Help needed on HVAC duct and moisture

[dadster4]

You need to get insulation BETWEEN the bottom of the joists and the top of the duct. I know what a pain working under a trailer is, so it will not be easy. Try to loosen the duct hangers enough to get a 1/2" gap to fill with insulation- vapor barrier on the outside. Even a 1/4" gap with the foil-faced "bubble wrap" insulation will help.
Another solution is to cover the ground under the trailer with heavy plastic sheeting to minimize moisture coming from the earth below- your source of most humidity problems. Make sure the space is well ventilated also.

 

[sld]

What about using treated plywood as a back up plan in case the moisture problem isn't solved?

 

[Ed of all trades]

What do you think of dropping the duct 1/2" and using spray foam between the duct and the floor joists? Spray foam might be good on the duct everywhere to. Ed

 

[Sigarms]

As mentioned, insulating the duct (preferably by wrapping it in R8 duct wrap) is the way to do it.

However, mobile / modular homes should use a blower with a high static blower drive due to the possible ductwork size. Guys installing equipment in mobile homes tend to use a standard air handler, and you can get away with it and never get bit. However, I can remember two jobs where guys got bit due to moisture/water issues, and it did come down to wrong air handler being used. It's a roll of the dice and generally you don't get bit (which is why a lot of guys do it), but it can happen.

If the HVAC system is old as the mobile home, this is probably not the case, but wanted to ensure that this possibility was eliminated.

 

[EddieWalker]

The HVAC system is around 6 years old according to my client. She had it serviced a few months ago, and there does not appear to be any moisture inside the duct that I can find. Bone dry.

There was enough room under the joists that I was able to slide 3/4 inch Styrofoam between the duct and the joists. Then I sealed up all the edges with HVAC sealant stuff that they sold at Lowes. There was two types, I went with the more expensive.





Then I wrapped the duct with R19 insulation with the paper towards the duct. Then I filled in the rest of the floor with the same insulation keeping the paper facing down. I also added a couple boxes of mothballs to the insulation. I think the main reason for this problem is from squirrels getting into the insulation and making their home in there. Most of it's all gone and I found acorns, leaves and sticks in there. She had the siding redone and put in all new skirting around the bottom of the mobile home a few years ago, so that probably stopped the squirrels. I didn't see or smell anything to indicate that they are still a problem.



Monday I will finish gluing and screwing down the 3/4 inch plywood that I'm using to replace the 5/8's particle board.


Thank you for the advice and helping me wrap my head around this.

 

[Billrog]

You've done a thorough job Eddie I can't see where there could possibly be any more condensation problem.

 

[SandburRanch]

I suspect there was an air leak at the round to rectangle junction because that area wasn't sealed properly.

Vapor barrier should go to the warm moist air side of the insulation which is the outside.

 

[tman1020]

That's a first for me to Eddie. I think you're on the right track for the repair, try to minimize porous insulation as much as possible sticking with the foam everywhere you can to prevent future troubles. A method that is becoming more popular around me is to insulate and condition crawlspace areas and not insulate ductwork there. Also a vapor barrier of plastic on the floor of the crawlspace area under the trailer should help minimize humidity levels.

 

[EddieWalker]

I suspect there was an air leak at the round to rectangle junction because that area wasn't sealed properly.

Vapor barrier should go to the warm moist air side of the insulation which is the outside.

I forgot to mention there was a quarter sized hole where the duct was installed for the room and most of the foil tape was coming off where the supply line attached to the duct. I sealed both up

 

[SandburRanch]

Already mentioned, dew point temperature was reached because of the leaks and caused the moisture problem. Here is an example. If the temperature under the house is 88° with a relative humidity of 72 percent the dew point temperature will be 78°. It's common for supply air in the ducts to be 60 or so degrees and even lower so we can see that if that leaking air cooled the surrounding material to 78 or lower there is a moisture problem that you and home owner have encountered.

When rebuilding I might even go as far as to drop a dial thermometer in a 1/4" hole of the sub floor in that area just to monitor it for a few days while the A/C is in operation.