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I have an existing finished basement where I'm in the process of replacing an severely damaged plaster ceiling with drywall. I've got the entire thing open, and as it's an old house (1925) there is no insulation at all. Heat in this room is provided via one large radiator that is suspended from the ceiling (not my favorite feature, but it seems tough to move).

Is there much to be gained by insulating the ceiling while I've got it opened? I'm not too familiar with what is or isn't worthwhile - obviously insulation on exterior facing surfaces is good, but does it matter between floors?

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Thermal insulation (and a vapor barrier) is applied between the interior and exterior surfaces of the home. Unless you plan on leaving your basement unheated, you wouldn't place thermal insulation between the floors. And leaving it completely unheated is a bad idea since pipes can freeze.

The only reasons to place insulation between floors is for sound or to slow the spread of fire. For fire protection, the places you'd install insulation are where fire could spread up an opening like a chase for running duct work and other utility lines.

What's left is sound protection, and that is best provided by using flooring materials like carpeting above, and then separating the drywall from the joists with something like resilient channel. Insulation in the ceiling cavity will only reduce the high frequency sounds from above. The low frequency sounds, such as loud footsteps, will travel directly through the flooring, the joists, and be rebroadcast by the drywall. Think of sound like electricity, the joists as a short, and carpeting and resilient channel as tools to prevent that short.

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+1 Great explanation. I'm always amazed when I see so many basements with insulated ceilings. This also tends to make the basement areas colder. Insulating the box ends along the rim joists is always a good idea, huge heat loss and air infiltration there. Foam in these boxes is best. At least around the edges and blanket fitted in rest of the void. –  shirlock homes Oct 28 '12 at 11:10

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