I don’t know what’s the future is. I’ve heard some talk about putting HEPA filters. It’s right now one of those unavoidable mold sources, so…


Tessa: How much does the filters on the air exchanger and the filters in your furnace remove the mold spores from the air if it’s coming through an HRV? If you have those filters in place, and they’re pretty clean, and you maintain them, are you still gonna get a lot of mold going through your furnace and being distributed throughout the house because of that? 


VS: Let’s ask Neil that. What do you think, Neil? 


Neil: I think you would.


Tessa: Really? Okay.


BO: As opposed…


Tessa: That’s unsettling.


BO: To just opening your window and allowing natural air into your house? What would be the difference? 


Tessa: Flowing over it and then being distributed by your furnace throughout the rest of the house.


Neil: Exactly.


Tessa: Yeah.


VS: I think some of them are gonna get pulled out eventually. Some are gonna settle in on your carpet. If you do your own mold remediation, you don’t run an air scrubber, you’re gonna have a lot of spores. Eventually they’re gonna dissipate mostly by settling out. But, these spores are tiny. And so all along, people have always wondered, “Is the mold gonna get sucked into your furnace?” And I always thought that was a bit over-exaggerated but recently, with testing, I just had a basement with a lot of stachybotrys in the basement, and I captured it in almost every room on the upper level. And, I did one where the mold was in the crawl space, and I swabbed the vent, and had that mold on the vent. So with more testing, I am starting to think that, yes, it does get pulled in through your HVAC system.


Tessa: Wow.


VS: It can spread around that way.


Tessa: Wow.


VS: In addition, just a stack of that. My understanding is 40% of the air in your house is coming up from your basement, some people say, “Well, I don’t go down my basement, the mold is down there.” But it can definitely just rise up to the upper levels.


BO: You mentioned something called an air scrubber. What are you talking about? 


VS: Yeah, so when you do mold remediation, I’ll make this short. You make every attempt to get rid of the mold, and it depends if it’s growing on a surface, that’s porous, non-porous or semi-porous, that dictates whether you could have to get rid of it or can clean it, most of the things we deal with porous would be carpet, fiber glass insulation or fabric. You have to get rid of that. If it’s on wood, concrete, use sand and scrape it off, use abrasive methods to get it off. So you’re not just spraying and killing it, you’re trying to get it out. But the two critical components of a mold remediation is to run a big HEPA Air cleaner, called an air scrubber, and that removes mold spores to a normal level, you’ll never get to zero, but then you use a HEPA vacuum, and all HEPA means is tighter filter traps smaller particles, so the HEPA air cleaner is gonna get your mold spores back to normal level.


VS: But then you take a HEPA vacuum and you vacuum every inch of that house, every vertical and horizontal surface, and then you wipe it and you vacuum it again. So, when you do mold remediation, you take up… ‘Cause this is one of my stories about carpet that I will bring in. There was a family that bought a town house in Wisconsin, and they sensed that there was a lot of cat urine in the carpet, so they got some sort of light and they could see cat urine everywhere. Well, that’s a moisture source, so all they did was pull up the carpet, put down new carpet, they didn’t do the air scrubbing and the vacuuming. And it was like nine months later, these spores were still there and really high level spores. So it’s not enough to take out your carpet, you have to… Do something with the air. An alternative is put a fan in the window and suck the air out of the room, create your own air purifier with a box fan and a HEPA air furnace filter, there are a bunch of videos online that show you how to do that, or rent or get your hands on one of these big air scrubbers, but you have to do something with the air when you remove mold otherwise the spores just linger for months.


BO: That was my next question. Is there just a way to scrub our air on a continuous basis in our home so we can try to keep mold under control? 


VS: Yeah, well, mold under control is really controlling your moisture, but yep, there are a lot of people that have allergies. I do believe in single room air purifiers, in fact, they sell a certain brand that I really believe in. I think Austin Air is great. I only believe in HEPA and charcoal filtration. Very passionate about air purifiers because most people that call me are caring about their air, so I always say what kind of air purifier are you running because I’m not fond of the ones with UV light, electrostatic plates, or ionizers, because that technology can put ozone into the air, so here you’re running an air purifier to clean your air and you’re putting ozone in which is killing your plants and giving you breathing problems, but a good HEPA and charcoal air purifier that just takes particulates and chemicals out of the air and puts clean air is… And a lot of people run these, they’ll have one in their bedroom, their break room. So I do believe that, yes, air purifiers of the right kind can better air.


BO: I believe in that as well. And I have one running in my family room.


Neil: You run it 24/7 or is it…


BO: Yeah 24/7, you just replace the filters every year and you wash the other ones or replace those as well. Yeah, that’s pretty simple. Just plug and go.


Neil: What kinda costs are you looking at for something like that? 


BO: A couple of hundred.


Neil: Oh, so it’s a low entry point or a low barrier to entry? 


BO: Yeah. Would you agree, Vickie? 


VS: Yeah, it depends. Yeah. So I trust… I don’t know if I can… I don’t wanna say product names on here, so this may be cut out, but just for you guys, I trust BlueAir, IQAir, Austin Air, Honeywell and Air Doctor, those are five that I trust. Austin Air the filter itself weighs 30 pounds, so the filter lasts for three to five years. Yeah, I do believe that air purifiers are a benefit.


Neil: How do you spell Austin Air? 


VS: A-U-S-T-I-N A-I-R dot com.


Neil: Okay, alright. Yeah, we’re gonna cut this out.


VS: So I was gonna to tell you something about the California Air Resource Board. It’s an organization in California test everything. So you might remember a company called Sharper Image, made really cool things, well, they’re not so much around anymore because their Ionic Breeze air purifier apparently made a… Caused so many ozone problems for people that they went out of business. So there’s a website called the California Air Resource Board, they test every air purifier. And if it emits too much ozone, you can’t even sell it in California, so if you’re online ordering an air purifier and says, “Can’t ship to California”, don’t buy that because it’s gonna put ozone in the air. So when I go to that list, they have a list of banned and approved, I’m like, “I’m amazed at how many air purifiers are out there that I’ve never heard of.” It’s a very long list.


BO: Well, you wanna talk about the wild west, when COVID popped up, there was all these conversations happening about how do we clean up houses and how do we kill this virus, and ozone was something that we’ve talked about. I’ve heard a lot of home inspectors talking about, “Well, can we start up a business where we would go in and kinda try to help clean these places?” And I don’t know anything about ozone, so I’m completely out of my league talking about this, but it just sounded like there was a problem. We were looking for solutions really fast, and I don’t know if the use of ozone was wanted or a good idea or not.


VS: So when it comes to mold ozone is not gonna help your mold because we’re trying to remove it, not kill; it could kill your mold. Ozone is an amazing deodorizer, it is chemically releases or destroys odors. So if you had a house fire and your whole house smelled like smoke, they would bring an ozone because it doesn’t cover up odors, it destroys them, so there is the use for it, so the three… But if you go into a house and they have a lot of cat smells or cigarette smell, these are the three deodorization techniques, ozone hydroxyl generators or chlorine dioxide bogging. So ozone does have a place, they’ll use them in hotel rooms for eight hours a night, they’ll use them in gyms, they have an amazing ability to clean the air, but you can’t breathe it in, and if you run it too long, its oxidation, it’ll start ruining your rubber gaskets on your refrigerator, so you have to be very careful. So I do believe in ozone as a tool for deodorizing in unoccupied spaces for the right amount of time.


BO: Perfect. Okay, thank you for the clarification. Let’s move down the list here, dirt floors and crawl spaces. I know that’s… Tessa disapproves of but…


VS: Why do we even have them? Why do people even build like that? Money? 


BO: Money, money, that’s why, ’cause it’s less expensive than pouring concrete.


VS: So, Neil, what do you think about the dirt crawl spaces? 


Neil: Yeah, very moldy areas, found tons of mold in those spaces. Pretty nasty, pretty stinky.




Neil: Not good.


VS: Yeah. The worst is when they have the fiber glass insulation in there. So, I did one in Inver Grove Heights and we brought in the city building inspector because, when you see the insulation under the floor joist and then some plastic under that, and the dirt, that just, that moisture gets up there, it traps. I mean, she had 60,000 spores in that crawl space. And when I asked the inspector, because it had wood foundation. When I see wood foundations I always try to go to the… ‘Cause I didn’t know if we could put celotex on a wood foundation. So I said, “Let’s have the city here and tell us if this is a good idea.” And I said, “When was a code to put fiber glass under the joist?” And he said, “It never was.” But I think it seems intuitive to some people. Like, let’s just put all this fiberglass under the floor and it’ll keep it warm. Those are really nasty.


VS: So, I do believe you can clean them. You have to get all the fiber glass. Sheet rock should never be down there. You see that once in a while. And I think that might have been down there for a fire barrier if they had foam. But my favorite way to do a crawl space is get everything out then use the celotex or some brand of foil lined foam at the perimeter, nothing under the floor, and then like a 12 to 20 ml plastic paper barrier, overlap five feet, taped, brought up to the edges. You can get those under control, but it’s expensive and I’ve kind of come to the conclusion that, after 20 years, you might have to do it again because the dust will land in there and you can grow a thin layer of mold. They will degrade over time. But, about every 20 years you need to do that. And you can make it really nice.


Neil: If done the correct way, they’re beautiful. I’ve seen some very nice crawl spaces.


VS: Well, and then there’s also company. There was a house that I worked in on a lake in Chanhassen. The whole house was three different crawl spaces. Well, one of the crawl spaces, the dirt was almost up to the floor. So the first thing they had to do was use five gallon buckets to make the floor level.


Neil: Right.


VS: This was a $28,000 crawl space project. But I went back a year later and it was performing so well because he put a de-humidifier hanging from the ceiling and had air blowing through. It’s like, so you can fix them but it’s just not cheap.


BO: One area I’d like to talk about is under floor ducts.


Neil: Under floor in this crawl space or, like? 


BO: No, no. Well, going past crawl spaces now. HVAC, under floor duct work, transite ducts. That’s a nasty area.


Neil: Where are you seeing this type of ductwork? 


BO: What we got for years, ’70s, ’90s, somewhere in there.


Neil: Yeah. And when you say transite, you’re talking about sub slab duct work.


BO: Thank you. Sub-slab duct work.


Neil: Yeah. In transite, we always like to reserve that term for when it truly contains asbestos in the duct work.


BO: Got it.


Neil: Most of this stuff is gonna be PVC. I think that’s what I heard, what we see for sub-slab duct work, right? 


BO: Correct.


VS: Yep, then if they do some sheet metal in between, you’ll see a house with a sheet metal which is totally rotted.


Neil: Putting this metal like right in contact with dirt? How else could this possibly end up? 


Tessa: Well, there was a worse material, wasn’t there? Like a cardboard? 


Neil: Yes.


VS: I saw that, yeah. What? What was that? I’ve only seen that once.


Tessa: About it, Reuben, we should link to it, but…


Neil: I do.


Tessa: Yeah, yeah. On a very rare occasion you might run into something that’s not metal and not asbestos and not plastic.


Neil: Yeah, you just talk amongst yourselves, you’ll figure it out.


Tessa: Thank you.


BO: Neil, I wanted to ask if you test sub-slab duct work, is it coming back high levels almost always? Or have you ever seen a clean…


Neil: Almost always, Bill. Take a swab of that, really high spore counts, right, Vickie? 


VS: Totally agree. And that again, I think is mostly from condensation. It’s the dust. So I don’t know if PVC is more prone to mold. We have a lake place that we use these PVC rollers to help with the dock in. And they sat outside for a year and I see mold growing on those. So, one thing I’ve kinda figured out, like maybe is PVC prone to mold growth, but it’s the dust and then the condensation. But these sub-slab ducts I’ve seen… One of my clients almost died because she lived near a lake and standing water was in there. And a bacteria grew that went through her furnace and into her lungs and it went septic infection. She literally almost died from. Now, that’s rare. I had one a couple of years ago in the middle of winter. She said, “I have a bad smell coming out and I thought it was a dead mouse.” So there’s a plumber that I like that uses a sewer scope and he can inspect the whole series. He found dead and hibernating snakes.


Tessa: Yeah.


VS: Then his camera woke up the snakes and all the snakes are coming out. I had a client who had them in a really nice town house ’cause you’ll see a lot of them in town houses next to a wetland. And she had live snakes coming up the whole time she lived there but the homeowners association said, “Don’t clean your ducts ’cause you’ll get more cracks and more snakes.” It’s like, I couldn’t live there. So I just… Those are rare problems where you get standing water and snakes, but it happens, and so I agree. Most of the time you’re just gonna see a thin layer of gray mold that, you’re right, if you swab it, it’s gray mold. So I just think we have to clean those. It’s really expensive to abandon them. That’s an option, but clean them every three years. And it’s one of those things that seem like a good idea but isn’t.


BO: Clean them, how? 


VS: So most duct cleaning is done with brushes and air. And this was probably 10 years ago, my client said, “Well, I just had them cleaned.” I’m like, “Well, I can still scrape the mold out with my finger nail.” So I searched and searched to find as everybody I knew, who can really clean them? And I was using a guy for a while but someone said, “Well, he worked his buns off but I can still see mold in there.” So I was on a job one day and I thought, “Who cleaned these because they are spotless?” And it turned out to be someone I know really well. I call it wet and scrubby, wet and abrasive. He actually puts a liquid down there and then puts a brig on his brush and just scrubs it. So it’s a wet abrasive. And I know some duct cleaners are like, “Well, you shouldn’t put moisture in your ducts.” But the way he does it, he gets them really clean.


Tessa: That sounds expensive.


Neil: Yes.


VS: I think it starts, yeah, it starts at $500. But what are you gonna do? These things are… It’s amazing, ’cause I was just in a house up in Albertville and super moldy but people just don’t look in there, just don’t think of it. They’re just living with this mold problem that just doesn’t… Like most problems. I think started out last year. I think everybody thinks mold is gonna be on your sheet wrap. They don’t know it’s in your ducts, in your attic, in your… That’s why we find so much mould that people are living with. But, it’s amazing how people don’t realize that these sub-slab ducts are a problem.


BO: I agree. That’s the first place I like to go when I see a house that’s got sub-slabs ducts. Go to that basement, take a look, pull that cover, and typically it’s gonna be moldy. And that’s a problem ’cause you’re breathing that air.


VS: Yup. So how do you advise people? Do you steer them away from that house or just say you have to get it cleaned, or you should get it inspected. What’s your advice? 


BO: I give them options. Yeah, for sure you can try cleaning it, then have it retested. I think your guy sounds like a great guy for doing the mitigation. See that, either they have to go over head, abandon it and go over head. It’s very expensive, probably minimum 20,000 that way.


Neil: But I’ll say from a home inspection perspective, one thing we’re always trying to shy away from… I shouldn’t say shy away. One thing we never do [chuckle] is tell somebody you go buy this house or don’t buy this house.


BO: Correct.


Neil: We will just give them the options and they gotta make those decisions for themselves.


BO: Right.


VS: Totally agree.


BO: I’m surprised that at the code level, there’s not conversations about creating bad environments inside of new builds, it feels like this should be addressed at the code level, and they should just say, “You shouldn’t do that anymore. It’s not a good idea.”


Neil: Well, we really don’t see it any more, Bill.


VS: And I agree. But my dad’s house burned down and he had a rental house in Lake Minnetonka for a while, and they had that in a three-year-old house because it was right next to the lake, I don’t know. So you will still see it once in a while and in the house.


Tessa: Slab on grade houses, town homes and stuff have it, and that’s always one big concern when you’re inspecting.


BO: Yes, it is.


VS: What’s the best way to heat and cool a slab on grade, do you think? 


Tessa: In-floor heat? 


Neil: The in-floor heat is absolutely the best way to heat anything.


Tessa: Yeah. [laughter] Yeah.


BO: For sure.


Neil: And then I got the answer to our quiz, what’s the cardboard tubing that they use? It’s Sonotube! 


Tessa: Yeah, Sonotube.


Neil: Yeah. It’s the same stuff they use for footings, for concrete footings.


BO: Oh! Oh, my gosh! 


Neil: Cardboard tube, high grade cardboard. And they used it for sub-slab ductwork, amazingly, and you can imagine what happens to it.




Neil: It disintegrates.


Tessa: Oh, man.


BO: Vickie, I don’t wanna start a fight here between you and Tessa, but you… We’re talking about the best way to heat, we say in-floor, right? But the best way to cool is from the top down, like ducts in the attic… That’s gonna make Tessa’s blood boil. So, just warning.


VS: I kind of agree. So Tessa, why don’t you like ducts in the attic. I had a guy who was so afraid of mold, he built on slab, he put everything up in the attic, this was up in Sandstone. Well, and they used all flex duct. And he was growing mold through his air exchange already. I said, “Well, just replace with metal ducts.” He goes, “Oh no, they stay in two different spray foam on this flex duct.” I’m like, “Is that even?” I don’t know, he moved to California, sold that house. But what are the issues with flex ducts in the attics? 


Tessa: Well, any time you run ductwork outside of the thermal boundary of the house, outside the insulation and everything, then you’re taking that conditioned air in the summer time, if you’ve got cold air going through that ductwork and it’s in a hot attic, then you’re more likely to have condensation form on the duct itself, and then you get all the issues with, obviously, with condensation and moisture, not to mention the energy efficiency problems with that too. Just losing cooling, and winter time, the opposite problem, you’re losing heat into the attic space and you can create ice dam issues, and if you’ve got leaky ductwork, then you’re pumping warm moist air into the attic too, which can create problems in of itself. So just running ductwork through an unconditioned space like that just increases the potential for condensation and energy efficiency problems and building performance issues. I always prefer to see ductwork inside the condition space, not in the slab, not in the attic, not in a tuck under garage ceiling, but in the house.


Neil: Yeah, that’s where it belongs. One we kind of skipped over was attics. I find a lot of mold problems in attics. And Pops, you to shared a photo with me, I think this morning, of an attic you were looking at. What do we usually see with attics? 


BO: The bath fan duck was not insulated and it was not hooked up to the outside, so it was pumping all kinds of warm moist air into that attic, condensing on the bottom of the roof sheathing, de-laminating the roof sheathing and all kinds of moisture issues, not good.


VS: And is that something they thought was a good idea for a while, or is that unintentional…


BO: This is unintentional.


Neil: The roofers came by and knocked the duct down, now it’s pouring in…


BO: You don’t mean to turn us under the bus, I’m not trying to do that.


Neil: Yeah.


VS: That’s what happens, though. Or they forget to put the ducts or the vents on the ceiling… The roof.


Neil: Well, and I know that there’s been a lot of homes built around like the ’70s and ’80s where you would have a first floor bathroom or just a powder room, you’d have a sink and a toilet, and they would build them without running a duct to the outside, they just run the duct up in in the attic space thinking, “Well, there’s no shower, it’s not a big deal”, and somehow these all passed inspection, they’re all just built that way. And it doesn’t matter if you don’t have a shower on there that’s taking warm humid air and you’re pumping it into an attic, so for a while, we got by with doing it, and it’s never been a good idea.


VS: One more thing while we’re on HVAC, I wanna talk about this round… Because a lot of people are using flex duct, you’ll see some of the newer homes have the round supply vent cover instead of the old rectangular vent cover. A few years back, I went into a house, you’ll sometimes see mold growing right around that vent cover, and this guy had every single one, and I’m like, I didn’t know what was happening at that time. So I researched and researched, I found on the internet, they said sometimes it’s like, if the duct isn’t connected to the vent that’s gonna happen. I think this is what’s really happening, because I saw one where this was happening and there was no duct feeding the vent. They were using a sheetrock cavity as the duct, so there was no duct, flex duct. So, I think what’s happening is those round ducts, for one thing they have a foam, if you pull them down, there’s a little layer of foam, so I think when condensation happens there, that foam traps the condensation so that’s…


VS: When I was trying to piece these all together, that’s when one of the remediators told me well the foam in there could create some moisture it spreads to the sheetrock. I think what’s really happening is gravity pulls those down and the cold air is escaping above the vent cover and condensation is happening there. So, I think it looks worse than it is, but… So you’ll see some that are screwed to the ceiling, but these ones that are just sort of up there and they’re not secured very well. I really think that’s what’s happening, and during air conditioning, that cold air is… The gravity is pulling the vent cover down, cold air goes out above, and causes condensation there. So it looks worse than it is. I don’t think it’s like anything wrong with your HVAC, I just think that vent cover is not securely attached to the ceiling.


Neil: Easy fix.


VS: Yeah. So one guy had them on everyone was like, I have no idea. It looked really scary, but I don’t think it really is that big of a deal.


BO: What you’re saying is periodically look at these vent covers and make sure there’s no obvious signs of mold growth or anything like that going on? 


VS: Yup, ’cause we don’t really see them so much. You don’t see that problem with the old metal vent covers. Occasional see mold on HVAC supply, and so, one thing that can cause that is if your air conditioner is oversized. So your air conditioner cools it and quits running, but isn’t running long enough to really take out the humidity. So I think some people think, “Oh, bigger is better”, but if you’re… Part of it is to remove humidity, so if your air conditioners shutting down before the humidity is gone, I think that’s when you will see some mold on the vent covers.


BO: Speaking of air conditioners, I think the last of the top 10 that we haven’t touched on quite yet, are window unit air conditioners and also mini splits. So, that made your top 10. What are you seeing on these units? 


VS: Well, I had a landlord, they had already taken the cover off, and you can just see the mold because they’re condensing in there, and they’re moving air with dust. So when they did some research on… You can clean these things. So I tell people, go online, find a YouTube video, look up your model that you really should clean these. The interesting thing about the mini splits is to hire a guy to clean it, it’s $400 or $450, and when you read online about cleaning mini splits, they said you should do it every four months. So it’s just another one of those unavoidable mold sources that… Mini splits seem like a good idea, but…


VS: You’re crushing my world, ’cause I… There are some situations I think mini splits solve a lot of issues in an affordable way, if you’ve got an old house that doesn’t have ductwork and you want some sort of air conditioning or something. But is there a way for a homeowner to clean it out properly, so you don’t have to spend $400 hiring someone? Is it a simple like, oh, cleaning off the filters and washing and putting back, or do you really need to dig in there? 


VS: I think you really needed to dig in there. So, I’m gonna do more research on it, but when I saw it and looked it up ’cause he said he couldn’t find anyone to clean it.


Tessa: I wonder what they do. You know, I’m just curious what the cleaning process is for something like that.


VS: I think it’s just cleaning every little thing, wiping everything up.


BO: But they do crush your soul.




VS: I agree. They seem so convenient, but… But you could also get mold in your A coil in your traditional air conditioner, and so I’m curious to hear what you guys say about this because originally… I can’t open the furnace. So when I have wanted to check someone’s A coil, I say arrange for an HVAC guy to open it up, and then I’ll come there and look at it, but one of my clients actually had mold growing in his lungs, and he’s pretty sure it came from his A coil. So his furnace guy had a little camera and he showed me. So the A coil is called that because it’s shaped like an A, it’s cold fins that condense your water into a drain pan, but the dust is moving up through that A, so the mold is in the A coil, and it’s really hard to look in there and clean in there. So, even when I’ve had the HVAC guy open it up, you can’t really look inside the A coil without a camera. And I don’t think it’s… You know, it happens in every case, but depends how much dust as we were talking about… How much dust in the HVAC, probably there’s… I don’t know under what conditions that happens, but it’s really hard to look under that A coil. So, what do you think about that? 


Neil: Do you know of any good way to clean those, Pops? 


BO: No, not right off the top of my head, I don’t.


Neil: Okay.


BO: I mean you gotta get in there, you gotta get to that pan, ’cause that’s probably where it’s… The drain pan under the A coil.


VS: Yep.


BO: That’s probably where it’s accumulating.


Neil: And that’s why we disclaim mold as home inspectors.




VS: Well, in some of these A coil… I don’t know if you call it plenum. I call it the box that surrounds the A coil ’cause I’m very layman, but some of them have fiberglass insulation in there, and then that’s just gonna trap dust and grow mold, so I don’t like that either.


Tessa: So, no safe way to cool your home, really. You can get mold growing in an A coil, your furnace, in a mini split, duct work, in your slab. I mean, any time you’re pumping cold air through you know, or you’re cooling air, you’re potentially causing condensation and got dust, you can have mold growth. It’s just the reality of it.


BO: I think we all have to move to San Diego where you don’t have to use any heating equipment or cooling equipment.


Neil: Yeah. Yeah.


Tessa: Dry, dry climates. Yeah.


VS: Yep. Yep.


Neil: Go somewhere where it’s 70 to 80 degrees all year.


Tessa: And dry.


VS: Probably in, yeah.


BO: Yeah. Hot climates, bad news. Really cold climates, bad news.




BO: Yeah, sounds like you’re gonna be busy for a while, Vickie.


VS: Mold is written about in the Bible. Leviticus Chapter 14. And so after hearing so many classes, every mold class starts with that, I finally got out the Bible to read it, and it’s really interesting because… Well, some people say it’s a metaphor for clean living. I’m like no, ‘I’m pretty sure it’s a health manual because Leviticus 13 will help you figure out if you had leprosy, and they literally called mold leprosy of homes.” And so, read it sometime it’s pretty interesting. So it’s part of the food chain. It’s a decomposer, that’s one thing that makes the mold business pretty solid because it will never go away.


Neil: Right.


BO: And with that, we’re gonna put a wrap on Episode Number Two of Mold. Vickie, why don’t you go ahead and tell us where you can be reached again.


VS: Yep. Well, my phone number is 612-508-2742. My little website is


BO: Awesome. Thank you very much for another extended session with you to talk about mold. You can see we’ve completely solved all the problems, and you can happily live in your house, in a clean environment and feel good every day that you’re not…


VS: We’re not trying to scare people. We don’t wanna scare people. I’m always trying to calm them down, so… But it’s a very moldy state here.


BO: Yeah, well, thank you, Vickie, we appreciate you and your time, and Neil, thank you for jumping in and providing your expertise. You’ve been listening to Structure Talk, a Structure Tech Presentation. My name is Bill Oelrich, alongside Tessa Murry and Reuben Saltzman. Literally today, alongside Tessa Murry, Reuben Saltzman, and you’ve been listening to Structure Talk. Thanks for listening. We’ll catch you next time.