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Community and Q&A

Insulate Supply Line in Attic?

Kevin Dorsey | Posted in Mechanicals on

Hi, I’m trying to get a little more efficiency out of my HVAC system and I’m wondering whether to bother with attempting to insulate some supply lines that are in the attic of a commercial building. The attic is within the thermal envelope but it is unconditioned. The attic is used for storage and therefore has a floor. All the supply lines have been mounted flush on top of the floor and I can feel major air leaks at all the seams/connections. My idea is try to slightly lift sections and apply mastic tape to the seams on the bottom as well as apply mastic to the sides and top. Then I started wondering if I could wrap insulation on the ductwork as well if I can slide it under. My concerns are: 1) If I prop the line up too far I risk causing some separation at the seams which may cause more air loss and 2) If the insulation is compressed much(and it will be) it will lose R-value and therefore a diminishing or marginal return.

Should I stick with trying the mastic only, mastic and insulation all around, or mastic and insulation on tops and sides? Or something else I’m missing? For reference I’m in Zone 5 bordering Zone 4, Hot summers, Cold winters.

My apologies if I left anything pertinent out. Thanks in advance!

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  1. Greg Follet | | #1

    I think you are on the right track. I guess my only question is if the floor between living space and the attic is insulated? You mentioned that it is within the thermal envelope, but not conditioned? Either way, I think the easiest solution is just as you suggested - try and get some mastic, or at the very least some foil tape on the bottom of those seams, then encapsulate with insulation. I would try with rigid foam? Whether or not you can get 1 or 2" of foam under the duct without disturbing the integrity of the duct is your call - but I would think it is worth a try. Good luck!

  2. Kevin Dorsey | | #2

    Thanks for the quick response Greg. The area between the living space and the attic is not insulated. The living space is offices with a drop-ceiling. It's a steel building with insulation over the purlins. When you say rigid foam I assume you mean like a sheet of polystyrene, maybe slide it under and cut to fit the bottom and then wrap the remainder? It looks like Lowe's has sheets that start at 1/4" and go up to 2". So I may be able to get some of that under without knocking everything out of kilter.

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