What is Dew Point?

by Erik North on December 16, 2011

What is Dew Point?

 

I’ve mentioned dew point in passing many, many times. You can tell from the name it has something to do with moisture. And moisture is a problem flash point in homes so it might be worth knowing a bit more about dew point. So what is dew point? How is it a driver of most moisture related damage in the home? And how can the least sexy phrase and concept possible be so important for home health?

What is Dew Point?

Dew point is the temperature at which the water vapor in the air will condense into liquid. Ok, who cares? It matters because dew point determines when (and where) air borne moisture condenses. And if moisture condenses where there are plant fibers (OSB, wood, paper, carpet, etc etc) odds are you’re heading toward adventures in mold.

Dew point determines the temperature at which moisture condenses. A real life example is a glass of ice water. The ice water is at 32 degrees F and the air borne water vapor condenses on the glass.

A similar mechanic happens throughout the house. Anywhere the temperature drops below the dew point, moisture will start condensing. The classic energy retrofit example is adding insulation without addressing air leakage.

What is Dew Point – In Your House

Suppose an older home has an uninsulated attic and moderate humidity (50%). The dew point at 50% humidity is 50 F (assuming sea level air pressure). With no insulation in the attic, the attic space is kept relatively warm. The warm moist air rising by stack effect doesn’t condense as the attic space is over the dew point.


 
The homeowners, tired of sky high heating bills, throw two layers of fiberglass batts without air sealing (check this article for why it matters). The warm moist air continues drifting into the attic but the new insulation cuts off most of the attic heat.

The roof deck is now much cooler (and below the dew point) and the air borne humidity condenses on the roof deck. Cue ice, mold growth and in extreme cases rotting wood. Celebration.

What is Dew Point – HVAC Systems

The other classic example is that of cold water pipes or air conditioning duct work. Air handling units in the South are often installed in the attic space. They are usually insulated but any flaw in the insulation means high humidity air contacting cool duct metal.

The same thing happens with cold water pipes. If the pipe temps are below the dew point, condensation will occur on the pipes. Imagine the mess if you’ve stored your old school texts in a box below a drip…

What is Dew Point – Comfort

It also matters for personal comfort. Another way of phrasing dew point is the temperature at which current air borne water vapor would be 100% relative humidity. The closer dew point temperature is to current air temp, the closer the air is to being saturated with moisture.


 
The human body uses sweat to cool down. You sweat, shedding heat and it evaporates into the air. The rate at which sweat evaporates (and how comfortable you feel) depends on how close the air is to being saturated. So … the closer dew point is to the air temperature, the more air will feel ‘muggy’. When the dew point is especially low, skin will dry out, lips will crack. Just super fun but illustrates the need for a healthy range of humidity.

So what is dew point? The technical definition is important but for homeowners the practical implications are more so. Dew point determines when moisture condenses out of the air. This directly impacts your home’s health and your personal comfort.

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