Humidity, Dew Point, And Your Home

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Ever wonder why your windows fog in winter or why the attic sometimes grows frost like a walk-in freezer? We unpack the simple physics behind relative humidity and dew point, then connect those concepts to the real problems homeowners face: condensation on cold surfaces, mold risk in damp basements, and hidden moisture damage in poorly ventilated attics.

We start by making humidity intuitive, showing how warmer air carries more water vapor because it has more energy. From there, we explain dew point in plain language: the temperature where air can’t hold its water anymore and it condenses. That single idea explains the beads on a cold soda can, the musty smell in a basement, and the reason roof sheathing can frost over on the coldest nights. Along the way, we talk through ideal indoor humidity targets (40–50%), why anything at or above 60% invites mold growth, and how device settings can work against you.

You’ll hear our take on common mistakes like running a whole-house humidifier at 55% while a basement dehumidifier fights to hold 45%. We dig into regional realities, why basements and crawl spaces often need dedicated dehumidifiers, and how small leaks in furnace-mounted humidifiers can quietly corrode ductwork. We also map out practical fixes: seal ceiling penetrations, ensure clear soffit paths, add baffles, balance ridge ventilation, and insulate cold spots such as rim joists. The goal is simple—align temperature, moisture, and airflow so dew point happens outside, not on your framing.

If you want a home that feels comfortable, smells clean, and resists mold, this is your roadmap. Tune in for clear guidance, actionable settings, and the inspector insights we rely on during real home visits. If this helped you dial in your humidity, subscribe, share with a friend, and leave a quick review telling us your target RH and what you’ll adjust first.

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Episode Transcript

SPEAKER_01 (00:00):
Hello, hello, hello.
This is Jim, and of course Laurais here with me.

SPEAKER_02 (00:04):
Hello, everyone.

SPEAKER_01 (00:06):
Alright, so Laura.

SPEAKER_02 (00:07):
So Jim.

SPEAKER_01 (00:09):
Dew point.
Yes.
And relative humidity.
Let's let's let's go throughthis a little bit.

SPEAKER_02 (00:16):
Yes, we we have a reason for all of this.

SPEAKER_01 (00:20):
So rele humidity is relative humidity is a
measurement of how much watervapor the air can hold relative
to relative to the temperaturethat it is.
So let's say you got some airtemperature is fifty degrees.

(00:46):
Fifty degrees can hold half asmuch water as 80 degree
temperature.

SPEAKER_02 (00:56):
Why not a hundred?

SPEAKER_01 (01:01):
No, seriously, why why is it half if it's because
the more all right, sotemperature is a measurement of
energy.

SPEAKER_02 (01:10):
Okay.

SPEAKER_01 (01:12):
Alright, so who has all right you're sleepy right
now.

SPEAKER_02 (01:17):
So yes, I am.

SPEAKER_01 (01:18):
So who has so energy is a measurement of of of
energy.

SPEAKER_02 (01:24):
So so it's not like potential or something like
that.
No, no, no.
Or is that work?
That was work.

SPEAKER_01 (01:30):
That's there's potential energy and there's
kinetic energy.
We're not talking about that.
Okay, never mind.
Yes, we don't want to get intothat right now.
Geek.
So because the heat is ameasurement of how much energy
you have.
So look at and visualize that asand as the measurement of how
much you can lift.

(01:52):
So the temperature is a measureof energy to lift things.
Use that to remind you that themore heat something has, the
more energy it has to lift andsupport things.

SPEAKER_02 (02:06):
So it's supporting the water that's in the air.

SPEAKER_01 (02:10):
Correct.
Okay.
80 degrees, if I remember right,can hold about twice as much as
50 degrees temperature air.
Okay.
So, all right, we're doing we'resaying this because we're coming
to go into a thing called dewpoint.
And dew point is just anotherterm for the the temperature in

(02:37):
which the air temperature airtemperature can no longer
support or suspend the waterthat's in in it.
So when you go outside and youhave a nice cold can of soda
outside and it's humid outside,the humid air says 90 degrees

(02:58):
out, it can hold quite a decentamount of water vapor.
But when that w that air vapor,that air and the which has the
water in it gets close to thecan, it cools off and it no
longer has the energy to supportthe water.
So the water condenses.
That is dew point.
Dew point depends upon thetemperature and then the

(03:21):
humidity level as well.
Okay.
Alright, so so we got thosedone.
We got that.
So here's the thing in yourhouse, some people will have
humidifiers.
Usually it's on the furnace, andif it's building that's on the

(03:45):
furnace on the return air, andwhat it does is it puts it
humidity into the air.

SPEAKER_02 (03:50):
So for like wintertime when it's really dry
and it gets down to like theteens or the low 20s, you want
some moisture in the air.

SPEAKER_01 (03:57):
Well, yeah.
Well, you'll look at yourhumidistact humidity gauge in
your house and go, dude, it's20% humidity in there.
It is way too dry.
Yes.
So you have a humidifier to addhumidity.
You also have your dehumidifierto take the moisture out of the
air.
Right.

(04:17):
Now, we had an inspection theother day.
They had all right for us, werecommend you really never want
your humidity to hit 60% orhigher because supposedly that's
the rate in which humidity,which some mold start growing.
And it starts deteriorating 40to 50.

(04:40):
That's a nice range.
That's a nice decent range.
It gives you some cushion aboveand low to not cause any issues.
Well, we had a house the otherday, they had the humidifier
which set at 55.
And the dehumidifier was set atand they also had a dehumidifier

(05:01):
that was set at 45.
Which means they have these twosystems running simultaneously,
battling each other as of wherethey want the humidity level to
be.
Now, if the basement is at adifferent temperature than the

(05:22):
rest of the house, maybe thatwould be okay.
Because the basements, if it'scolder, can't hold as much
moisture as the warmer upstairs.

SPEAKER_02 (05:35):
Right.

SPEAKER_01 (05:35):
So they kind of needn't need to look at that.
But but still, you shouldn'thave two systems battling each
other because it's just notenergy efficient for for it's
gonna it's gonna rack up youryour heating bills.
Well, unless you're gonna beable to get it.
Well your electricity works.

SPEAKER_02 (05:50):
It's also gonna wear them out faster.

SPEAKER_01 (05:52):
It could it can, yes.
So we recommend every basementin Ohio have a dehumidifier.
If you if you have crawl space,I'd put a dehumidifier there
also, especially if it's sealedup to the exterior.
And they have it set at acertain humidity level, like 45,
50.
So as soon as the humidity getsto that level, it turns on,

(06:13):
takes humidity out of there.
That's it.
I'm not a real big fan ofhumidifiers because if they're
leaking, you may not notice itfor quite a while.
Because if it leaks, it's gonnabe leaking inside the furnace
ductwork, and you you won't seeit.
By the time you see it, it mayhave caused some rusting and may

(06:34):
have been gone on for a longway, you know, a long time
already.
So, dew point is the temperaturein which the moisture can no
longer be suspended by the airand it condenses, which is why
during the wintertime, if youhave a damp basement, you are

(06:57):
likely to, and you don't haveany insulation on your walls or
up in the seal plates, like onthe joist on top of the
foundation and outside, thethat's gonna get cold.
That's a good potential spot forthat moisture to condense on
that wood, and now you havepotential for more growth and
some deterioration.
So that's where dew point comesin to uh a house.

(07:22):
It also takes place also in theattic space if you don't have
good ventilation because youalways have moisture vapor
coming through your house.
Right.
You're cooking, you're cleaning,you're taking showers, you're
breathing out moisture vapor,and it makes its way through the
drywall or the like openings inthe light, fixtures, ductwork,

(07:45):
and makes its way up inside theattic space while having enough
humidity, it can get up thereand start condensing up inside
the attic space during thewintertime because that roof is
really cold, especially onreally cold nights, and it is
really cold in there, and thenyou can then it can condense on
there, which is why you wantgood ventilation in the attic

(08:06):
space.

SPEAKER_02 (08:07):
And that's why we've seen icicles in some attics,
right?

SPEAKER_01 (08:10):
Yes, yes.
We have seen frost up in theattic space.
There was one I remember, and itwas all white.
Usually only on really cold daysdo we see that, but when we see
that, it is pretty much alwayscomp you know combined with uh
poor ventilation, or somehowthey have gaps in the ceilings

(08:34):
in the house, and the vapor isgoing up inside the house.

SPEAKER_00 (08:48):
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SPEAKER_02 (09:05):
So look, I'm pulling this is Oh, I remember doing
those psychometric channels.

SPEAKER_01 (09:11):
A psychometric chart, yeah.
And we're not and I'm not gonnatry to explain this to you if
you're gonna be able to do it.

SPEAKER_02 (09:18):
I'm tired, I'm not even s no, no, I can't even
remember what I did this for.

SPEAKER_01 (09:24):
What did you do a white a wet bulb and a dry bulb?

SPEAKER_02 (09:27):
No.

SPEAKER_01 (09:29):
So, wet bulb and dry bulb, you're got a thermometer,
you get the bulb of it, the partthat holds all the liquid, get
it wet, spin it around, okay,and as the water evaporates, it
it cools off and lowers thetemperature.
You can use that temperaturedifference between that and the

(09:51):
dry bulb that had noevaporation, and that gives you
a clue to the humidity.
And there's a very confusingpsychrometric chart that shows
you all this stuff.
It really is.
There's so many different waysand things they can they can add
to it.
But we are not gonna get intothat.

(10:12):
No, please, we don't need toknow that.

SPEAKER_02 (10:14):
I think we should be done.

SPEAKER_01 (10:16):
That's this is all we need from this one.

SPEAKER_02 (10:17):
Yeah, but what don't don't have competing devices,
use one or the other.

SPEAKER_01 (10:23):
Yeah, and I can understand having one and then
the say the basement one, like,hey, we we do not want to get
too moist down here, so alwayskeep that running.
I get that.
But kind of you, you know, thinkabout it.
Think about what's going on,make sure it's done
intelligently.
And that's about it.

(10:44):
And then we do I mean, duringhome inspection, we do think
about dew point and when that'sgonna cause issues on things.
It's not it's really nothing weput into the report too much,
but we do see uh indications ofcondensation formed up in the
attic space.

SPEAKER_02 (11:00):
We'll talk to that.

SPEAKER_01 (11:01):
That's because the moisture is hitting dew point up
in that attic because that roofsheathing gets cold enough to
make the water vapor, even witha very not much, if it gets if
it's cold enough, it willcondense and turn frosty up
there.
So that's it.
Thank you, everybody.
Bye bye.