Building Science-Humidity

What is humidity?  According to Wikipedia, humidity is the amount of water vapor in the air.  A nice, simple explanation that all of us can understand.  Now look at the chart below.

Not so simple.   A psychrometric chart indicates when the dew point will be reached given a specific temperature and humidity level at a constant pressure.  We aren’t going to dive deep into the science of wet and dry bulb, temperatures and dew points, but I think it’s important to understand humidity and the effects it can have on a building.

Let’s start with the basics, humidity wants to move from a wet area to dry area, warm air can hold more water vapor than cold air(this moisture can be drawn out of the air at a certain temperature called the dew point), and cold building products(such as plywood or OSB) tend to be wetter when they are cold.  The cold, wet building products become an issue as we increase insulation levels within the home, reducing the available energy for drying.  I will be addressing this issue in a future blog posting.

As I write this post, the temperature outside is 14 degrees, the temperature in the house is 68 with a humidity of 21%.  This little bit of information can tell a lot about a house, in fact, as an energy auditor, the information can be an important clue to how a home is constructed.  The low humidity tells me there may be a high air exchange rate, or the home is leaky.  It might be a good idea to conduct a blower door test.  Low humidity can also be caused by an air exchanger that is not operating correctly, or over ventilating a home.

Most people become uncomfortable when humidity drops to below 30%, but some building material manufacturers like the humidity low.  Low humidity might be good for the house, but people living in low humidity can experience health issues.  Irritated sinuses and throats, itchy eyes, dry skin and rashes all cause discomfort.  I personally experience more headaches during the winter, which I contribute to the humidity level.

So, what about high humidity?  High humidity in a home during the winter, especially in a cold or very cold climate, can cause building issues.  Any building air leaks to the exterior with elevated humidity present can cause the water vapor to “piggy back” and condense on the colder building surfaces.  That surface will be either damp, or if the temperature is below freezing, frost will form.  This phenomenon is the result of moist air reaching a lower temperature where the moisture is condensed out of the air.  The air has reached the dew point temperature.

Controlling humidity within a home is very important.  Humidity levels tend to increase when a home’s tightness is improved, (less natural air exchanges). There are a lot of sources that can lead to this increased humidity level, a good topic for a future blog post.  Minnesota building codes require all new homes to have balanced mechanical ventilation, usually achieved by installing an air exchanger.  The air exchanger is the best way to control humidity levels and the rate of ventilation in the home.

Health issues can also be present during periods of high humidity. Mold and mildew like damp areas, and they can cause breathing issues in sensitive people. Critters like dust mites and some bacteria also like higher humidity, which also leads to breathing complications.

Where should humidity levels be?  As an energy auditor, I tell people as low as you can comfortably stand.  Some people can tolerate levels below 30%, some, closer to 40%, which is the highest humidity level in the winter I recommend. Humidity levels should never exceed 60% during the summer.  70% is where the bad stuff will start to grow.  Buy a gauge, called a hygrometer, it’s the only way to verify the humidity level.

I have a great dew point example.  Several years ago, I was asked to replace four windows on the upper story of a 100-year-old farmhouse.  The space was previously 2 bedrooms.  Now the homeowner only used the space for storage, and partially sealed the area during the winter.  There was no heat source on the upper level, so the temperature probably dropped into the 30’s and 40’s.  The windows that where replaced were old and very leaky.  The air leakage through the windows helped keep the humidity level low during the winter.  The first spring, after the windows were replaced, I was called to the home to investigate water damage in the ceiling.  One look in the attic and I could see where the water was coming from.  The entire roof deck was covered in frost.  There had never been any issue with frost or water before the windows were replaced, so what was the cause.  The window replacement tightened the space just enough to raise the temperature and humidity level.  Science says warm air can hold more moisture.  There were several areas of attic knee wall storage where I could easily reach through the insulation and touch the roof deck.  The slightly warmer, moist air was leaking into the attic because of the “stack effect”, the warmer air holding more moisture was meeting the cold roof deck, achieving the dew point and turning to frost.  All because 4 windows were changed raising the temperature of the space a few degrees, who would of thought?

So, what is the solution?  Either the temperature of the storage space needed to be lowered, the humidity level lowered, create better ventilation in the attic space to clear any moisture before it condensate, or air seal the warm side of the attic.  Trying to control temperature and humidity in a home to avoid a dew point would be is difficult.  Increasing ventilation in the attic would solve the problem of the frost in the attic, but the potential for the frost would still be available in the storage space below.  I always view attic ventilation as the back-up plan, only used if there is  higher than normal humidity in the home or a hole opens in the air or vapor barrier as the home ages or is remodeled.   Air sealing is the best choice.  Stop the air movement into the attic space, you’ll stop the frosting problem.

That’s it for this week.  Want to discuss a topic, let me know in the comments below.


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