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Question DetailsAsked on 7/28/2014

Air duct sweating in between floors.

I have mold in the celling and found that the A/C ducts are sweating (I took part of the ceiling). What are my best options?

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6 Answers


If the floor above is a conditioned space,not an attic, warm moist air must be getting between the ceiling and the floor above to allow the ducts to sweat. Assuming there is insulation on the ducts.

I'd get a Pro, in heating and cooling to take a look at this.


Answered 5 years ago by BayAreaAC


I am going to take the opposite tack from BayAreaAC - you are always going to have room-temp and humidity air in the floor spaces except in crawl spaces - so I am going to assume your ducts are NOT insulated, which they should be in that environment if carrying cooled air. You have two "easy" ways to attack this - the difficult way being replace the ducts with insulated ducts, or to replace the one duct with two parallel smaller insulated ones if space is too tight.

The "easy ways" best is to insulate the outside, commonly with about 3 inches of foamed-in-place closed cell foam. Easier in some cases but restricts airflow, is a thinner (hence less effective) sprayed in place foam on the inside (assuming these are metal box ducts rather than the smaller corrugated plastic ducting), which also reduces airflow and can alter the performance of your HVAC system in both heating and cooling mode.

Note that the sweating is for the same reason as a cold window condenses moisture in a house in the winter - warm moist air contacting a cold surface (the duct), then dripping off it - so using fiberglass insulation is not as good a way to insulate it, because with normall workmanship there are lots of air gaps where the air can get to the ducting and condense, creating mildewy fiberglass - plus the moisture in it then almost totally eliminates the insulation value of the fiberglass.

As BayAreaAC says - you need an HVAC contractor to assess the situation - the Search the List category for local contractor reviews is Heating and A/C.

Answered 5 years ago by LCD


Thank you for the response. I have checked the situation with HVAC people and got better understanding of the problem now. My airducts connects from HVAC in the ceiling of my powder room (in inter floor space, I cut the ceilling to see it). Here all the ducts are not insulated but the ducts that goes to diffrent room through attic (I have one over the living room) are insulated ducts. This attic over living room is connected with the interfloor space where the ducts are and this are is not insulated (so the attic air can go there). I changed the suytem 3 years back (my house was buikd in 1992). With the new system, more cold air going to those ducts and the attic air is getting into the ineter floor space where the ducts are. My take is totally insulate the area connecting the attic with interfloor space so no hot air from attic can go. This might solve the problem or I need to change some ducts (to insulated one's) which will be difficult and expensive too. What you experts think? I appreciate all your suggestions.

Answered 5 years ago by PPC


I don't think insulating above the interfloor airspace will help - the interfloor airspace would just be colder than it is now due to the cold ducts and less heat from above to warm the area and that area will still have the same amount of moisture in it probably, so the humidity will increase and you will have MORE condensation. Short of putting ventilation with low humidity air into that airspace to remove the humidity, I think your only solution is going to be to insulate the ducts so they cannot cool that airspace - either put retrofit insulation on them, or swap out for insulated ducts depending on accessability to put insulation on.

Another thing the HVAC contractor might have check but might not have - what the air temperature through the ducts is - if they are too cold, then they will sweat more readily. Generally, a well-functioning A/C unit should drop the air temperature about 15 degrees in high humidity conditions or early in the cycle to about 20 degrees in lower humidity and once the house is basically at the desired temperature. However, your air in the ducts should be no colder than about 55-60 degrees or it will promote condensation both inside and outside the ducts. I have seen units that, once the house cools down to desired temperature or at night as outside air cools, will actually frost up the ducts right behind the evaporator - DEFINITELY excess cooling there, but it can happen when A/C is run at low ambient temperatures or if your system is significantly oversized for the house.

Answered 5 years ago by LCD


Thank you LCD. My house id 1950 sqft and my A/C is 2.5 ton (one unit). The duct temperature is 55 degree. The space between the flloors are tight, so I might be able to install one or two insulated ducts. I cut the ceilling left it open. I saw that there is now sweating during night time (i keep my termostate at 74 degree). I amthinking making a vent in ceilling so that the cold air can get in to the space, insulating one or two ducts and insulating the interfloor space so that no air can get from attic might solve the proble. Please feel free to put your thoughts. Thanks.

Answered 5 years ago by PPC


There are three reasons why it would be sweating (more) at night:

1) the incoming outside blending air is a bit cooler because the outside air is cooler, so it is able to cool the return / blended air down a bit more than during the hot daytime, resulting in cooler ducting.

2) the outside air humidity is higher because the outside temperature is lower, so the air coming into the house is higher humidity (same water load but higher humidity as air cools) so it takes less of a temperature drop to reach the saturation point, which is what causes condensation and fog, which is basically what you are getting at the duct surface

3) people are in house at night so there is more moisture from body evaporation, cooking, showers/baths, cooking, etc

I think you need a good HVAC contractor to look at this - or even an HVAC designer for an architecture or engineering firm, because I am afraid what you are talking about doing may not work - it requires balancing humidity and airflow in an area that normally has very little airflow. A part of your problem may be leakage of cold air conditioned air into that area too - you have not said if the interfloor area is warmer or cooler than the attic or under-ceiling area, or what the relative humidities are.

if the humidity is getting in from the attic then vapor barrier at the attic level could solve the problem - but generally at the attic floor level the airflow is up through the ceiling, so it is just as likely or even more so that the problem is the cool, humid indoor air getting in that space and condensing - in which case vapor barrier/insulation above it would reduce airflow and temperature in the flooring area, and INCREASE condensation.

Insulating the interfloor area is likely to aggravate the problem because it will reduce airflow, and drop the overall interfloor temperature because the duct will have less surrounding air to cool, so it will cool it more effectively.

Letting more conditioned air into the air through a ceiling vent could aggravate the problem - warmer, less humid air is what is needed. Also,this would be putting household moisture into the interfloor area which could lead to mold / mildew not only onthe duct, but also at outside wall surfaces in the winter if it gets cold in your area. It is possible, depending on your situation, that a very small interflow vent into a RETURN duct in that area would promote airflow enough to move enough air to solve the problem, even though that would be an inefficient use of the airflow from an energy conservation standpoint. This is common when this problem occurs in suspended ceiling areas in large buildings where concentrations of A/C ducts start forming condensation - to promote circulation in the area. Use of scavenger fans like "Robin Hood" fans also is a solution in situations like this, running on a humidity sensing switch - pulling a low airflow from the interfloor area and venting it back into the room below.

I'm sorry I can't be more help and that is why I am suggesting getting a local pro, but I misplaced my 1000 mile glasses (I suspect the dog borrowed them for squirrel chasing purposes) so I just cannot give any more detailed response without seeing the situation, and I am not at all confident photos would help - though you can try if you want, using the add left-most photo yellow icon above the Answer box in Answer This Question right below your question. If you do photos, interfloor and attic "floor" photos would be the most useful.

Answered 5 years ago by LCD

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