August 29, 2018

High or low humidity levels can make your indoor air feel heavy and sticky in the summer or bone dry in the winter. Did you know that this humidity imbalance usually stems from home performance issues?

This blog post will dig into what could be causing your home's whacky humidity levels, and how you can fix these issues.

What is relative humidity?

Humidity is the amount of water vapor in the air. Relative humidity refers to the amount of water in the air compared to the total amount it could contain at a specific temperature. For example, 100% relative humidity during the summer would feel like the air is made of pea soup. However, 100% relative humidity during winter would feel less uncomfortable. This is because 80°F air can hold more water than, say, 20°F air. See the example and chart later on in the post for more information.

How humid should my home be?

This is the golden question. The "best" humidity level for your home depends on the season, your climate zone, and more. According to the Environmental Protection Agency (EPA), it is best to keep your home below 60% relative humidity, and preferably within the 30-50% range. Your indoor relative humidity levels should fall below the following levels at the corresponding outdoor temperatures:

  • Outdoor temperature over 50°F, indoor humidity shouldn’t exceed 50%
  • Outdoor temperature over 20°F, indoor humidity shouldn’t exceed 40%
  • Outdoor temperature between 10-20°F, indoor humidity shouldn’t exceed 35%
  • Outdoor temperature between 0-10°F, indoor humidity shouldn’t exceed 30%

What causes humidity levels to be out of whack?

Psychrometric chart showing humidity versus temperature

The more outdoor air brought into your home – through infiltration through cracks and gaps around your doors, windows, light fixtures, or through mechanical ventilation – the more your indoor humidity level will resemble the outdoor humidity level.

Using the example offered by Allison Bailes, based on his psychrometer (a tool that measures the relative humidity in the atmosphere through the use of two thermometers) data in the chart to the right, we'll explain how infiltration from cracks and gaps in your home can really affect your indoor humidity level.

For example, it's late winter in Upstate New York. It's a rainy day and the outdoor temperature is staying steady at around 32°F, with a relative humidity level of 100% (Point A). You're keeping your heating system at 70°F, leading to around a 20% indoor relative humidity level (Point B).

This means that, in the process of the air leaking into your home, it has lost about 80% of its relative humidity. Your heating system works overtime to heat this intruding outdoor air up to 70°F and, as a result, this colder, dryer air mixes in with your indoor air, making it dryer inside your home.

How can I fix these issues?

Now, imagine if that cold air didn't leak into your home through cracks and gaps. Your heating system wouldn't have to work as hard and your relative humidity level would stay in a comfortable range. This is how a home performance issue fits into the grand scheme of things. If you find that your home is leaking like a sieve, air sealing could really help with your humidity issues.

If your indoor winter air is still dry after your home is all buttoned up, then you can add moisture to the air with a humidifier or by adding plants to the mix. If it's still too humid inside during the summer months, you might consider using an air conditioner or dehumidifier to lower the humidity level in your home. However, (de)humidifiers, and the like, should be added only after the core home performance issues are fixed to get the most benefit out of them.

A BPI GoldStar Contractor or BPI Certified Professional can perform a home energy audit to help you figure out if your leaky home is contributing to your humidity issues.

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