[tireshark]

I removed my entire duct system (6" rigid round) due to rust holes, no tape/mastic anywhere, and other poor installation issues. I brushed/washed the interiors out, and will be using mastic to seal every seam/joint.

The insulation on it is 30+ years old... (faced fiberglass) some is tattered and torn and needs replaced, while some is still intact. Is there a reason why leaving the old insulation on wouldn't be a good idea?

I've read where some have said it's ok to leave the old on, and others say that having a 2nd vapor barrier between the insulation may cause moisture issues.



What if I put the old insulation on backwards, so the faced side is next to the duct? This way all of the insulation would be between the two faced sides... would this get around that moisture issue that some bring up (assuming it's a valid concern)?

Thanks for any input.

 

[chandler48]

If you removed the round hard pipe, why not just replace it with modern flex? I can't see the benefit of using the old duct back. If it had moisture problems causing holes, it hasn't stopped and will continue. You don't want insulation with two vapor barriers separating them.

 

[Bud9051]

Temperature moves from inside out and with the ducts well sealed there will be no moisture from inside during heating season.

Only moisture risk is from summer cooling and a warm moist attic and as always air sealing is the best protection. In summer the vb is typically located on the exterior.

The conflict is, all assemblies need a direction in which to dry, but the importance of that is related to "how much" moisture. Boils down to conflicting advice.

Personally, I hate old fiber insulation so I would replace it, but it is still insulation and will do its job. I would also add a lot more than typical duct insulation. The thicker you can get it the better. Note, even adding all new you will still have a vb on the outside and the duct will be a vb on the inside.

Bud

 

[dj3]

I'm with Cheap and skilled on this one.
You want to repair rust holes and spots?
Are you serious?
Get new flex vents - known as tunnels.

 

[tireshark]

If you removed the round hard pipe, why not just replace it with modern flex?

I guess mostly because I already have it, plus from what I've read, rigid duct provides better airflow.

You don't want insulation with two vapor barriers separating them.

Yes, I was asking if what I proposed would get around this issue. Perhaps I didn't explain it clearly... here's a diagram.

This would put the VB from the old insulation next to the duct, so that there is nothing between the two sections of fiberglass.

 

[tireshark]

I'm with Cheap and skilled on this one.
You want to repair rust holes and spots?
Are you serious?
Get new flex vents - known as tunnels.

Well, I was going to replace anything with holes, which is primarily a few elbows under the registers.

There is a bit of surface rust in a few areas, that I was going to sand and treat with a Rust-Oleum product, and maybe cover with foil tape.

 

[tireshark]

Get new flex vents - known as tunnels.

I hadn't really thought about it, because I thought rigid was preferred.


Replacing them with flex would be about the same price as new insulation, though, and a lot easier to deal with.


Is there no performance loss with flex? Or other drawbacks compared to rigid?


My unit will be a 2.5T, and the ducting is all in the crawlspace.

 

[Bud9051]

When a flex duct ends up looking like a sea serpent and is strangled bu support wires there is a huge difference. But, if you (being a crawlspace) provide continuous support and avoid too many bends it can work fine. I believe there is a better insulation version so go with the most insulation you can find. But I prefer even more. But if that is a under the house crawlspace as opposed to an attic crawlspace, less critical.

Bud

 

[tireshark]

When a flex duct ends up looking like a sea serpent and is strangled bu support wires there is a huge difference. But, if you (being a crawlspace) provide continuous support and avoid too many bends it can work fine. I believe there is a better insulation version so go with the most insulation you can find. But I prefer even more. But if that is a under the house crawlspace as opposed to an attic crawlspace, less critical.

Bud

Thanks Bud.


Yes most of the runs are straight, and I will install it with proper support methods. There is an R4.2 and R8 version, so I will go with R8 if I do flex, and right now I'm thinking I will go that route.