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XPS insulation around HVAC duct

stryped1 | Posted in General Questions on

I have a cabinet in my kitchen and one in each bathroom that has toe kick registers. The HVAC air/heat vent comes out at the floor under the cabinet, but not directly to the vent. I do not want to remove the cabinet and have limited acess to it. I am noticing condensation in the sub floor in these areas, I assume because of condensation due to the entire floor under the cabinet being cooled. My idea is to cut strips of xps 1 inch or so insulation to lay on the floor to insulate the floor. Then use strips of the xps turned on its side between the floor and the bottom of the cabinet to create a sort of “vent” to vent the heated and cooled air directly out from under the cabinet.

My question is, is it ok to use rigid xps in this way during the heating season? Will the hot air from my propane furnace be too hot for the xps? The xps will not directly contact the HVAC pipe. It will just sit on top of the floor and around the HVAC vent opening.

Thanks for any help!

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  1. user-2310254 | | #1

    Are the ducts running between floors or through a crawlspace or basement? Where do you live so we can understand your climate conditions.

  2. GBA Editor
    Martin Holladay | | #2

    First of all, you need to verify that the condensation hasn't yet rotted out the subfloor.

    If the subfloor isn't rotten, the best solution is to install a register boot and a register in the toe kick board. Then connect the register boot to your duct, sealing all seams. (I'm not sure if you have enough access to do this, but that's the proper way to proceed.)

    If you want to try to create a duct plenum under your cabinet, don't use XPS. Use duct board. Here is a link to a web page that shows you what duct board looks like:

  3. stryped1 | | #3

    I know what the best answer is, but I would have to disconnect the cabinet and tear off the toeboard to do it so I would prefer not to do that.

    I live in Kentucky. I have a crawlspace.

    I would use duct board, but it seems no one local to me carries it. (I checked).

  4. user-2310254 | | #4

    Since you have access through the crawlspace, perhaps you or an HVAC contractor could relocate the ducts. In any case, it would be prudent to check the subfloor to rule out rot and/or mold issues. You also might want to exam your duct runs to make sure they are completely sealed.

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