Insulating Duct Work

john916

My house has rigid metal ducting that suffering from the effects of aging. The insulation is fiberglass with a dark paper looking outer covering, stapled and taped. The paper is torn, the staples have rusted and the insulation is starting to come apart in many places. Also, it appears that the joint were taped using plain ole duct tape so the joints are failing too. (Although this may be a previous owners attempt at repairs)

My questions:
1- Can I screw the joints together with sheet metal screws and then tape them with aluminum foil duct tape?

2- Can I cover the metal duct with flex duct for insulation or must I wrap the ducts instead?

3- The main trunk needs to be re-wrapped too. Should I tape the joints on the main duct or is that necessary?

I've had to re-hang a few ducts that had separated at the joints and/or fallen off the boots. I'd prefer to keep the metal duct. The few sections I've had off and looked inside are fairly clean. The only dirty sections are right at the registers were dirt and toys have fallen in.

Thanks in advance for any suggestions.

DuMass

If you’re removing all of the old insulation, I would probably use duct mastic on all the joints of the metal ducts instead of tape. The mastic seals the joints and can help prevent them from loosening over time due to vibration and expansion/contraction.
You could use some short zip screws if needed for sections that don’t fit together securely.
The boots themselves are usually secured through the inside edges to the floor cutout where they pass through. Hanger straps are needed to support most horizontal duct runs.
I would probably use either a reflective insulation roll like Reflectix or the SoftR fiberglass foil duct wrap. Same thing on the main trunk. You may want to use a stapler for these to make installation easier. I would then use the foil backed tape on the joints of this insulation.

Texahoma

I agree the mastic is a good idea for sealing up the seams of your duct. I would definately use a 1/2 zip screw then mastic over the seam. Also use a foil wrap as mentioned above and staple your ductwork. A little formula I use. Example if you are going to wrap a 8" pipe you would multiply 8" times 4 plus 4 inches up to a 10" pipe. Above a 10" pipe it would be the size times 4 only. I hope that helps.

flashheatingand

It would be good to mastick the metal seams it's relatively cheap and seals like a m.f.. Use zip screws to make sure that the pipe is securely fastened. I think the job would be done quicker if you used the insulation (r-8) that is used with flex duct on the branch lines. Use foil-back duct wrap (r-8) on the trunk.However, as for the cost of materials, you may get more bang for the buck with a roll of duct wrap.

Don't forget to wear a mask. Another thing, by spreading baby powder on your body, you will limit the itchy feeling from working with the inuslation.

Tinmanfab

These guys are right for the formula for ductwrap (at least R-4 and R-6 work with these numbers). For R-8 I have been adding 6-8" extra on the circumference.

There is a special stapler which has an outward clinch. It might help.

Definetly wear a mask and long sleeves.

I found wrapping steel branches (hardpipe) is quicker by pulling flex over it and then pulling the guts out. If you use foil flex the seams are less visable.

You can't go wrong with mastic. Tape may not stick to older duct if it has dirt on it. Mastic dries hard as a rock.

In large jobs I would wrap with foilwrap and sprayglue. This eliminated the need for wire baling. It holds the ends of the wrap tight as you tape it too.

Hope this helps.