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Old 05-18-2009, 02:26 AM   #1
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insulating duct work in basement


I am curious about the right/wrong ways to go about this for a future finished basement.

I have metal duct work that runs in the joist bays. I occaisionally notice sweating on these. Lately humidity has been up and I have been trying to reduce the indoor humidity. I recently changed my thermostat with a unit that will run my a/c to dehumidify the house if it gets to a certain point. I think I currently have it set at 55% RH, but am thinking of reducing to 50%.

In any case most of the seams in the ductwork aren't taped or sealed, so I've been trying to go through and seal the seams. Is the metal tape ok, or should I be using mastic?

I am trying to get ready to hang sheetrock and finish the basement off, but I am worried about condensation driping onto my finished ceiling. I am also using fiberglass batting as a sound barrier in the joist bays, so that I don't get sounds from the 1st floor to the basement and vise versa. I recently purchased some duct wrap that is fiberglass with a metalic foil outside cover. Is this stuff ok or should I use something else?

I am also curious if I can just use regular fiberglass batting to insulate these ducts instead of the special made duct wrap? Again, the entire joist bay would be filled with the batting, so it would surround the duct. If that is not preferred, does it hurt to use the batting around the duct wrap, so that I gain my "noise barrier"?

Thanks for any input...

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Old 05-18-2009, 04:24 AM   #2
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insulating duct work in basement


Mastic works better on old pipe/duct work.
Use the foil faced wrap, the foil is the vapor barrier so it doesn't sweat and soak the insulation.

You can still insulate the bay for noise.

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Old 05-18-2009, 07:14 AM   #3
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insulating duct work in basement


Don't expect the fiberglass insulation to provide much of a sound barrier.

Also, wrap your water pipes with foam tubes before you hang drywall.
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Old 05-18-2009, 07:58 AM   #4
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Also, wrap your water pipes with foam tubes before you hang drywall.

+1 on that tip. Do not skip this step. I nearly did. I've been in my house for almost six years and never saw my water lines sweat. Then one day this late winter the temperature spiked into the low 70s with fairly high humidity. The 3/4" cold water line coming in to the house had a constant drip all along the length of it.
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Old 05-18-2009, 09:37 AM   #5
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insulating duct work in basement


Before one half of my basement was being finished, any time the sprinkler was on outside on a hot day, the cold water pipe in the basement dripped along the full length of it. I'm glad I put the foam around them and taped it. Spending 10 minutes and a couple bucks to do that was well worth it.
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Old 05-18-2009, 10:25 AM   #6
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insulating duct work in basement


Thanks for the advice.

After I started this thread I thought, wonder if I should put the foam on the cold water pipes. I am already installing it on every hot water pipe I can get to. So thanks for that advice before I asked!

I know the fiberglass matting isn't a great sound proofer, but it has actually helped in the areas I've put it. My basement is going to be my TV/Game room area. The area above where the TV is going is where my daughters' bedrooms are. I've been installing 6" thick batting, but because I'm running 2x4's perpendicular to the joists to level them out (dropping ceiling another 1-1/2") so that I don't have to re-work things like the a/c line and other misc items. I am actually thinking of putting 1-1/2" thick foam board between the added 2x4's. My thoughts are it would add one more layer of sound proofing for a minimal cost.

Is there a better idea? This is an old house (c1915) so it has the 1x4 sub-floor and then solid wood floors above.

Any references on the foil wrap for the ducts? Are some better than others?
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Old 05-18-2009, 04:23 PM   #7
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insulating duct work in basement


Unlike thermal insulation, which requires loosely packed insulation materials, sound insulation works better with densely packed material. So you'll need to decide which takes priority.

Edit: Re the foil wrapped insulation - that's what most pro's use to insulate metal duct. What brand did you get (I hope it was R6/R8 or better). Look up CertainTeed, Johns-Manville and Owens-Corning!

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Old 05-18-2009, 05:10 PM   #8
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Unlike thermal insulation, which requires loosely packed insulation materials, sound insulation works better with densely packed material. So you'll need to decide which takes priority.

Edit: Re the foil wrapped insulation - that's what most pro's use to insulate metal duct. What brand did you get (I hope it was R6/R8 or better). Look up CertainTeed, Johns-Manville and Owens-Corning!
I'm not worried about thermal insulation between floors, so a sound insulation is the route I'd like to take. This may be a question better suited for a different section of the forum, but thinking out loud, could in order to gain better sound insulation could I overpack the fiberglass batting in? All the stuff I seem to find for sound deadening seems to be 1/2" sheets of this or that. I don't want to add another 1/2" drop to my ceiling. The stuff also seems to go up in price when you add the words "sound insulation". I'm not looking for total deadening, just decreased noise. Mainly so someone upstairs doesn't bother someone downstairs watching a movie or vice versa.

As for the brand of duct insulation, I'm not sure. It was from Home Depot. I actually found some today that is the foil type and adhesive. I'm not out much $$ on either, so I'll see how it works and look up what you suggested.
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Old 05-18-2009, 05:22 PM   #9
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insulating duct work in basement


If you live in or near Canada, you can get the Roxul 'Safe and Sound' insulation. I hear that works pretty well to cut down on sounds. There is also a specific kind of drywall you can buy for this. I can't remember the name of it right now, but it's mentioned in a couple of other threads if you want to do a search.
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Old 05-18-2009, 05:34 PM   #10
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insulating duct work in basement


Sprayed in foam insulation is pretty good for both. Yes, specialized "sound insulation" solutions tend to cost an arm-and-a-leg!
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Old 05-19-2009, 10:28 AM   #11
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Sprayed in foam insulation is pretty good for both. Yes, specialized "sound insulation" solutions tend to cost an arm-and-a-leg!
I love the idea of sprayed in foam, but it is still on the very pricy side. Especially since most of it is professionally installed. I know you can buy the DIY kits, but again, too expensive. At least from past searches.....

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