Sealing flex duct joints - I forgot one

Steve Garson_2

Today I was in a hot attic installing a 10-inch flex duct to a new take-off from an AC plenum. A super tight, awkward, back twisting job, flat on my back,

I used two zip ties with a tensioning tool, but in my discomfort, later realized that I forgot to use mastic on this joint, after wrapping and taping the insulation.

Should I not worry about this mistake? How much air leakage can there be with two tight zip ties. Obviously not the right way to do things, but it was such a painful task, I would need a good motivation to fix the mistake.

P.S. It's my own home.

Thoughts?

Steve

rick in Alaska

My vote is to leave it. If you have an infrared camera, such as the ones for I-phone, you could see if it had any leakage. I doubt it would be much though, if any.
Rick

ChrisJ

I've been trying to figure out how to seal these on my own system that I haven't started yet.

I'm using silicone on most joints, and really good tape on joints that are fairly tight on their own.

I don't see a good, easy sure way to seal flex to pipe tho?


@Steve Garson
Here's the thing...........I'd go re-do it.
But I'm nuts, and I'd redo it for the simple fact it would drive me crazy every day until I finally had enough and redid it. I know how I am.

lchmb

I'd leave it... I bet a camera would show it's not enough to make any difference..

Steve Garson_2

Chris, I agree. I'm going to open the insulation and seal the joint. It would bother me not to.

JUGHNE

Chris, my method of madness I developed over the years is:
A 1 1/2 wrap of duct tape on the metal pipe or takeoff protruding about 1/2" past edge of tin (this adds a little thickness as a gasket and also protects the inner vinyl duct from any tin cuts....I have redone some flex and often found the inner liner cut open from the raw tin edge with vibration over the years.)
Then slip the duct over the tin pipe end 2-3" and a good spiral wrap with duct tape again overlapping to seal it tight to the tin.
Then 3 sheet metal screws with plastic washers cut from plastic hole strap, the screws are placed to capture the metal spiral of the liner to prevent pull off.
The outer insulation flex is then pulled up and the outer vinyl barrier is rolled under the FG and held in place with flex tie.
I use a good quality of the grey duct tape, as long as the tape is tacked with the screws and tie it will stay in place.

You can buy a gallon of grey or white duct sealer, it applies with a brush or foam brush. Water clean up and dries solid. It is waterproof as I have patched rusted AC coil drain pans with it.
Would be quicker and cleaner than silicone caulk.

Jack

Duct tape is good for everything except what it is named for. It dries up and blows away in surprisingly short time. The best method of sealing I have seen is a layer of duct seal, scrim tape laid into that and then a sealing coat over the scrim tape. Probably not the best for your pretzel position on this job. Choice of duct seal can be either the smooth or with fiber. The smooth is a bit easier to work with, but I've always thought the duct seal with the fiber helps hold things together and bridges gaps better.

ChrisJ

Jack said:

Duct tape is good for everything except what it is named for. It dries up and blows away in surprisingly short time. The best method of sealing I have seen is a layer of duct seal, scrim tape laid into that and then a sealing coat over the scrim tape. Probably not the best for your pretzel position on this job. Choice of duct seal can be either the smooth or with fiber. The smooth is a bit easier to work with, but I've always thought the duct seal with the fiber helps hold things together and bridges gaps better.

This is the tape I'm using, and so far I'll have to respectfully disagree, this doesn't dry up or blow away.

Hardcast foil grip 1402 rolled mastic.

JUGHNE

The Big Box HO duct tape will always end up coming loose.
I have opened some flex conn I did 20 years ago and the tape was still intact. It being tacked down with screws/washers and ties keep that first edge from coming up. IMO
I use a above average grade of tape. 20+ years ago there was no user friendly sealer/mastic.

Steve, for the tough spots I cut the inner flex short by 10-12" then would put a 6 to 10" length of round pipe into the flex by your methods and then stick it onto the TO. Screws and foil tape, mastic etc. Then pull the insulation up to the main duct and cable tie it.

ChrisJ

@JUGHNE
Why screws and washers when a zip tie likely clamps down better, and more evenly?

They go awfully tight using the tool.

JUGHNE

Just an old guy .....belt and suspenders.... who doesn't want to go back into the attic

I will put the tin connector sleeves on premeasured flex before going to the attic. Most of mine are crawlers and not stand ups like yours.

In basement jobs I use that grey brush on sealer on the take offs which are usually on the top of the main duct, also inside the plenum as the air pressure is against those seams. It can get looking bad if put on the outside. If the duct will be boxed in then the tape and brush will be applied to rectangular joints. All round joints get foil tape.
The RA only gets serious sealing between the filter and furnace.
These are conditioned spaces. Probably will not be approved if a serious air test. The basement gets SA's and at least 1 small RA.

ChrisJ

Mine is only kind of stand up in the center, though even there I'm slightly hunched over. I can stand up in between rafters in the dead center, but I still get concerned about nails.

A lot of it, is going to be laying on my stomach, or side, in a pile of dirt and slate. It's not exactly the nicest situation, tho not the worst either I suppose.

JUGHNE

In those cases near the outside wall, if your register boot is top entry, I put a 90 on the end of the flex. Then the serious sealing can be done first with the tin to tin joint made in the tight location.
Again screws, foil tape, sealant etc and then pull the insulation "hoodie" over the 90 and tie.
We have a lot of 4/12 rooflines here. I use round ceiling regs as they may end up 3-4' from the outside wall.

AMservices

Foil tape and ductseal ever seam, I've never failed a pressure test.
In Massachusetts( depending on the town) I'm not allowed more than 4% duct loss.
The trick to making ductwork go in easy is the prep work.
Do as much as you can out of the tight spaces. Work smarter, not harder and a fast pace makes wast ( of time and blood that is). Knife jugglers cut themselves less then tin knockers.
The first guy that taught me how to put ductwork together said
" you need to be smarter than the metal".
I think of that every time I see somebody struggling to make a connection.