Relative humidity plays a critical role in how we experience temperature, how buildings age, and how
materials respond to the environment. Yet, it’s a concept that many homeowners don’t fully
understand. If you’ve ever noticed foggy windows in winter, static electricity in dry air, or mold growth
in damp areas, you’ve seen relative humidity in action.
What is Relative Humidity?
Relative humidity (RH) is a measure of how much moisture is in the air compared to how much it can
hold at a given temperature. It is expressed as a percentage — the more moisture is in the air, the
higher the percentage.
- RH at 100% means the air is fully saturated and can’t hold any more moisture, leading to dew,
fog, or condensation. The higher the RH is the closer the dewpoint to the ambient air
temperature - RH at 50% means the air contains half of the moisture it is capable of holding at that
temperature. - RH at 0% means the air is completely dry, though this is nearly impossible in natural conditions.
How Temperature Affects Relative Humidity
RH and temperature are directly linked – as temperature increases, air can hold more moisture; as
temperature decreases, air holds less moisture. This is why humidity levels fluctuate with the daily and
seasonal temperatures, affecting both indoor comfort and home maintenance.
Cold Weather = Lower Humidity
Imagine it’s winter in Bellevue and the outdoor temperature is 35°F with 80% RH. When that same air
enters your home and heats up to 70°F, the RH drops significantly because warmer air can hold more
moisture. This creates a drier indoor environment, leading to common winter discomforts, including:
- Dry skin and cracked lips due to low moisture levels
- Increased static electricity caused by dry air
- Condensation on windows as warm indoor air meets cold glass surfaces
This explains why homes often feel drier in the winter, even when it’s damp outside – the air’s actual
moisture content remains low despite the increase in temperature.
Warm Weather = Higher Humidity
In contrast, if it’s a humid summer day in Snohomish with 75°F and 70% RH, the air is already carrying a
lot of moisture. If this air cools suddenly, the RH rises because cooler air can’t hold as much moisture.
This often leads to:
- Foggy windows in the morning
- Sticky, heavy air indoors if ventilation is poor
- Mold growth in damp basements and crawl spaces
This is why proper ventilation and moisture control are critical for homes in the Pacific Northwest,
where damp conditions persist year-round.
The Impact of Relative Humidity on Buildings
Relative humidity doesn’t just affect how we feel – it plays a major role in the health and longevity of
buildings. For example, a vacant home can collapse from humidity. Because a vacant home has no
climate control, it will slowly deteriorate over time due to fluctuating humidity levels. When there is too
much moisture, the wood swells, warps, and rots, and mold takes over. If there is too little moisture, the
wood shrinks, cracks, and becomes brittle, weakening the structure. For this reason, historic buildings
require constant humidity regulation to preserve materials like wood, plaster, and stone.
The Coldest Place on Earth is Also the Driest
You might think of deserts as the driest places on Earth, but Antarctica holds that title. Despite being
covered in ice, Antarctica has an extremely low RH because cold air holds almost no moisture. This
explains why metal rusts slowly in Antarctica but rapidly in humid environments like the Pacific
Northwest.
This concept applies to homes as well. A damp crawl space accelerates wood rot and mold growth, while
a dry home in the winter can cause hardwood floors and furniture to shrink and crack.
Why RH Matters for Homeowners in the Puget Sound Region
The Pacific Northwest, especially cities like Bellevue, Kirkland, Snohomish, and Redmond, experiences
high levels of humidity year-round. Since RH affects indoor air quality, comfort, and building integrity,
maintaining proper humidity levels is essential for preventing moisture-related damage.
At Hausmeister Home Services, we specialize in moisture management, attic and crawl space
ventilation, and whole-home humidity control.
If you’re dealing with mold, condensation, or fluctuating RH levels, we’re here to help.