Advice on replacing plenum insulation in attic

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Old 04-15-19, 11:59 AM
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Advice on replacing plenum insulation in attic

I need to re-insulate the plenum in my attic - it is about 35 years old and the old insulation is getting brittle. I made the mistake of having the air ducts cleaned last summer and the company did a lousy job putting it back together so this winter I had a lot of condensation wherever there was a joint. Since water ruins the insulating properties of fiberglass I'm thinking it needs to be replaced. It's a run about 50 feet long.

Any advice on how best to handle this? I have 2 options: First option is to remove all the existing ductwrap, then seal all the joints with mastic and foil tape, then put new R8 foil-faced fiberglass on the whole thing (Maryland climate.) If I do this, is it best to work in 4 foot sections since the insulation is 4 feet wide, or better to try to run it longways as much as possible? I'll be doing it myself since I got what I think is an outrageous price from an HVAC guy to do it. He even suggested going right overtop the existing foil-faced insulation with another layer, but I don't want 2 layers of foil to trap moisture.

My other option is to remove all the insulation and have the whole plenum spray-foamed. This will seal and insulate in one step and I got what I think is a fair price to have it done. My concern with this method is that it would make it much more difficult to service the ductwork in the future if it ever needs it.

The plenum is elevated off the floor and rests on wood supports so I have pretty good access to all sides.

I'm open to other solutions if anyone has any. Thanks in advance
 
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Old 08-28-19, 11:38 AM
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I guess since there are no replies after 4 months I'm on my own!

Anyone have an opinion on the bubble-wrap style foil insulation to wrap my attic ductwork, rather than fiberglass?
 
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Old 08-28-19, 01:04 PM
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No bubble wrap! Get the low itch R-8, FSK tape. Stapling the seams is best practice before the FSK tape. Will need to purchase a duct wrap stapler.
 
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Old 08-28-19, 01:15 PM
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I am planning to buy an outward-clinch stapler for this project. Thanks!
 
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Old 08-28-19, 01:43 PM
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Hi Dan, sorry you were left hanging but long threads often get passed over, especially by myself. I wait for others to read and reply to judge whether I should jump in or not. My bad.

If it happens again just post a bump so it shows up as a new post.

In addition to the insulation use some foil tape of duct mastic to seal all seams. Then seal up your insulation very well.

Check all penetrations into the living space to be sure they are sealed as well.

Bud
 
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Old 08-28-19, 04:38 PM
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No worries, Bud. I kinda forgot about the post myself since there was no way I was going to start this project in the summer heat. Now that fall is on the horizon I need to start buying my materials so I can do it when the AC is not needed but before the heat is needed, either.
 
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Old 09-03-19, 08:02 AM
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Since I'm replacing all my flex ducts I will upgrade them to R8 while I'm at it. The ones at the big box stores seem kinda flimsy - is there a better flex duct I should consider? I know that's a loaded question because flex duct is not the best product in general, but the old ones I'm replacing seem pretty robust and have lasted at lwast 30 years if not longer. I can't make out any identifier markings on the old stuff.

I want to to spend my money on the right products!
 
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Old 09-04-19, 06:11 AM
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airman - Is an outward clinch stapler the right tool, or should I be using a plier stapler? The website where I am buying the insulation says I should use a plier stapler, but I've seen folks recommend an outward clinch style. I will buy whichever is best suited for the job.
 
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Old 09-04-19, 06:50 AM
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Outward is all I have every used. Just a simple Duct wrap stapler.
 
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Old 10-12-19, 08:26 AM
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OK, I finally got this project started last weekend. Stripped off all the old plenum insulation, removed all the flex duct branches and sealed the seams with mastic and/or foil tape. Next step is to wrap the plenum with new R8 fiberglass insulation.

My question is regarding how to handle a wiring box mounted on the plenum - it supplies power to a powered damper that closes off the air to one side of the house and has 2 wires going into the box. The old insulation was cut around the box, perhaps because it was installed long after the system was originally installed. The box does get warm to the touch, but not hot.

So, can I insulate right over this box, or would that be a fire hazard? I'll opt for safety first, but if it's not a hazard I'd rather not have a break in the insulation creating a condensation point.
 
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