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Discussion Starter #1
I'm replacing my old sheet metal returns in the ceiling with Thermopan duct board. My question is, can I place xps rigid foam board in the rim joists inside the return before putting the new duct boards up? I would seal them with Great Stuff or is that not advisable.
 

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Why would you want to insulate a return duct?? I don't see why you couldn't put in some XPS but don't see the point of doing so.
 

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Only supplys need to be insulated.
 

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Discussion Starter #4
not insulate the duct themselves. just the rim joist that the plenum buts up to. This is a basement ceiling btw. I have all the other open rim joists properly insulated and was just wondering if I should do the same that the plenums butt up against in the ceiling as they follow the ceiling joist.
 

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Looks like panning on bottom of joist.If this panning is going all the way to the outside bond, then chances are there are openings at the exterior walls for R/A going to upper floors.So trying to put insulation in the joist space around the bond probably wont be possible without covering cutouts.
 

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Discussion Starter #7
Correct but there is about 3 inches of space between the cutout and the outside wall where 2"xps could feasibly go. I just need to know if it will helop with insulation at all and that kind of insulation will be ok in a return(health reasons
 

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If it were me I would use pieces to insulate the bond and leave the metal up.Or buy couple cans of great stuff and foam the space for a better air seal.
 

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Discussion Starter #12
Why is that? The reason i'm doing this is because it's old, filthy, and I'm sound proofing the basement. Will xps in that area hurt or not?
 
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