Stop Using Your AC To Dry The Basement or Crawl Space

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Cold air can fool you. A basement can feel crisp at 60 degrees while humidity sneaks past 70% and turns prized gear into mold bait. We bring on Alex Hamilton from Aprilaire to demystify why HVAC systems are built to control temperature, not moisture, and how that gap leads to musty smells, surface mold, and ruined finishes in basements and crawl spaces.

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We start with the classic contractor mistake: finishing or encapsulating a space without adding a dehumidifier. Alex explains how added ductwork without resizing the system reduces coil contact time and causes short cycling, which means the AC removes less moisture even if the upstairs feels perfect. Then we dive into a real-world case of a stunning, brand-new basement with humidity near 80%—leather pockets and felt on the pool table blooming with mold—because the basement sat below the thermostat setpoint and the AC rarely ran.

From there, we break down how dedicated dehumidifiers prioritize relative humidity and dew point, measuring inlet and outlet conditions to condense water efficiently and reheat the air so comfort rises without overcooling. We cover ducted installs from a mechanical room, RH targets for healthy homes, and why sealed basements and crawl spaces need active drying year-round. You’ll learn how reducing moisture can help the AC run less, why the stack effect moves damp air through the whole house, and the simple metrics to watch so you never get blindsided by hidden humidity again.

If you care about indoor air quality, building durability, and true comfort, this conversation gives you the clear path: seal smart, dry proactively, and stop asking your AC to do a job it wasn’t designed to do. Subscribe, share this episode with a homeowner or contractor who needs it, and leave a review with your biggest humidity headache—we may tackle it next.

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Episode Transcript

SPEAKER_01 (00:00):
The Dhumidifier is designed to control humidity.
The HPEC system is designed tocontrol temperature.
You're listening to the CrawlSpace Ninja Show where Healthy
Homes begins from the ground up.
I am Michael Church, the founderof Crawl Space Ninja, and your
host today.
And we've got a great show foryou.
Many years ago, when I gotinvolved in doing crawl spaces

(00:22):
and basement waterproofing, oneof the biggest things that the
HPC industry was trying to do isregulate humidity in crawl
spaces and basements with HCequipment.
And we found that was not a goodplan and wound up causing a lot
of mold problems.
So I've got Alex Hamilton herewith me from April Air, and we

(00:43):
go over that with you so youunderstand if you're getting
told to use your HC unit toregulate humidity.
It's probably not a good idea,especially in that crawl space,
but in other parts of your homeas well, whether you have a
finished or unfinished basementor even an attic.
Also, make sure you check outour description of this video

(01:04):
because we got some greatinformation for you.
Let's get started.
Even in basements, I've evenseen this.
If you can touch a little bit onbasements, I think this is one
of the biggest overlooked thingsthat basement waterproofing
companies uh have is they willnot install a dehumidifier when
they uh when they waterproof abasement.
And I think that's a hugemistake.
And we've even had instanceswhere you have you know the

(01:26):
mechanical room, and you caninstall that uh that
dehumidifier in the mechanicalroom and then duct it in a way
to where you're blowing in andout of that mechanical room so
it's not an eyesore in themiddle of the basement.
Tell us a little bit about thattoo, if you don't like.

SPEAKER_00 (01:40):
Sure.
So and just just over the lastcouple of years, I've I've run
into that exact thing where youknow they're they're going in,
they're encapsulating everythinglooks really pretty.
Like it looks, it looksefficient, but they're not
adding a dehumidifier.
And something that I thinkhomeowners and contractors need
to understand is if you're notan HVAC contractor, okay, and
someone finishes a basement andthen they add ducting to that,

(02:03):
but not a larger airconditioning unit to the home,
you're actually going to sufferin the fact that it's not gonna
be able to return enough air todehumidify it.
So what happens is you need morecoil space.
That's that's essentially wherewe get into sizing issues and
things of that nature.
You need more coil surface areato remove that humidity.
So to do that, the only thingyou can do is add a larger air

(02:23):
conditioning system, or the moreefficient and more proactive way
to do that would be to add adehumidifier.
So for those instances whereit's not necessarily a finished
basement, um, if you're leavingthe space fully encapsulated
without a dehumidifier, whetherit be a basement, a crawl space,
or anything like that, it's kindof like closing the doors and
the windows on your car and nothaving the air conditioner on.

(02:46):
You know, it's going to changethe entire ecosystem inside that
space.
But you know, you you know, alot of people will say, oh, we
roll the window down, you getthe fresh air and things of that
nature.
With the basement, when weencapsulate, we want sealed.
With a crawl space, we wantsealed.
So the only way to introduceconditioned air into that space
is with a dehumidifier.
Um that is that's somethingthat's heavily overlooked

(03:07):
because with you know, stackeffect and things of that
nature, stack effect doesn'tjust affect crawl spaces, it's
gonna affect the basement.
Um and with most basements,you're gonna see a concrete
floor, a block wall, uh, in somecases you'll see a dirt floor uh
with a block wall.
And when we're sealing that up,if we're not actually taking
that humidity out, it's stillgonna find a way to change that

(03:30):
ecosystem as far as thehumidity, the temperature, and
things of that nature.

SPEAKER_01 (03:35):
Well, and I want to just advance what you just said
a little bit because I'vepersonally had some experiences
where you're talking aboutadding more coil, right?
Uh I forget exactly how youphrase it, but that's a good way
to say it.
You're basically adding morecoil surface by adding a
dehumidifier to an HPEC system.
But there's also a problem.

(03:55):
I had a buddy, if I could tell aquick story, he's a he's a home
builder and he built thisbeautiful home, finished
basement, you know, justimmaculate, right?
I mean, uh it was it's the mancave, you know, there's a pool
down there and a bar and allthat kind of stuff.
And he calls me up and he goes,Michael, I don't know what's
going on.
I got uh the homeowners got moldgrowing on their on their pool

(04:16):
table.
This house is less than a yearold and they've got mold growing
on it.
So I, you know, he calls me up,I go over there, and uh, you
know, the the thing that peopledon't understand is a lot of
times a basement's temperatureis lower than the set point of
the AC.
So, you know, I get over thereand I'm in the house and it's
immaculate.

(04:36):
They've taken care of it, it'svery nice.
I open the basement door, and assoon as I open the door, I feel
a chill as well as a humid airsmack me in the face.
And I kid around about when Itell this story, I could almost
hear like monkeys screaming downthere because it was so humid,
right?
Right.

(04:57):
So, and and and so I walked downthere and it was just
ridiculous.
It was almost 80% humidity.
There was surface mold gettinggrowing, and he had like the
pool table, it had like theleather pockets, you know, that
were really nice.
That leather was growing mold,the felt was growing mold, all
this other stuff.
And the problem was is the steppoint or the temperature was so

(05:19):
comfortable because of how wellthe basement was built that the
AC never turned on to helpdehumidify the basement, and
that's where you need somethinglike an A-layer dehumidifier,
even if it comes on twice ayear, you still need it, right?
Could you could you kind ofshare that a little bit?
It's not that it's it has to runconstantly, it just needs to be

(05:39):
there in case you need it aswell.

SPEAKER_00 (05:42):
Well, I think we get confused, like, you know, you
can be you could have thetemperature exactly where you
want it, and we think thatthat's where comfortability lies
is strictly temperature, andthat's just not the case.
Um with high humidity,obviously, if the air
conditioner is going to runconstantly in the summertime.
I mean, we're in the southeast,I mean, we know how this works.
Across the country, you want tobe able to make sure that you're

(06:02):
comfortable.
I think that's priority one.
Well, when you walk into abasement like that where it's
very cold, but it's highhumidity, what we have going on
is it takes less um, what's theword to put cooling, I guess you
would say, less BTUs to changethe temperature of a basement
than it would a main floor of ahome or a, I guess, a second
story of a home, even still.

(06:24):
Uh but when you have adehumidifier put in place, what
we usually run into, this iswhat I see 99% of the time, is
they will have an airconditioning system that's
oversized for the entire space,which means that it's short
cycles.
And to your point, if there'snot a need to cool, then the air
conditioning system's not goingto come on, which means that no
humidity is going to getremoved.

(06:46):
Uh now, the dehumidifier, it's alittle bit different.
So if you've ever seen, and youcan go to their website if you
want to see it, on the frontportion of all of our
dehumidifiers, it's a standardthat it has a user interface
control that's going to readtemperature, humidity, inlet and
outlet temperature and humidityas it goes to that unit.
So you're getting a full uhspectrum, I guess you would say,

(07:08):
on what that room is actuallydoing.
So where it would read, let'sjust say it was 60 degrees in
that basement, you know, it'scold down there, but it's 80%
humidity, we're gonna prioritizethat humidity and we're gonna
get that reduced.
So, what it would do toacclimate for that is it will
try to cool that air to thepoint where it's going to uh
basically condense and then it'sgonna heat that air and pull it

(07:31):
back out.
So it all happens in one in onesetting, one spot, and it's
calculating dew point.
Whether we want to see RH or noton the front of that unit, we
have to know the dew point inorder to produce that.
Most thermostats in homes do notdo that.
Uh, there are some in themarket.
I know we actually wemanufacture a thermostat that
can that can do both, um, butyou're either going to run the

(07:53):
air conditioner a lot longer,which means it's gonna make it a
lot colder down there.
Uh so that's not efficient andthat's not comfortable.
So that's where the dehumidifieris a that's the prize winner for
that.
Because, like to your point, itmay only run one or two times a
year just to acclimate thespace.
But once it's acclimated, we'rewe want to stay in the spectrum
of how do we get this monitoredand changed actively, not over

(08:17):
the course of how long the airconditioner runs.
Yeah, that's a good point.

SPEAKER_01 (08:20):
I I appreciate you sharing that because uh, you
know, even in the crawl spaces,there was this phenomenon for
and it probably still goes on,but there was a lot of HVAC
contractors that were, oh, wedon't need to put a DHU down
there.
Let's duck, you know, uh asupply and a return in the crawl
space.
And again, it was the samething.
We'd go into these crawl spaces,and because the thermostat was

(08:41):
in the living space, you know,either the DHU would short cycle
or not cycle long enough tocontrol humidity in the crawl
space.
You know, the dehumidifier is isdesigned to control humidity.
The HBAC system is designed tocontrol temperature.
You know, they're two totallydifferent things in a lot of
ways, and they they can feed offeach other a little bit.

(09:02):
You know, the dehumidifier canhelp the AC run less if the
humidity is lower and all that,but but they are two different
mechanical devices that have twodifferent purposes, right?

SPEAKER_00 (09:13):
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