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airheatman, HVAC Technician
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what can I do against condensation in the ducts in the crawlspace? Then

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what can I do against condensation in the ducts in the crawlspace?
Then insulation of some flex ducts in the crawlspace is soaking wet. Also there are puddles under the connection boxes.

Jerry :

Welcome to Just Answer.

Jerry :

Does your crawlspace have any vapor barrier on the ground?

Jerry :

If it is condensation, then the humidity level in your crawslpace must be extremely high. You will need to address this problem to prevent not only the ductwork from getting wet, but all of the other members under the home.

Jerry :

My first recourse would be to ventilate the crawlspace well, by adding vents if needed, or unblocking the existing vents, then replacing or repairing the ground cover with a good quaility vapor barrier.

Jerry, Service Technician
Category: HVAC
Satisfied Customers: 3361
Experience: HVAC Technician. Hearth Products Specialty
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Customer: replied 5 years ago.

I live in NC, so we have a lot of humidity. crawlspace is - in my opinion - well ventilated. Have even read the suggestion that in such a humid climate as NC ventilation would bring in more humid air.

Ground is not very well covered. Only some loose plastic sheets at the entrance.

Sorry about the delay, the site is having technical issues. If your ground cover is not intact, then the humidity level will be out of control, you need to get a good solid vapor barrier down and sealed.
Customer: replied 5 years ago.

read a bit up about crawl space insulation and vapor barriers. There are many options.


Would you suggest to insulate the complete crawlspace or first put a proper ground cover down?

The ground cover vapor barrier needs to be done. This is regardless of whether or not you decide to put in insulation. It will stop the ground moisture from coming up into the crawlspace area. Then if you don't currently have any insulation in the crawlspace, there are several ways that you can accomplish this. You could provide a conditioned space, and insulate the perimeter, blocking all vents, OR insulate up under the deck of the floor, leaving the vents open. In your high humidity environment, you may be better served by blocking all vents, and insulating the perimeter walls.
Customer: replied 5 years ago.
Relist: Other.
Just want a second opinion


What previous expert mentioned is good information and very important.

Another thing that needs to be addressed is your air flow.It sounds as though the condensation seems to be more towards the end of the runs. This is often caused by low air flow.

How many flex ducts to you have total? What sizes are they?Do you have a trunkline? And lastly, how far is each one ran from the connection box?

Customer: replied 5 years ago.

Yes, condensation is closer to the register than to the connection box. Also it's always at the lowest point, but it might just be that this is where the water flows to and then it sags even more.


All ducts in the crawlspace are flexducts. A14" from the HVAC to the first connection box and then a 10" to the second connection box. Both boxes have 6 runs each which are 6"


Length of the runs varies from 8' to something around 25'


Not sure what yo mean with 'trunkline'. Can you please clarify?



It sounds as though you do not have a trunk line.graphicThis picture shows the trunk line with the flexible branch ducts coming out the side. It sounds as though you just have junction boxes with flex attached, This is most likely why you have the condensation problems. Very common. A couple of ideas would be to get rid of those junctions and install a solid trunk line with transitions to keep the velocity equal as the air travels through to each register or if this is not feasible, you could install some duct boosters to help with the air flow. These would be controlled by a control to thermostat to bring the motors on when thermostat calls for cool.Either of these would probably solve your condensation issues.graphic
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8385
Experience: Furnace, A/C and heat pump specialist.
airheatman and 2 other HVAC Specialists are ready to help you
Customer: replied 5 years ago.

Would you recommend this instead of installing the vapor barrier or in addition to that?


What would be the most economical way of finding out? And could I install such a trunkline myself or do I need a pro doing it? Do I need to look for the right sizing, angling, etc.?


And how would I connect the trunkline to my 14" flex duct from the hvac?

Most hvac supply houses sell a square to round transition.graphicgraphicEither of these will work. The vapor barrier is a great idea.Both will help. but I believe getting the air moving will help the most. It is something you can do if you have the tools, but you need to have the duct sized properly. For example if we have 1400cfms of air leaving the unit, the duct will need to be sized for 1400cfms. Then after you drop off say 3 of the 6" ducts, you need to transition the trunk line down to handle 1070 cfms, etc. This is determined using a ductulator which can be obtained on line .Here is a link to a ductulator for proper sizing (listed on Ebay).

Customer: replied 4 years ago.

When looking at my crawlspace insulation of the sub-floor the batts are not tight against the floor, but there is about a 10 inch gap and the batts are held up by some wobbly wire struts.


Shouldn't the insulation sit snug against the floor?



Yes , your insulation should be against the floor with no gaps.