Humidity, Dew Point, And Your Home

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.

Chapters

• 0:00 Humidity Basics And Temperature
• 2:20 Dew Point Explained With Real Examples
• 3:28 Home Humidity: Tools And Targets
• 4:13 The Humidifier vs Dehumidifier Problem
• 5:23 Basements, Crawl Spaces, And Settings
• 6:16 Winter Condensation And Dew Point Risks
• 7:22 Attic Moisture, Frost, And Ventilation
• 9:08 Psychrometrics Without The Math
• 10:22 Practical Takeaways And Inspector Notes
• 11:18 Closing And Resources

Transcript

SPEAKER_01: 0:00

Hello, hello, hello. This is Jim, and of course Laura is here with me.

SPEAKER_02: 0:04

Hello, everyone.

SPEAKER_01: 0:06

Alright, so Laura.

SPEAKER_02: 0:07

So Jim.

SPEAKER_01: 0:09

Dew point. Yes. And relative humidity. Let's let's let's go through this a little bit.

SPEAKER_02: 0:16

Yes, we we have a reason for all of this.

SPEAKER_01: 0:20

So rele humidity is relative humidity is a measurement of how much water vapor the air can hold relative to relative to the temperature that it is. So let's say you got some air temperature is fifty degrees. Fifty degrees can hold half as much water as 80 degree temperature.

SPEAKER_02: 0:56

Why not a hundred?

SPEAKER_01: 1:01

No, seriously, why why is it half if it's because the more all right, so temperature is a measurement of energy.

SPEAKER_02: 1:10

Okay.

SPEAKER_01: 1:12

Alright, so who has all right you're sleepy right now.

SPEAKER_02: 1:17

So yes, I am.

SPEAKER_01: 1:18

So who has so energy is a measurement of of of energy.

SPEAKER_02: 1:24

So so it's not like potential or something like that. No, no, no. Or is that work? That was work.

SPEAKER_01: 1:30

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 into that right now. Geek. So because the heat is a measurement of how much energy you have. So look at and visualize that as and as the measurement of how much you can lift. So the temperature is a measure of energy to lift things. Use that to remind you that the more heat something has, the more energy it has to lift and support things.

SPEAKER_02: 2:06

So it's supporting the water that's in the air.

SPEAKER_01: 2:10

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're saying this because we're coming to go into a thing called dew point. And dew point is just another term for the the temperature in which the air temperature air temperature can no longer support or suspend the water that's in in it. So when you go outside and you have a nice cold can of soda outside and it's humid outside, the humid air says 90 degrees out, it can hold quite a decent amount of water vapor. But when that w that air vapor, that air and the which has the water in it gets close to the can, it cools off and it no longer has the energy to support the water. So the water condenses. That is dew point. Dew point depends upon the temperature and then the humidity level as well. Okay. Alright, so so we got those done. We got that. So here's the thing in your house, some people will have humidifiers. Usually it's on the furnace, and if it's building that's on the furnace on the return air, and what it does is it puts it humidity into the air.

SPEAKER_02: 3:50

So for like wintertime when it's really dry and it gets down to like the teens or the low 20s, you want some moisture in the air.

SPEAKER_01: 3:57

Well, yeah. Well, you'll look at your humidistact humidity gauge in your house and go, dude, it's 20% humidity in there. It is way too dry. Yes. So you have a humidifier to add humidity. You also have your dehumidifier to take the moisture out of the air. Right. Now, we had an inspection the other day. They had all right for us, we recommend you really never want your humidity to hit 60% or higher because supposedly that's the rate in which humidity, which some mold start growing. And it starts deteriorating 40 to 50. That's a nice range. That's a nice decent range. It gives you some cushion above and low to not cause any issues. Well, we had a house the other day, they had the humidifier which set at 55. And the dehumidifier was set at and they also had a dehumidifier that was set at 45. Which means they have these two systems running simultaneously, battling each other as of where they want the humidity level to be. Now, if the basement is at a different temperature than the rest of the house, maybe that would be okay. Because the basements, if it's colder, can't hold as much moisture as the warmer upstairs.

SPEAKER_02: 5:35

Right.

SPEAKER_01: 5:35

So they kind of needn't need to look at that. But but still, you shouldn't have two systems battling each other because it's just not energy efficient for for it's gonna it's gonna rack up your your heating bills. Well, unless you're gonna be able to get it. Well your electricity works.

SPEAKER_02: 5:50

It's also gonna wear them out faster.

SPEAKER_01: 5:52

It could it can, yes. So we recommend every basement in Ohio have a dehumidifier. If you if you have crawl space, I'd put a dehumidifier there also, especially if it's sealed up to the exterior. And they have it set at a certain humidity level, like 45, 50. So as soon as the humidity gets to that level, it turns on, takes humidity out of there. That's it. I'm not a real big fan of humidifiers because if they're leaking, you may not notice it for quite a while. Because if it leaks, it's gonna be leaking inside the furnace ductwork, and you you won't see it. By the time you see it, it may have caused some rusting and may have been gone on for a long way, you know, a long time already. So, dew point is the temperature in which the moisture can no longer be suspended by the air and it condenses, which is why during the wintertime, if you have a damp basement, you are likely to, and you don't have any insulation on your walls or up in the seal plates, like on the joist on top of the foundation and outside, the that's gonna get cold. That's a good potential spot for that moisture to condense on that wood, and now you have potential for more growth and some deterioration. So that's where dew point comes in to uh a house. It also takes place also in the attic space if you don't have good ventilation because you always 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 in the light, fixtures, ductwork, and makes its way up inside the attic space while having enough humidity, it can get up there and start condensing up inside the attic space during the wintertime because that roof is really cold, especially on really cold nights, and it is really cold in there, and then you can then it can condense on there, which is why you want good ventilation in the attic space.

SPEAKER_02: 8:07

And that's why we've seen icicles in some attics, right?

SPEAKER_01: 8:10

Yes, yes. We have seen frost up in the attic space. There was one I remember, and it was all white. Usually only on really cold days do we see that, but when we see that, it is pretty much always comp you know combined with uh poor ventilation, or somehow they have gaps in the ceilings in the house, and the vapor is going up inside the house.

SPEAKER_00: 8:48

Trusted licensed home inspectors for your name. For a great home inspection, you really can't come. Visit home inspections in oh.com.

SPEAKER_02: 9:05

So look, I'm pulling this is Oh, I remember doing those psychometric channels.

SPEAKER_01: 9:11

A psychometric chart, yeah. And we're not and I'm not gonna try to explain this to you if you're gonna be able to do it.

SPEAKER_02: 9:18

I'm tired, I'm not even s no, no, I can't even remember what I did this for.

SPEAKER_01: 9:24

What did you do a white a wet bulb and a dry bulb?

SPEAKER_02: 9:27

No.

SPEAKER_01: 9:29

So, wet bulb and dry bulb, you're got a thermometer, you get the bulb of it, the part that holds all the liquid, get it wet, spin it around, okay, and as the water evaporates, it it cools off and lowers the temperature. You can use that temperature difference between that and the dry bulb that had no evaporation, and that gives you a clue to the humidity. And there's a very confusing psychrometric chart that shows you all this stuff. It really is. There's so many different ways and things they can they can add to it. But we are not gonna get into that. No, please, we don't need to know 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 always keep that running. I get that. But kind of you, you know, think about it. Think about what's going on, make sure it's done intelligently. And that's about it. And then we do I mean, during home inspection, we do think about dew point and when that's gonna cause issues on things. It's not it's really nothing we put into the report too much, but we do see uh indications of condensation 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 roof sheathing gets cold enough to make the water vapor, even with a very not much, if it gets if it's cold enough, it will condense and turn frosty up there. So that's it. Thank you, everybody. Bye bye.