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Old 06-19-2008, 01:08 PM   #1
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Can I add insulation to a 6" flex duct?


The whole house is 6" flex duct. I am not sure about the R value but it is not R-8. I was thinking about wraping it with R-13 because it is fairly cheap compare to regular "duct" wrap insulation.

Should I wrap it with faced or unfaced insulation?

If faced, should the kraft paper be on the outside or inside?

Any other suggestions?

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Old 06-20-2008, 04:50 PM   #2
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Can I add insulation to a 6" flex duct?


Just to warn you i cant imagine a way that that would come out looking good , that being said it will work . If I had to make a recomendation I would say use faced on the outside so you can at least tape it

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Old 06-20-2008, 10:45 PM   #3
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Can I add insulation to a 6" flex duct?


If the duct is relatively straight, and is supported by something, you might want to use pipe insulation. This stuff is expensive, tho. It will make the installation look nicer, but you do need sufficient strength to support the weight of the insulation.

On the other hand, pipe insulation comes in anything from 1 to 3 inches thick, so you could buy the thinner stuff for less cost and less weight.
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Old 06-21-2008, 08:08 AM   #4
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Can I add insulation to a 6" flex duct?


Here are some helpful links:

http://www.expressinsulation.com/

http://www.jmhomeowner.com/insulatio...acductwork.asp
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Old 06-22-2008, 10:42 AM   #5
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Can I add insulation to a 6" flex duct?


Would adding more insulation with the vapor barrier on the outside cause a problem with moisture getting trapped and therefore mold growth?

After looking at the duct work closer I realized that it was pinch in three different places from sharp bends preventing air flow. The insulation was R4.4. I decided to redo the flex duct with a next size larger flex duct (8") that was long enough to not cause the sharp bends. I also bought the flex duct with R8 insulation. The run will be about 20' to one room. I know flex duct should not be ran that far but the rest of the 1000 sq ft house is using 6" flex duct and seems to work fine. This room was the southern most facing room too.

Thanks for all of the help.
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Old 06-22-2008, 04:19 PM   #6
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Can I add insulation to a 6" flex duct?


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Originally Posted by EE playing with HVAC View Post
Would adding more insulation with the vapor barrier on the outside cause a problem with moisture getting trapped and therefore mold growth?

After looking at the duct work closer I realized that it was pinch in three different places from sharp bends preventing air flow. The insulation was R4.4. I decided to redo the flex duct with a next size larger flex duct (8") that was long enough to not cause the sharp bends. I also bought the flex duct with R8 insulation. The run will be about 20' to one room. I know flex duct should not be ran that far but the rest of the 1000 sq ft house is using 6" flex duct and seems to work fine. This room was the southern most facing room too.

Thanks for all of the help.
I wouldn't expect a huge improvement going from R4 to R8. See for example this page and the graph under the "Duct Insulation" section in particular: http://ducts.lbl.gov/distribution.html#insulation. (I can confirm that going from R4 to R6 is a waste of time; for under $30 for one duct, at least it wasn't an expensive experiment!) As for 20' being too long a run, I wish someone would tell that to all the builders here in south Louisiana. I've yet to see a house under construction in the 200-400K range that doesn't suspend an evil octopus of flex duct from the rafters.
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Old 06-23-2008, 08:30 AM   #7
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Can I add insulation to a 6" flex duct?


I think that it is worth going to R8 insulation if you were going to change out the duct anyway. The R8 insulation cost about $10 more at HD than R6. The savings from R8 according to the graph would be about 3%. There are 9 ducts in the house so the savings on the electric bill would be 3%/9 per duct. The electric bill is around $150. So $150*3%/9 is $0.50. So it would be a 20 month pay back.

It may still be worth it to change out even if you didn't need to. Going from R4 to R6 is a 5% savings. Going from R6 is an additional 3% savings. So you will get around a 8% savings going from R4 to R8. 6" R8 is $35 at HD. The savings would be $150*8%/9 = $1.33 per month per duct. Which makes it about a 26 month pay back.

What should be considered a good pay back? 1 year, 2 year?
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Old 06-23-2008, 03:45 PM   #8
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Can I add insulation to a 6" flex duct?


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Originally Posted by EE playing with HVAC View Post
I think that it is worth going to R8 insulation if you were going to change out the duct anyway. The R8 insulation cost about $10 more at HD than R6. The savings from R8 according to the graph would be about 3%. There are 9 ducts in the house so the savings on the electric bill would be 3%/9 per duct. The electric bill is around $150. So $150*3%/9 is $0.50. So it would be a 20 month pay back.

It may still be worth it to change out even if you didn't need to. Going from R4 to R6 is a 5% savings. Going from R6 is an additional 3% savings. So you will get around a 8% savings going from R4 to R8. 6" R8 is $35 at HD. The savings would be $150*8%/9 = $1.33 per month per duct. Which makes it about a 26 month pay back.

What should be considered a good pay back? 1 year, 2 year?
Sure, if you're going to replace a duct anyway, use the highest R-value available. When my house was built 2 years ago, code called for R4.2, and that's what they used. It went up to R8 briefly sometime after Katrina, but then for whatever reason, it got knocked down to R6, where it stands today. I just happened to see a bunch of R6 duct recently at Home Depot and bought one as an experiment. After replacing one R4.2, I didn't observe any difference in the temperature of the air coming out of it; it appeared to gain as much heat as the original. I guess there might have been maybe a couple of tenths of a degree difference, but I don't know if that's going to significantly improve the comfort level in my home. For the time being, at least, I'm focusing on sealing the ductwork, caulking all the gaps around ceiling penetrations, touching up underinsulated spots, etc. I think fixing conditioned air going into the attic and attic air coming into my house has a better bang/buck ratio as a first step.
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Old 06-23-2008, 07:53 PM   #9
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Can I add insulation to a 6" flex duct?


I guess something else someone might want to factor in is their time. If you factor pay for your time you most likely would not replace the duct unless it needed it. But I think it is worth paying for the extra insulation if you are already going to spend the time to replace the duct.

Just my $0.02

Thanks for the help everyone.
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Old 06-23-2008, 08:34 PM   #10
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Can I add insulation to a 6" flex duct?


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Originally Posted by tima2381 View Post
...I just happened to see a bunch of R6 duct recently at Home Depot and bought one as an experiment. After replacing one R4.2, I didn't observe any difference in the temperature of the air coming out of it; it appeared to gain as much heat as the original...
I should have mentioned that the duct I replaced was about 21' long and connected directly to the plenum. Both the original R4.2 and replacement R6 gain several degrees on very hot days compared to a 6' run also connected directly to the plenum. Anyone make R30 flex?
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Old 06-24-2008, 06:07 AM   #11
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Can I add insulation to a 6" flex duct?


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Originally Posted by tima2381 View Post
Anyone make R30 flex?
That is why I was asking if you could insulate flex duct and not cause a mold growth problem with a double vapor barrier.

i,e. duct - insulation - vapor barrier - insulation (mold growth?) - vapor barrier

The run I have is 20'. I asked this because R-13 is very cheap. I could insulate a 6" duct 20' long with R-13 for $9 plus tape if I use conventional wall insulation.

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