Duct and Grille Sweating
© Kevin O'Neill
February 6, 2006
1
With the ever increasing popularity of putting the air conditioning ducts outside of the
conditioned space; many contractors and homeowners in humid climates are now experiencing
sweating ducts. Most of these sweating ducts are carrying cool supply air, although sometimes
return air ducts can sweat, especially if the house is kept very cold and the return ducts run
through cool vented crawlspaces.
The reason that ducts sweat is the outside surfaces of the ducts are below the dew point of the
air in their immediate surrounding. The primary ways to stop ducts from sweating is to raise
the surface temperature of the ducts or to lower the dew point of the air surrounding the duct by
some form of dehumidification. The dew point of air is the temperature at which the amount of
water vapor in the air is the maximum amount the air can hold. The dew point is the same as
air at 100% Relative Humidity. For a brief comparison of air at different temperatures and the
same dew point, see Table 1.
Make ducts warmer:
There are several ways to raise the outside surface temperature of the ducts. The first is to raise
the temperature of the air inside the ducts. One of the easiest ways to do this is to raise the
room temperature or the return air temperature. Homes with low thermostat settings are more
prone to having ducts sweat. If the room temperature is 70 degrees, and the thermostat is
turned up to keep the room at 75 degrees, the chance of the ducts sweating is reduced. If the
return air temperature is raised 5 degrees, and the temperature drop across the indoor coil is
kept at twenty degrees, the supply air temperature will go up five degrees.
For supply ducts, another way to raise the outside surface temperature is to increase the air flow
in the system. I have been measuring air flow in air condition systems with a flow hood since
1988. Half of the systems that I measure the air flow of only have 200 CFM per ton or LESS.
When a system is operated with low air flow, the cooling coil gets colder; hence the supply air is
colder than normal. This makes the outside surface of the duct colder also. So if the supply
ducts are sweating, measure the system air flow to be sure it is adequate for the system.
Make the environment warmer:
Another consideration is the temperature of the air surrounding the ducts. Raising the
temperature of the air surrounding the ducts will raise the outside surface temperature of the
ducts. If the ducts are installed in a cool crawlspace, they are more likely to sweat than ducts
installed in a hot attic. Many times consumers are advised to add exhaust fans to their attics.
By running these fans excessively, bringing in cooler outside air, this can cool the air
temperature in the attic. The temperature at the duct surface can be cooled to below the dew
point and start forming condensate on the duct surfaces. Adding exhaust fans to the crawlspace
can also aggravate the problem, if the ducts are in the crawlspace. Because crawlspaces are
typically cool, if the air outside has a dew point above the surface temperature of the ducts, more
moisture is added to the crawlspace in the form of humidity. The duct condensation will become
more rapid, increasing the amount of liquid water present in the crawlspace.
Duct and Grille Sweating
© Kevin O'Neill
February 6, 2006
2
Dehumidify the environment:
Another way to decrease the amount of duct condensation is to dehumidify the space around the
duct. This would normally be done by installing a dehumidifier, such as when the ducts are in a
crawlspace. By closing the crawlspace and installing a dehumidifier, the air in the crawlspace is
dried out, lowering the dew point temperature of the air. With a lower dew point of the air
surrounding the duct, it is less likely for the ducts to sweat.
Calculations:
The way to determine the outside surface temperature of the duct is with the following formula:
Tsurface= Tout-(Tout-Tin)*Rout/(Rin+Rduct+Rout)
Tsurface = The outside surface temperature of the duct
Tout = The air temperature around the outside of the duct
Tin = The air temperature inside of the duct
Rout = The R factor of the outside surface film of the duct
Rin = The R factor of the inside surface film of the duct
Rduct = The R factor of the duct insulation
Values used for the above equation:
(Source: ASHRAE Fundamentals, Chapter 25)
Inside Surface Film
15 MPH wind
7 MPH wind
Rated R-Value
R=0.17
R=0.25
Duct Insulation
Outside Surfaces
Still air, horizontal surface, heat flow down, shiny surface
Still air, horizontal surface, heat flow down, dirty surface
Still air, horizontal surface, heat flow down, some condensation
Still air, sloping surface, heat flow down, shiny surface
Still air, sloping surface, heat flow down, dirty surface
Still air, sloping surface, heat flow down, some condensation
Air spaces by ducts
0.5 inch space to building component
1.5 inch space to building component
3.5 inch space to building component
0.5 inch space between shiny ducts
1.5 inch space between shiny ducts
R=0.92
R=1.09
R=1.08
R=2.25
R=3.50
R=2.7
R=0.92
R=1.50
R=1.67
R=0.76
R=1.0
Duct and Grille Sweating
© Kevin O'Neill
February 6, 2006
3
3.5 inch space between shiny ducts
R=3.25
By using this formula, you will find that you can increase the outside surface temperature of the
duct by raising the air temperature inside of the duct, by raising the temperature of the air
surrounding the duct or by adding insulation to the duct. This can be as simple as using proper
installation techniques when installing the duct wrap on sheet metal ducts. Often when
installers wrap the duct, they compress the insulation to “make it look nice.” By pulling the duct
wrap tight during installation, duct wrap with an R-6 rating can be effectively reduced to R-3 or
less.
Be careful when adding insulation to an already insulated duct. If the vapor barrier on the
outside of the new insulation is not well sealed, moist air can get between the layers of
insulation. Then condensation can occur inside the insulation, destroying the insulation R-value
and putting you back where you started.
Room Humidity:
Be careful when increasing the air flow in an air conditioning system to increase supply air
temperature. Doing so may reduce duct sweating, but will also reduce the dehumidification
done by your air conditioner or heat pump.
Reduce Duct Leakage:
Another Problem is caused by poorly sealed sheet metal ducts with duct wrap over them. When
air leaks out of the duct, it is trapped by the duct wrap vapor barrier. The cold supply air flows
through the insulation on its way to the nearest tear or opening in the duct wrap vapor barrier.
In the process, the surface of the vapor barrier assumes the supply air temperature. Therefore
the duct sweats over the path from the duct leak to the duct wrap leak. If the metal ducts are
not sealed at all, the entire surface on the duct wrap vapor will sweat. For this reason I
recommend ductboard and flex over wrapped sheet metal ducts in high humidity areas, as they
are less likely to sweat. Properly fabricated ductboard has tight corners, keeping the supply air
away from the outer surface of the duct. Therefore the full rated R-value of the duct is achieved.
The same with flex ducts. When installing flex ducts, you only need to seal it at the ends, where
it connects to the starting collar and the boot. You don’t need to seal three to five elbow gores at
every bend and the pipe every five feet. As long as you don’t crimp the flex, or use very narrow
straps to support it, the full rated R-value of the flex is achieved. It is also possible for ducts to
sweat at the exit point of the air leaking from the duct. This is common at poorly fitting or
poorly sealed flex duct starting collars. This is especially true in cool crawlspaces. The trunk
duct surface around the leaking starting collar will be chilled and will sweat.
Duct and Grille Sweating
© Kevin O'Neill
February 6, 2006
4
Another thing that causes ducts to sweat is when they are run through trusses or supported by
the building structure in a crawlspace. It the duct is supported by the web of a truss, air is
trapped between the duct and the truss. Because no new heat is added to this air, the cool
supply air in the duct gradually cools the pockets of air outside the ducts. Running ducts next to
insulation or other ducts can have the same effect. These ducts will then sweat where they touch
or come close to other supply ducts or building insulation.
Registers Sweating:
Here in Myrtle Beach SC we have some very bad problems with humidity. One way this
manifests itself is in sweating supply grilles and registers during the summer. I have had this
problem a number of times and I have found several solutions for it.
The first thing that I found to cure it is to increase the airflow to the affected register or grille.
When the cold air comes out of the register, it cools the metal surfaces of the register. If the
velocity is too low then moisture from the air in the room can condense on the register. I don’t
know what the magic velocity number is, but opening dampers can sometimes fix the problem.
The second thing I have found is if the register boots are not sealed to the floor, wall or ceiling.
Depending on air velocity, duct pressure and room pressure with respect to the outside, unsealed
boots can cause the registers to sweat, because outside air is drawn inside around the boot. This
condensation can also cause the adjacent surface to get wet. If the register is mounted on a wood
or drywall surface, the surface can be degraded to the point that it must be replaced. This
moisture condensate can also promote mold growth, a real problem these days! Sealing the
register boot to the wood or drywall with silicone caulk can eliminate the problem. Just be sure
to leave the register off for a day to allow the silicone to cure, or the register can become glued in
place, creating a service headache. If the register is in the floor, moisture can drip down into the
duct. I have seen flex ducts fill enough that no air will flow through them.
One last thing that causes registers to sweat is a high humidity environment. The thing to do is
to remove the source of humidity. I had one customer who left the French door partly open in
her living room because she liked the “fresh air”! This was while the A/C was running! The
registers in her living room were sweating and she wanted us to “fix it”. It took me a half hour to
convince her to keep the door closed (she didn’t want to pay for the call either!?). Another time a
customer had a register in a bathroom that was sweating. I found that the bathroom exhaust
fan was poorly installed and had a large gap around it in the drywall ceiling. I bridged the gap
with sheet metal in the attic. Then I sealed it with silicone from below and urethane foam from
above. The register stopped sweating. A poorly fitting damper in a bath exhaust fan can cause
the same problem.
Duct and Grille Sweating
© Kevin O'Neill
5
February 6, 2006
Dew Point Comparison (Table 1)
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130 F, 20% RH = 75 DP (Dew Point)
120 F, 28% RH = 75 DP
110 F, 33% RH = 75 DP
100 F, 46% RH = 75 DP
90 F, 63% RH = 75 DP
80 F, 86% RH = 75 DP
o
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75 F, 100% RH = 75 DP
Duct Outside Surface Conditions (Table 2)
Duct Air Temperature =
Surrounding Air Temperature =
Duct Insulation =
Outside Surface R-Factor =
Inside Duct Surface R-Factor =
Duct Outside Surface Temperature =
55
78
2
1.67
0.2
68.1
60
78
2
1.67
0.2
70.2
65
78
2
1.67
0.2
72.4
55
78
6
1.67
0.2
73.1
60
78
6
1.67
0.2
74.2
65
78
6
1.67
0.2
75.2
Duct Air Temperature =
Surrounding Air Temperature =
Duct Insulation =
Outside Surface R-Factor =
Inside Duct Surface R-Factor =
Duct Outside Surface Temperature =
55
78
8
1.67
0.2
74.1
60
78
8
1.67
0.2
75.0
65
78
8
1.67
0.2
75.8
55
95
8
1.67
0.2
88.2
60
95
8
1.67
0.2
89.1
65
95
8
1.67
0.2
89.9