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AU_Prospector 01-20-2009 03:48 PM

Is it worth it to add insulation to HVAC distribution tunnel and ductwork?

I have two combination heat pump/ac systems in my new to me home. A smaller unit controls the upstairs and a larger unit controls the downstairs. They both consist of a long distribution tunnel and several round flex duct lines that run to the vents in the ceiling upstairs and in the floor downstairs.

All flex duct is a combination of 6" and 7" round and insulation rated R-4.2 is covering it.
Some flex duct runs are short and some long at just about 30 feet.

The upstairs unit has a metal distribution tunnel that is 12 feet long and has
9 ducts coming off of it. The flex duct is laying on top of and in some places halfway buried in the blow in fiberglass insulation in the attic. The distribution tunnel appears to be lined with insulation. I pulled off a duct and was able to see the words CT INSULATION 1/2". There was more, but I couldnt see it. The insulation appears to be a black dense matted material with the feel of rough felt.

The downstairs unit is in my crawlspace. It has a metal distribution tunnel that is 24 feet long which tapers down to 11" square at the end. It has 15 ducts coming off of it. The tunnel is also insulated as described above. All flex duct in the crawlspace is hanging from the floor joists and is otherwise exposed.

Things I have done so far in the crawlspace.
1) Straightened out the flex duct and re hung it to create as straight of path as possible.
2) Reconnected loose duct to the distribution tunnel as they were not all tight.
3) Some of the longer runs involved the joining of two ducts to create a longer one. This was not done cleanly and created restrictions in the line. I have fixed this to the best of my ability.
4) Retaped some of the seams in the distribution tunnel as some were leaking.

I am considering two new steps... 1) Wrapping insulation around the distribution tunnels in both the attic and the crawlspace and securing it with HVAC tape. 2) Adding more blow in fiberglass insulation in the attic and burying the flex duct to add to its R-Value.

Is it okay to bury the flex duct in the attic with pink blow in insulation? Added insulation in the attic is a good thing, further insulating the duct work is a bonus right?
Will wrapping the distribution tunnels increase the effeciency enough to warrant the cost?

Thank you if you have made it this far. I can be a bit long winded, but want to explain the situation and what I am doing as much as possible.


glennjanie 01-20-2009 09:16 PM

Hello Prospector:
If you have read many of my threads you know I am death on flex duct. I was around when they started making flex and the companies said not to use more than a 3' run of it. The flex duct is made around a wire spring with a wire every inch or less, each one causing the air to have to jump over it; after so many feet the air is worn out and can't jump any more.
On your 30' run I would venture the flame of a match or candle would not be affected by the air at all. Change your runs to galvanized duct pipe with sleeve insulation on the outside and the long run may blow the match out. There really is that much difference in the performance of the two. Galvanized pipe is smooth inside and just lets the air slide right on through.
Now, the insulation issue: yes, you could wrap your main ducts with 2" duct insulation (which has aluminum foil on the outside) tape the joints with aluminum tape 3" wide. Also the blown in insulation on top of the runs will give you a better R value.

jdougn 01-24-2009 11:54 AM

Hey Prosepctor,

Glenn's advice is good. However, I wonder if there is a problem you are trying to correct? Let us know since it will increase the accuracy of our advice!
Doug :)

AU_Prospector 01-26-2009 05:43 PM

Hi jdougn and Glenn,

Thanks for the advice. I am not trying to correct any problems, though as Glenn guessed I do have decreased airflow to 2 of my longest runs. I would estimate it at about a 40% decrease over a shorter one. I am simply trying to maximize effeciency. I have a rather long primary tunnel in my crawlspace in particular and if I could limit the loss of cold air in the summer and the warm air in the winter is all.

I will look into the cost of replacing the two long flex duct runs with smooth metal and get back to you all.

I am kind of anal about stuff like that for instance I have 60 feet of 3/4 inch uninsulated copper pipe underneath the house from the hot water heater to my master bath shower. Takes a full 2 minutes for the hot water to get there. I decided in an effort to minimize heat loss to buy $10 bucks worth of pipe sleeve insulation and spent the time cutting and trimming to fit all the corners and hangers etc and insulated the whole run. Dont know if I will ever realize any savings from that, but I suspect I will and besides it made me feel better about it. . .


jdougn 01-27-2009 11:02 AM

Hey Prospector,

Adding pipe insulation is a good step. If nothing else, the water at the shower head will be hotter thus allowing the temperature of the water heater to be reduced, which can save a significant amount.

Also, if you really want to find out what's going on with the energy efficiency of your house and your ducts get a Blower Door test done by someone that also has a Duct Blaster. If you're not familiar with these tests you will be amazed. The duct blaster is a specially calibrated fan that forces air through your duct system. This allows the tester to identify how "leaky" the ducts are, how much air is being lost to the outside, and then to correct these problems. The blower door test does much the same for your entire house. People are very pleased at how easy it is to find then stop unconditioned air from outside the house from getting into the house. Of course, this saves energy and makes the home more comfortable by stopping drafts!

One way to find an energy rater in Georgia, or any state, that does Blower Doors tests is to go to Residential Energy Services Network. Go to the bottom of the page to search your state for Energy Raters.

Hope this helps, Doug

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