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Understanding Relative Humidity: It's All Relative

But some buckets are *more* relative
December 14, 2015

But some buckets are more relative

Chris Schumacher of Building Science Labs explains what relative humidity is and why it matters using buckets to paint a picture.

Relative Humidity

relədiv/ (h)yo͞oˈmidədē | noun

"How full is my bucket?"

—Chris Schumacher

The overview:

Air can hold a certain amount of moisture.

If a bucket represents how much water the air can hold, then 100 percent relative humidity means that the bucket is full.

Fifty percent relative humidity means the bucket is half-full (or half-empty, depending on how you look at it).

Where it gets complicated is that the size of the bucket changes depending on the temperature of the air. When you raise the temperature 20 degrees F (~10C), the bucket doubles in size.

Heating the air in your bucket doubles the amount of water that your bucket can hold. So, while the amount of moisture in the air hasn't changed, the proportion of water in the bucket has. The air is now 25 percent RH because the bucket is bigger.

Heat the air another 20F, and double the size of the bucket again, and lower RH to 12.5 percent.

Practical applications for buckets of air

Chris walks us through the real-life hypothetical scenario of ventilation a house in a cold climate. When outside air, which is around freezing (32F, 0C) at 100 percent RH moves inside, it is heated to 68F (20C) the RH in that same air becomes 25 percent.

That is why when you bring outdoor air into the house and heat it up, you get lower relative humidity.

But indoor moisture sources can add to the bucket. Cue the spigot diagram over the bucket. Cooking, bathing, breathing, drying clothes in the basement, all of these things pour water through the spigot and into the bucket. Let's pretend the spigot dumps enough moisture into the bucket to fill it halfway so the indoor air is at 50 percent RH.

This is bordering on becoming a mold problem, so we ventilate the air out, bring new air in, and the RH decreases to a reasonable level.

What happens when indoor buckets go outside?

Shrinkage.

The stud cavity is somewhere between the inside and the outside—not just literally, but also thermally. Let's say it is 50F (10C) in the stud cavity. Sliding that half-full (or empty) bucket through the wall toward the outside means the bucket sees colder air. The bucket shrinks to half its former size because the temperature went down about 20 degrees.

Now, the pessimist's bucket is full. Great news for the pessimist, bad news for the wall cavity. Because as it turns out, the temperature is a gradient in the wall cavity, with the coldest part being the part that touches the outside, or the wall sheathing, which is mighty close to the outside temperature.

Cue the smaller bucket.

When you pour the water from the 50F bucket into the 32F bucket, you get a full bucket and a big puddle.

Puddles in the street = fun to jump in.

Puddles in wall cavities = trouble.