The building envelope is the border between your comfy indoors and the chilly/sweltering outdoors. I say this a lot. The building shell controls the air, heat and for the purposes of this discussion, the vapor controls retard moisture flows in the house. Humidity in the air affects people, paper, the wood, concrete and OSB of the building frame. So what is humidity and how does it relate to your house? How does humidity relate to your health and comfort?
Yeah …. What is Humidity
Humidity is the amount of water in vapor form in the air. There are a whole bunch of different measures for humidity: absolute humidity, relative humidity, specific humidity, vague humidity (Ok, I made that one up). The one we’re most interested in and the one reported by meteoreologists and your humidity gauges is relative humidity.
Warning: ‘What Is Humidity’ Tech Talk!
OK, not super techy. Relative humidity is often spoken as air’s capacity to ‘hold’ water. This is conceptually wrong, as air is a mixture of gases like nitrogen, oxygen and yes, water vapor. When air temperature rises, it doesn’t hold more water but rather a higher percent of the gas mix can be water vapor.
End of Warning: ‘What Is Humidity’ Tech Talk
Relative humidity is how much water vapor is in the air at a given temperature, expressed as a percent of the highest possible water vapor content. This varies with temperature and pressure. At higher temperatures, there can be more moisture in the air (aka higher humidity).
70% relative humidity means that given current temperatures and pressure, the air has 70% of the water vapor content that could be possible.
What is Humidity – Why This Concerns Us
And why do we care? What does moisture have to do with the price of oil? Nothing but everything to do with homeowner comfort, air quality and building durability.
The human body sheds heat by sweating. Have you ever been in a crowded club? It’s hot and humid. The human body is basically a walking humidifier. When the air is drier (a low relative humidity), sweat can evaporate more easily and you stay more comfortable.
If the relative humidity is very high (close to as much water vapor as is possible at that temp), the sweat on your skin won’t evaporate. You’ll feel plenty uncomfortable not being able to shed heat and sweat. Conversely, if the relative humidity is overly low, the sweat and moisture will wick out very quickly. Skin cracks, lips and mouths will dry out … just super not fun. A moderate range of humidity is important for personal comfort.
Air quality and building endurance tie very closely to humidity. Mold needs two things for growth: food and moisture. Food means cellulose and other organic material and most of your house is made of loads of mold food. OSB, plywood, drywall, wallpaper, carpeting all can grow mold. As relative humidity creeps over 50%, the conditions become more conducive to condensation and mold growth.
Here’s a photo from the roof deck from an audit last November…yep, that’s all mold.
The higher the relative humidity, the closer the dew point is to the current air temperature (dew point which I’ll jot up at some point is the temperature at which water vapor condenses into liquid). The higher the relative humidity, the more likely moisture will condense. Yuck. Condensing moisture means mold, rot, mushy wood, poor air quality, cats and dogs living together…mass hysteria.
So what is humidity? It’s ridiculously far more complicated than most folks think, that’s what. Humidity is such a complicated topic that a few hundred words cannot do justice. But understanding a little bit about humidity can help you think more clearly about your house.