DeWitt, principal of RLC Engineering, LLC, Clemson, S.C., pointed out that in more humid climates, venting the attic and crawl space could spell disaster. "Code says to vent the heck out of them," De Witt said. "If that doesn't work, vent some more. However, once we get condensation in ductwork, it is self-propagating."
In a paper he authored, DeWitt re-counted, "At an Affordable Comfort meeting that I recently attended, a speaker from Canada said that venting crawl spaces in the southeastern U.S. was lunacy. I have to agree."
His argument is based on psychrometrics.
"Mold is a symptom of a moisture issue," DeWitt explained. "It takes moisture to have fungus." Symptoms of moisture include fungi (mold, decay), condensation, and wood changes.
"Mold is everywhere," he pointed out. "We can't make spaces sterile environments. The spores are already there." By addressing condensation moisture, however, HVAC contractors address "the one [element] we can control." There's no need to become overly concerned, say, if the only evidence of moisture is a little condensation on the windows. "I don't see any reason to call in a Haz Mat team."
In order to understand how ventilation can lead to moisture problems, DeWitt said contractors should understand a few rules:
"Psychrometrics are the thermodynamics of just plain old air," he explained. Relative humidity (rh) equals the amount of water over the capacity of water the air can hold. Mathematically,
rh = amount of H2O/H2O capacity
The dewpoint is the temperature at which the amount of H2O = H2O capacity (capacity of the air to hold water). Below this temperature, condensation forms.
According to DeWitt, "From a psychrometric standpoint, venting a crawl space to remove moisture works when the outside air is drier than the crawl space air. â€˜Drier' does not mean a lower relative humidity, but rather a lower absolute humidity. Relative humidity is a ratio of the amount of moisture in the air relative to the amount the air can hold at that temperature. Absolute humidity is the amount of moisture in an amount of air.
"Air at 85 degrees F and 60 percent rh has the same absolute humidity as air at 70 degrees and 100 percent rh. So venting a 70 degrees/100 percent rh crawl space with 85 degrees/60 percent rh air will not remove moisture."
DeWitt explained that the dewpoint temperature is the temperature at which condensation forms as air is cooled. "At the dewpoint temperature, the air is saturated and any further cooling will result in condensation. In the above example, both the 70 degrees/100 percent rh crawl space air and the 85 degrees/60 percent rh outside air have the same dewpoint temperature: 70 degrees. If we vent a crawl space with air that has a higher dewpoint temperature than the crawl space air, we will actually be adding moisture to the crawl space rather than removing it."
To have condensation, you need moist air and a cold surface. Keep in mind that "moist," "cool," and "cold" are all relative.
So, let's say the supply air is 55 degrees, there is some condensation on the coils, and the ducts are uninsulated. Attic conditions are 80 percent rh, 80 degrees on a sunny day; the dewpoint temperature is 75 degrees. Any bare duct will get condensation on it.
Getting MADIn addition to understanding the fundamentals of psychrometrics, DeWitt pointed out that HVAC contractors need to know something about the moisture and air dynamics (MAD) of wood, especially in crawl spaces and attics. Certain wood conditions can indicate moisture problems, even though moisture might not be present when the contractor is inspecting the system.
Sweating ducts in the crawl space, for example, can create a cupping problem in wood floors directly above. Bowing can occur during the heating season.
Moreover, fungi "like to eat biological things" such as wood, he said. Mold and mildew need 80 percent rh or higher on a surface, where they grow. Decay fungi, erroneously called dry rot, needs liquid water. "There is no such thing as dry rot," De Witt said. "It's the result of a wet decay." By the time the damage is discovered, the fungi have used up whatever moisture there was.
A sweating duct in contact with wood beams and thermal insulation can lead to these problems. The damage can still be visible even though the duct may not be sweating when the contractor sees it.
To get condensation on a surface, DeWitt reiterated, the surface temperature has to be lower than the dewpoint temperature of the air.
Let's say you're in a crawl space with a 78 degrees ambient temperature, 90 percent rh; you need about a 3 degree temperature drop to go to 100 percent rh. "If we start seeing 90 percent rh any place, we are very close to having a disastrous situation," DeWitt said. With insulation, the duct surface temperature is 74.2 degrees (60 degree air inside the duct). The dewpoint temperature is 74.5 degrees; that duct will not sweat.
Now let's say the air being moved is 50 degrees and the duct surface temperature is 74 degrees. That duct will sweat. You might think that higher temperatures in the attic would prevent duct sweating; the ambient summer temperature there is 130 degrees during the day, with a dewpoint of 74 degrees. At first glance it looks pretty good, but "there is a flipside," said DeWitt. "It does get cool at night." Temperatures in the attic can drop to 75 degrees at night, and the surface duct temperature drops to 72 degrees. With the 74 degree dewpoint temperature, the result is condensation.
At 90 percent to 100 percent rh, if the temperature is dropped about 3 degrees, you add 16 grains of moisture per pound of air, he said. Therefore:
"If we have ducts in contact with insulation, we have more of a potential for condensation," he added. "If you've got insulation blown around ducts, you're asking for trouble."
Sources Of MoistureLeaks from air conditioning condensate probably are not the worst moisture problem in a humid-climate home, DeWitt said. However, it is one of the moisture problems that falls within an HVAC contractor's domain. In a 1,500-square-foot house, during one day:
SolutionsDeWitt cited the following "real solutions to condensation on ducts" and other attic and crawl space surfaces:
For A/C condensate control:
You can also dry the air by reducing the ventilation rate, DeWitt said, and by installing mechanical dehumidification.
Cool soil, ducts, and the floor can handle the sensible load, he said. "What handles the latent load? Nothing."
In summary, he asked, "Why in the world would we want to vent an attic or a crawl space?"
To prevent condensation, what's needed is to prevent outside air from contacting inside surfaces. In short, don't let warm, humid air contact a cold surface. "It's not something we as a trade should handle," DeWitt said, "but it affects what we do."
What about adding insulation?
If you decide to warm the surface:
The Wrap-UpIn summing up, DeWitt pointed out the following.
If you vent your attic or crawl space:
If you do not vent your attic or crawl space:
Who's responsible for duct condensation? The HVAC contractor, that's who. "You add water to the space" via A/C condensate problems, and "You create a cold surface" - that is, uninsulated ductwork. If that moisture causes mold spores to breed and multiply, the contractor bears the responsibility.
Sidebar: Ventilation - Facts vs. MythsAccording to Craig DeWitt:
It's a myth - A research basis exists for our current ventilation codes and standards.
It's a fact - Some "current" codes date back to the 1940s, and did not take air conditioning into consideration, said DeWitt.
It's a myth - We build houses the same today as when current crawl space ventilation guidelines were established.
It's a fact - No we don't, said DeWitt. "The most significant change we have made in the last 50 years, in my opinion, is air conditioning. In many parts of the country, we make a standard practice of creating artificially cooler temperatures in our homes. Now we easily create temperatures that are near or even below the dewpoint temperature of the surrounding air."
It's a myth - Venting will reduce crawl space moisture levels.
It's a fact - Not all the time. It can reduce crawl space moisture levels in wintertime, when the outdoor air is drier than the indoor air; "but why would you want to vent in wintertime?"
ASHRAE literature backs up DeWitt's points:
It's a myth - Airtight construction and inadequate ventilation cause problems.
It's a fact - Watertight construction and inadequate ventilation cause problems.
Again, he assumed some typical summer conditions (70 degrees/90 percent rh crawl space, 90 degrees/50 percent rh outside air). The crawl space volume is 4,500 cubic feet. Under these conditions:
- B. Checket-Hanks
Publication date: 12/15/2003