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  1. #1
    Join Date
    Apr 2018
    Maryland 21122
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    Metal Duct Insulation

    Hi all,
    I have a small home (1200sq/ft). The home was built pre-war around 1939/40. It has the standard HVAC challenges found in a home from that era. Main trunk ran down center of the house with supplies on the interior walls and returns on the exterior walls using boxed joists in the basement for the returns.

    When I retrofitted a new oil furnace and A/C 20 years ago I sealed off the boxed joists (made them air tight) and combined the return and supply trunks into one big trunk so all vents in the house would act as supplies. It worked well because the original main trunk was severely undersized for the 1000cfm~ 2 ton AC/Furnace I was installing. The combined size of the two ducts was a tad larger than what was needed for the 1000cfm~ blower which was fine. I then ran one main return to the center of the house hallway.

    Fast forward a few decades and I have recently installed a new Rheem Heat pump that performed great over the summer for our A/C needs and even eliminated some warm spots normally found in an upstairs cape cod home. We are now into heating season and all has been good so far but I am looking for ways to further improve the heating efficiency of the unit which is the point of the post now that I am 3 paragraphs into the story.

    Would I see enough of a benefit vs labor involved if I wrapped my metal ducts in the basement with insulation? The basement is partially heated (one small vent in the main trunk) so the ducts do get cool (maybe even cold sometimes) when warm air is not blowing through them. If so, what product and method would you recommend? The ducts are tight to the floor joists so it may be tough to get to the tops but the sides and bottoms would not be terrible.


  2. #2
    Join Date
    Feb 2014
    Roanoke VA
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    Make sure you seal up the duct with mastic paste before you wrap them. If it were me I would even go so far as to have a manual J load calc and balance the system based on that. Also air seal the home. I'm no expert compared to some others on this site but I have sealed up a fair share of houses or advised people on what to do with varying degrees of success. The main thing is air sealing and then duct sealing. Since your duct is in somewhat conditioned space wrapping them wouldnt improve by much IMO.

  3. #3
    Join Date
    Nov 2001
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    First try closing the vent into the basement, as it would achieve the same thing. Both that and insulating will reduce the quantity of heat entering the basement.

    Is the ceiling of the basement insulated? If not, then I’d guess that neither action would save much. A lot of heat “saved” will be pulled back into the basement via your colder floor. I had a cold basement with uninsulated floors at my old house. It was just like having radiant cooling panels in the floor. Insulating made the first floor a lot more comfortable.

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