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Reverse bulkheads for ducts in kneewall attic?

kneewall | Posted in General Questions on

My name is Mat, and I live in Zone 5 (Indiana). My 1.5 story house (circa 1960) has unconditioned kneewall attics with ducts in that unconditioned space. I know from reading this site that that’s a problem.

After reading this article (, I was curious if a sort of reverse bulkhead for each section of duct would allow me to bring the ducts inside the thermal envelope. I don’t have the ability at this time to move the thermal boundary to the roofline. I’m envisioning encapsulating each run of ductwork (of three total) in a sort of air-sealed, insulated box. The ducts come up through the “floor” of the attic, so it seems like I would only need to insulate three sides, since the bottom would be in contact with the conditioned space below. The length of each run of ductwork in the unconditioned space is not very long.

So my questions:

1. Would this accomplish my goal of bringing the ducts inside?

2. Are there any practical issues with this approach?

3. Is it likely that the savings from bringing the ducts inside would make this cost-effective?

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  1. GBA Editor
    Martin Holladay | | #1

    Answering the cost-effectiveness question is close to impossible -- especially if you are doing the work yourself. We don't know how you value the per-hour cost of your labor, nor do we know how many hours the work will take.

    Your plan will work, as long as you remember that (a) your new insulation assemblies need to be airtight, without air leaks at the edges (where your new insulation assemblies meet existing insulation assemblies), (b) duct insulation needs an exterior polyethylene jacket to avoid condensation problems in the summer (assuming that these ducts are used for cooling) -- especially if you are using fiberglass or mineral wool as your insulation. (If you are assembling the duct insulation from rigid foam, you don't need the polyethylene jacket.) You don't want any warm, humid air to have access to cold ducts.

    1. kneewall | | #2

      My tentative plan would be to build each "box" from rigid foam, which seems like it would be pretty simple, from a labor time perspective. I could supplement the r-value by adding batt insulation to the outside of the rigid foam box.

      If this approach works for HVAC ducts, it seems to me that it might also make sense to do it for the bathroom fan vent ducts, which are also in the attic. Thanks for your help.

      So what type of rigid foam product would be appropriate for this application?

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