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HVAC duct insulation

rf_engineer_5 | Posted in Energy Efficiency and Durability on

My question is about duct insulation and if you really need it. This assumes that the insulation is fiberglass around metal duct work and not the bubble stuff. I live in western NY. For my particular case, a lot of my duct work runs in the basement but the basement is unfinished (at this time). I have several lengths that are 25′ long, maybe a bit longer, and based on my measurements of temps, I lose roughly 20-25°F for that length of run. My furnace has an output temp of appx 120°F. At the end of a 25′ run, I see about 100°F. Roughly this is a loss of 1°F/foot drop. I did not do measurements during summer in relation to AC conditions. My duct work has been sealed up with mastic. The only thing left is insulation but I am not sure there is a cost/benefit analysis in spending the money on this. If the basement was more finished, would it still be a good idea to insulate longer runs of duct work even within “conditioned” space? Regardless, should all duct work be insulated or are there guidelines for such things?

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  1. GBA Editor
    Martin Holladay | | #1

    Like you, many energy experts in northern states -- states where ductwork is located in basements, not attics -- question the need for duct insulation and even the need for mastic. As you have correctly noted, as long as the ducts are located inside your conditioned envelope, your arrangement does not necessarily result in an energy penalty.

    Here are the caveats:
    1. Since you ducts are heating your basement, you need to be sure that your basement is air-sealed and well insulated. That means you have done a good job of air sealing your rim-joist area, and you have basement wall insulation that meets or exceeds code.

    2. If you have long uninsulated duct runs, the air reaching rooms at the end of these long runs may not be hot enough in winter or cold enough in summer, leading to the possibility of comfort complaints. If these far-off rooms are too cold or too hot, you may want to rethink your plan to skip duct insulation.

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