In general the AC in our house works reasonably well (though I haven't done performance tests on it yet). It's a single stage 3.5 ton heat pump/AC original to the house, which was built in 2002. Everything is in the basement with the condenser being about 25 ft away outside.

The house is 2 stories sitting on a basement/garage, and the floors are 2 separate zones. The basement is not insulated beyond 1.5 of the cinder block walls being underground.

All of the branch lines that feed the first and second floor are insulated, but the trunk lines that feed those are not. There are two separate trunks, one for each zone. Both trunks are 6-7 ft long.

On humid days (we're in the south, so every day) I notice a lot of condensation on the trunk lines sometimes to the point of seeing small puddles on the basement floor in some spots.

My question: Should these be insulated (yes, probably?) and if so would there be a noticeable efficiency gain to make it worth the effort to wrap them?

I know there are a lot of variables here so I'm just trying to get a basic understanding or rule of thumb.

  • Why is the basement so humid? How does air circulate around the house - specifically when the furnace draws air in, where does it come from, and when conditioned air is blown into rooms, what happens to the air it pushes out? If you open a window upstairs, does air want to blow out it? If you open a basement window, does air want to suck in? Does it help the humidity problem if you leave the basement door open so it is effectively drawing from the first floor? Commented Aug 1, 2016 at 23:12
  • @Harper while its called a basement by everyone, it's really a garage to me and it isn't sealed very well. Lots of leaking through garage doors, etc. Leaving the door to the house open doesn't affect humidity in the basement. The return vent is on the main floor a few feet from the basement door, so that will definitely bring in the humid air. There is a little airflow when the windows are open.
    – Steve
    Commented Aug 2, 2016 at 14:15
  • Ok, so it's quasi-outdoors in a way you can't do anything about. Your #1 concern is condensation happening somewhere destructive, that would rot out your timbers for instance. After that, yes, there's an efficiency issue. Water has a high heat of vaporization, so that condensate is stealing your cool. Commented Aug 2, 2016 at 15:01
  • Yeah, it's not a great spot. Eventually we'll get to insulating and finishing the room, but that's a ways off. I think we're good-ish on damage. There isn't anything below them, and they're only in contact with wood via hangers. There's a concern about rusting out the metal, but I see no evidence of that after being this way for 14 years.
    – Steve
    Commented Aug 2, 2016 at 15:23

1 Answer 1


One thing to check: measure the return and supply air temperature inside the ducts. A rule of thumb is a temperature difference of no more than 20 degrees. If it is more than 20 degrees the air delivery is too low ( you need more air flow ). Yes, the trunk lines and all the supply duct work in the unconditioned space should be insulated since you cannot control the humidity in that space. Adding insulation to the trunk ducts will do very little to change the efficiency of the overall system. Since You have a single return grill which you said you had in one of your answers, then all return air from rooms with doors will be from the space at the bottom of the door. If that gap is too small air delivery will be reduced. Think of each room with a door as being a balloon; you can only put in a certain amount of air. I would insulate the duct work and use insulation designed for that purpose available at some Home Depot, lowes or hardware stores and measure the temperature difference in the ductwork.

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