Why Your Relative Humidity and Temperature Decisions Matter

What relative humidity is ideal for your home? Sounds simple enough to answer. But can sometimes be tricky to understand.

You are right when you blame moisture for the mold, corrosion, decay, rot, and other moisture-related issues. Microbiologists would agree. But low relative humidity can lead to a whole host of problems too. Discomfort, shrinkage of wood products, paint cracking, electrostatic discharges, and etc.

The key is not to be extreme on both ends. High enough to provide comfort but low enough to avoid moisture problems. Unfortunately, finding out the correct range is not straightforward. It depends on climates, seasons, thermal properties of building surfaces, exposure time and personal sensitivity.

So how high and how low you can go?


What is Relative Humidity?

Okay. Which smartass came up with this? Why not just dropped the word “relative”? And humidity can simply mean humidity…

It turns out to be an important factor that mother nature depends on to stay intact. Relative humidity tells us two things:

  1. The water-holding power of the air, in relation to temperature
  2. The water content in the air, though it will not change, in relation to temperature

Relative humidity is expressed in RH%. 0% means the air is completely dry. 100% means the air is completely saturated. The percentage changes as the temperature go up or down.

There are two RHs to look into when studying mold growth. First is the room air RH. Second is the RH at the building surfaces (boundary layer) such as wall and ceiling.

This makes sense. Temperature plays a role in changing a medium’s RH. But why only care about interior RH and not building surfaces RH?

How Does Relative Humidity Change with Temperature?

Imagine the air is a sponge that gets bigger when warm and smaller when cold. A big sponge sucks more than a small sponge. Then if you pour a cup of coffee over it, it disappears. The sponge is heavier right now.

But how do you get the water out without squeezing it?

The only way is to make the sponge smaller than what it can suck. Suddenly, it spills out the liquid without warning. So just by changing the size, the same amount of coffee can no longer hold in the small sponge.

In reality that’s how nature works. Temperature changes the ability of air to hold water. As the temperature goes up the sponge size becomes bigger. Thus the power to suck more water and vice versa.

If spilling means anything it’s condensation.

What is Dew Point?

A dew point tells at what temperature moist air starts to condense.

As the temperature drops, the sponge that holds the water becomes smaller until a point where it can no longer contain the water. It must spill.

Dew point is never fixed as it continuously changes according to climate and seasons. A higher dew point would mean there is more moisture in the air.

How to Measure Relative Humidity in Your Home?

The good old days of sling psychrometer has gone. Folks in the industry have found an easier way to measure relative humidity and temperature without doing manual calculations. It’s named multi-function hand-held meter.

It has high accuracy, multi-function (temperature, relative humidity), calculated psychrometric values (dew point and etc.), data capture and logging, thanks to advanced sensors technology and integration.

There’s not really much else to say about this instrument. Just swing the probe in the air and you can instantly get the readings.

But it has a price to pay. If you want cheaper options… read on…

digital hygrometer thermometer

The most common one is the digital indoor thermometer and hygrometer. It has an LCD that shows current temperature and RH. Accuracy is okay.

Or if you want a totally FREE option, no worries…

Just Follow Your Senses

You have 5 super sensors to perceive the world. Forget about the rest.

Can you touch relative humidity?

Our skin uses ambient air to remove sweat. When RH is high, the air is saturated with moist and our sweat won’t go away. Thus making us feel muggy and hotter than the actual temperature.

When RH is low, our sweat goes away quickly and making us feel cooler than usual.

Can you see relative humidity?

person in black hoodie riding swing while raining

Yes, You can see condensation, wet, mold, electrostatic sparks, sticky powder, slow drying laundry, and etc. It tells lots about the current air RH.

Can you smell relative humidity?

Noses inhale air so the lungs can exchange oxygen with carbon dioxide. It functions countless times everyday conditioning the air before it even reaches the throats.

When the air is cold or dry, the nasal passage shrank to allow adequate contact for heat and moisture transfer. And widen and broaden when the air is warm or humid. This dynamic gives an indication of the current RH level and temperature.

Signs of high RH: musty, odor, stuffy, off-gassing smell, VOCs, ozone scent before it rains, and etc.

Can you hear relative humidity?

Yes. When you hear the sound of the falling rain two things happen. Air temperature drops as rainwater evaporates especially in summer. We call it evaporative cooling. It can potentially raise outdoor air RH up to 95%

Can you taste relative humidity?

Maybe not. Because moist air doesn’t taste like Buffalo chicken?

What are the Effects of Relative Humidity On The Human Body?

man wiping nose

Many people believe the best relative humidity for health is 40% to 60% for reasons explained below. Under- or over RH is not okay. But comfort is different than health. ASHRAE recommends 25% to 60% with two concerns in mind:

  • homeowners in very cold climates tend to over humidify (thus wasting energy). There is a big difference between 25% to 40%
  • most people can’t sense relative humidity swing within 25% to 60%

There is no real consensus on the lower limit yet. However, the interior RH as agreed should never rise above 60% to avoid building surfaces reaching 70%. Experts said 70% is where the mold story starts.

Studies have shown bacteria and viruses are not happy to exist in that range. And it links respiratory infections or absence from work and school to RH. Very low or very high RH is bad. Mid-range RH is perfect.

They found the size of allergenic mite and fungal populations is directly linked to RH. But died away when RH is below 50% and maxed again at 80%. Most species of fungi would not grow unless the RH is more than 60%

RH also affects how fast formaldehyde is off-gassing, how fast acids and salts are forming, and how fast ozone is forming, from indoor building materials.

To a normal human body, the health effects caused by these microbial and chemicals are serious. Will you hold on if you have allergies and asthma?

How About When Air is Too Dry?

We heard lots of bad stuff about mold fungi in wet conditions. But how about when air is too dry?

dry climate

The human body is about 65% water. It has secrets to help body fluid balance.

Below 30%. The skin gets dry and starts to itch, crack, and chap. Ouch… The eyes irritate. Throat inflames. Nasal dried up and dirty air gets into the lungs…

Low RH also creates smaller particles that can get into the lungs deeply. It tells why influenza is more common in fall and winter.

We’ve not even talked about electrostatic discharge resulted from low RH below 20%. And how dangerous it can be to you and your small kids.

We can’t drift too far away from these truths. Because the real hard facts are there. The mid-range relative humidity is where the bull-eye is.

What lessons can our senses teach us about our home indoor air quality and our wellness?

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