Why Dew Point Matters For Your Home

Ever wonder why your windows sweat, your basement smells musty, or your attic grows frost on the coldest nights? We break down the simple physics behind moisture at home—how temperature fuels relative humidity, why dew point decides when water appears on surfaces, and what that means for comfort, energy costs, and long-term durability.

We start by making humidity intuitive: warmer air carries more moisture because it has more energy to “lift” water vapor. From there, we translate dew point into a practical tool you can use to predict where condensation will strike first—on cold cans, uninsulated rim joists, basement walls, and roof sheathing. Along the way, we share the ideal indoor target of 40–50% relative humidity and explain why that sweet spot reduces mold risk without drying out wood or sinuses.

Then we get tactical. If your humidifier is set to 55% and your dehumidifier to 45%, they’re battling and burning cash. We explain how to stop the fight, when to run a dedicated basement dehumidifier, and why cooler spaces need tighter set points. You’ll learn how sealed crawl spaces, air leaks, and cold bridges push surfaces below dew point, creating hidden wet zones. We also cover attic ventilation—balanced intake and exhaust—plus ceiling air sealing to move vapor out and keep roof decks dry on subfreezing nights.

By the end, you’ll know how to read the signs—sweating ducts, damp corners, frosty sheathing—and choose the right fix: adjust RH, insulate, air seal, or ventilate. It’s a straightforward plan to keep mold at bay, protect your structure, and lower energy bills with fewer gadgets running overtime. If this helped demystify dew point and home humidity, follow the show, share it with a friend who battles a musty basement, and leave a quick review so more homeowners can find it.

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Transcript

Humidity vs Temperature Basics

SPEAKER_02 0:00

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

SPEAKER_01 0:04

Hello, everyone.

SPEAKER_02 0:06

Alright, so Laura.

SPEAKER_01 0:07

So Jim.

SPEAKER_02 0:09

Dew point.

SPEAKER_01 0:10

Yes.

SPEAKER_02 0:11

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

SPEAKER_01 0:16

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

SPEAKER_02 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_01 0:56

Why not a hundred?

SPEAKER_02 0:58

This is just No, seriously, why why is it half if it's Because the more Alright, so temperature is a measurement of energy.

SPEAKER_01 1:10

Okay.

SPEAKER_02 1:12

Alright, so who has Alright You're sleepy right now.

SPEAKER_01 1:17

So yes, I am.

SPEAKER_02 1:18

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

SPEAKER_01 1:24

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

SPEAKER_02 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 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_01 2:06

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

Dew Point Explained With Real Examples

Home Humidity Gear And Best Ranges

SPEAKER_02 2:10

Correct. Okay. 80 degrees, if I remember right, can hold about twice as much as 50 degrees temperature air. Okay. So alright, 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 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 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_01 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.

The Humidifier–Dehumidifier Battle

SPEAKER_02 3:57

Well, yeah. Well, you'll look at your humid 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.

SPEAKER_01 4:56

And the dehumidifier was set at.

SPEAKER_02 4:59

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_01 5:35

Right.

SPEAKER_02 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 heating bills. Well, and look at the electricity works.

SPEAKER_01 5:50

It's also gonna wear them out faster.

Basements, Crawl Spaces, And Settings

Condensation Risks In Winter

Attic Ventilation And Frost Problems

SPEAKER_02 5:52

It could it can, yes. So we recommend every basement in Ohio have a dehumidifier. If you have crawl space, I'd put a dehumidifier there also, especially if it's sealed up to the exterior. And 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 mold 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_01 8:07

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

SPEAKER_02 8:10

Yes, yes. We have seen frost up in the attic space.

SPEAKER_01 8:14

There was one I remember, and it was all white.

SPEAKER_02 8:16

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

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Wet Bulb, Dry Bulb, And Charts

SPEAKER_01 9:05

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

SPEAKER_02 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_01 9:18

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

SPEAKER_02 9:24

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

SPEAKER_01 9:27

No.

SPEAKER_02 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. And 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.

SPEAKER_01 10:12

No, please.

Practical Takeaways And Inspector Notes

SPEAKER_02 10:13

We don't need to know that.

SPEAKER_01 10:14

I think we should be done.

SPEAKER_02 10:16

That's this is all we need from this one.

SPEAKER_01 10:17

Yeah, but what don't don't have competing devices, use one or the other.

SPEAKER_02 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_01 11:00

We'll talk to that.

SPEAKER_02 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 all right, that's it. Thank you, everybody. Bye bye.