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Why Is My Second Floor More Humid than the First?

Why Is My Second Floor More Humid than the First?

Wondering why it’s more humid upstairs than downstairs?

Well, it’s actually normal—to a certain degree. Indoor air that's warm and moist (humid) is less dense than dry air, which makes it rise.

So here’s a good rule of thumb: If your second floor’s relative humidity* is 1–5% greater than your bottom floor, that’s normal.

However, if the relative humidity is greater than 5%, then you may have problems you should address like...

  1. Poor insulation
  2. Leaky ductwork
  3. An oversized AC

We’ll go into more detail about each of these problems and how to solve them.

*Note: Not sure of your home’s humidity levels? You can measure relative humidity using a Hygrometer. Learn how to use a Hygrometer in this article.

Problem #1: Poor insulation

Is it summer? Then poor insulation may be what’s causing your upstairs to feel more humid.

Here’s why: During the humid summer season, outside air enters through leaks in the top of the house (attic) and pushes cooler, air conditioned air out through the cracks in the basement or bottom story of your home. This process is called the stack effect.

But if your attic and bottom story are both well-insulated and sealed, then your home won’t bring in as much outdoor air. And if you live in a humid place like Minneapolis-St. Paul, the last thing you want is to bring that sticky, outdoor air inside.

Solution:

Have an insulation contractor inspect your home to see if your current insulation levels are up to par.

Energy Star recommends different R-Values for specific regions (if you’re in the Twin Cities area, your attic should be insulated with R38 to R49 and your floor-level should be insulated with R25 to R30).

Problem #2: Leaky ductwork

An example of leaky ductwork in the attic

Many 2-story homes have a separate AC unit that delivers cool air only to the 2nd floor. If you have a second AC controlling your second floor, then leaky ductwork could be what’s causing it to feel so humid.

Here’s why: Your AC is designed to remove a certain amount of moisture from your home’s air. However, leaky ductwork pulls in excessively humid, hot air from your attic into your AC system (seen in the picture above).

Over time, the extra moisture from attic air will exceed your AC’s dehumidification abilities (especially if your AC short cycles—see problem #3), leaving your upstairs hot and humid.

Solution:

Contact an air conditioning specialist to inspect your ductwork for leaks.

If they find you have leaks, don’t be surprised: According to Energy Star, the average home loses 20–30% of the air that moves through the duct system to leaks or poorly connect ducts.

Problem #3: An oversized AC

For a quick refresher on air conditioner size, read our article, “What Size Air Conditioner Do I Need for My House?

One of the purposes of your home’s air conditioner is to remove humidity from the air it cools. But if the AC that controls your second floor is too big, it can’t do that properly.

Here’s why: An oversized AC will run for only a short time then shut off. This is called short cycling, and it’s a problem for humidity because it means your AC never runs long enough to properly remove humidity from your home’s air.

Solution:

Have a professional perform a Manual J Heat Load calculation to determine if your AC is too big. If it is, they can help you find one that is the correct size for your home.

Need an AC pro to help get rid of 2nd floor humidity?

Contact MSP to schedule an appointment with one of our trusted technicians. We’ll take a look at your home and AC system to see why your upstairs is humid and provide recommendations on how to fix it.

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