Insulate Ductwork?

[Dave]

What are the different options to insulate ductwork?

I am installing central air ductwork in unconditioned space above my ceiling.

Thanks for the advice!

[Don]

I'm all for it. Don

[Art]

Do not use the insulation that goes inside the ductwork. It is crap. A great place for mold to grow. Also don't use that cardboard self insulating ductwork. That stuff is crap too. Insulate metal ducts by wrapping them with fiberglass insulation or use flex duct but that stuff isn't as good.

[malamilia]

[buffalobill]

1. pick out your climate pdf at this site for the r-factors:
2. find the material specified for your zipcode by insulation manufacturer such as:

[hallerb]

they have plastic flexible line that comes pre insulated wrapped in a blanket. easy to install too

[Dave]

Thanks guys!!! I will make sure we use the right stuff.

[Stretch]

Installing ducts is not a good DIY project.

1) Sheet metal has the lowest resistance to air flow, but it tends to be noisy and is more difficult to seal than other types of duct. It is more resistant to physical abuse than most other types of ducts..

2) Flex is the easiest to install and easiest to seal. You only have to seal the two ends. It has higher resistance to air flow than ductboard or sheet metal. It is the quietest of all ducts. It is prone to damage from abuse. It is easy to kink, which reduces air flow. The insulation around it is fluffed up and is better insulated than wrapped sheet metal in most installations.

3) Duct board is more resistant to abuse than flex, but less resistant to abuse than sheet metal. Ductboard is Thermally superior to wrapped sheet metal in most installations. 1-1/2" Ductboard is R-6. It takes 2-1/4" duct wrap over sheet metal for R-6. If the duct wrap is pulled tight over sheet metal duct for good appearance, the R-factor is reduced to as little as R-2 or R-3. Ductboard with tight corners leaks very little.

NOTE: Regular duct tape does not meet code for either ductboard or flex. Tape for ductboard should be rated UL181 A-P or A-H. Tape for flex should be rated UL181 B-FX. The rating should be written continuously on the outside of the tape along with the model of the tape, the brand and the last test date that it passed UL..

Note that if you install sheet metal, you should SEAL EVERY SEAM. With round metal duct, in an elbow, that is 3 joints in the middle plus two at the ends where it attaches to the round duct. Every longitudinal seam should be sealed as well. Use mastic, not duct tape. If the sheet metal is not well sealed, the cold supply air will leak out into the wrap, making the outer vapor barrier of the wrap cold, making it more likely to sweat and less effective as insulation.

So you see, it is not that straight forward. There are tradoffs for any installation that must be considered.

Also, if you don't size the ducts properly, you will not get the right amount of air flow, reducing the performance of the system. Many contractors have a hard time with that. Look for a contractor with NATE certified technicians and installers. You stand a better chance of getting a good job that way.

Good luck

Stretch