Coast Resorts Open Roads Forum: RV style AC attic duct work


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JBarca

Radnor, Ohio, USA

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Posted: 10/05/22 09:54pm

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I restore older campers and there are times the AC ducts in the attic gets damaged beyond using thick HVAC tape to seal the air leak, the whole duct needs to be replaced.

I am trying to track down who makes them, or at least what they are called in the RV industry in attempts to buy some.

Here are pics of what I am talking about. This size has to fit through the rafters.

The ceiling registers are the older standard round turn type to adjust air flow. I know where to buy them.

This is the attic with the roof off showing the AC ducting I'm trying to locate.

The cieling air box on a older Dometic AC unit
[image]

Curved ducts come out of the air box and head down the camper through the rafters
[image]

A close up of the space the duct fits through the rafter. You can see the white round ceiling register poking out of the ceiling board into the duct
[image]

Thanks for any leads on this.

John

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Roger10378

Goodrich, MI.

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Posted: 10/06/22 06:21am

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I think that they are made of foam board. They are cut to size for what is needed and taped together. Most any foam board will work but the aluminium faced is the best choice. If the big box building supply stores do not have the thinner foam board any HVAC supply will.

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BobsYourUncle

Calgary Alberta Canada

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Posted: 10/06/22 07:36am

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John, I can't directly answer the question for sourcing the original product, but I could offer a suggestion for fabbing your own.

There is a construction product called Fome-Cor. I used to use it extensively years ago when I was doing reno siding on old homes. It is a quarter inch thick foam with perforated foil on one side, kraft paper on the other. 4 feet wide by 50 feet long, and it comes in a 2 foot increment fanfold.

If you have access to an aluminum brake, like the 12' 6" one I carry on my work truck, you could cut this product to size, run it through the brake and create your own duct. One side would have an overlapping flap that gets taped together with foil or duct tape.

It would be flexible enough to sideways flex it to a trapezoid shape so it could be flexed to insert into the narrow opening of the trusses, then returned to a rectangle shape. You could further insert rigid foam blocks on each side, at the trusses, to help it retain its shape.

I know I could do this as described, but you need a brake to do the folds. The brake has to have a gradual cam lock system to put gentle pressure on the foam board, not a snap lock system which would crush it.

Just thinking out loud with this idea, I have always been one to create my own things when none are available...

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dougrainer

Carrolton, Texas

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Posted: 10/06/22 07:39am

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The ONLY place I am aware of to get Ductwork is from the OEM's. Jayco(and others) use the type in your pics. The few times I had to replace a duct(Newmar and Jayco), they sent the ducts by freight. Since you have the rafters exposed, the ducts just bend and slide in place and you cut to fit. You could buy thin foam board and make your own. Just use trial and error to figure out how to cut and bend to form the rectangle. Then use the foil tape. You could also make it out of 1/8(3/16) luan and have a good strong duct system. Doug

Gdetrailer

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Posted: 10/06/22 09:49am

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Looks like "wall stack" ductwork might be usable..

[image]

Wall stack dimensions are typically 3 1/4" x 10" and in 24" lengths and you can also get different elbos and is available in galvanized steel or aluminum. Snaps together fit.

Of coarse, you will need to wrap the metal ductwork in some foam insulation to reduce sweating.

You should be able to find it at Lowes, Home Depot and Amazon..

nickthehunter

Midwest

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Posted: 10/06/22 10:57am

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It's called "Duct Board"

Clicky

Video

* This post was edited 10/06/22 11:27am by nickthehunter *

LMHS

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Posted: 10/06/22 11:02am

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You can get metal brakes for cheap at Harbor Freight. I have an 18 inch ($13) handheld that I used when I was repairing/upgrading an old truck camper. I wish I had the 18" mountable one ($45) so I could have used a heavier flashing. The ones at Harbor Freight run from handheld 6" seamer ($18) up to a 36" floor mounted brake ($300). The 18" brake comes in very handy.

Gdetrailer

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Posted: 10/06/22 11:28am

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nickthehunter wrote:

It's called "Duct Board"

Clicky

See the problem with "Duct board" in a RV that I circled in red?

1" is the thinnest I seen on that first page of the link.

Unless you can find thinner ductboard than 1" wall it will be useless in a RV ceiling as it would only give about a 1" inside dimension.. That would be like trying to drink your coffee through a coffee stirrer (it can be done but at a cost of efficiency and not worth the hassle).

For duct board to work reasonably, it would have to be very thin, not much more than 1/4" wall which would take 1/2" of the open framing dimension the OP has to work with.

Thin ductboard also has a bad habit of sagging unless it is fully supported which is what the OP is fighting with..

Gdetrailer

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Posted: 10/06/22 04:33pm

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nickthehunter wrote:

Video

Picture snip from video in link above.

[image]

Not practical in a RV ceiling, look how thick that material is.

Might work fine in a sticks and bricks where you have virtually unlimited space to build a duct as large as you want.

Not so much when you have a very small opening such as the webbing in the roof joist that may be not much more than 3".

As I mentioned before, you would have to find fiberglass duct with a wall thickness of no more than 1/4" if you want to have a duct of any useful inside size..

JBarca

Radnor, Ohio, USA

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Posted: 10/06/22 07:32pm

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Hi Everyone,

Thanks for your input. It all helps.

A few thoughts, it appears I may have to make up the new ducting, which is not a real problem, I can do this. The issue now is to finding the foam board or duct board in 1/4" and foil outside warped. Not sure if Jayco will sell it direct, it's an option if I can't find the board.

I did find this which could work, but I'm not sure of the thickness, yet. I have to call them, Thermopan, by the connector strips they sell, it appears it might be more like 1/8". https://thermopan.com/collections/thermo-pans

I can find it for sale on the web, https://www.supplyhouse.com/Thermopan-16........-Ultra-Fire-Resistant-Duct-Panning-Sheet

I will also check at local HVAC places.

Here are some good duct pics from a similar camper to the pics above that I am working on restoring now. I restore mainly Sunlines, but have worked on other brands too.

It is 1/4" foam board of sorts. Foil over the outside, none on the inside. It appears is was made from flat sheet, then V cut at the corners, folded and taped at the one and only seam.

This section is close to 8ft long. I have the roof all apart and can pull it out of the rafters.
[image]

The inside
[image]

You can see the angle cut corners
[image]

1/4" thick, 2" outside depth
[image]

Here is the plugged end. 2" tall x 7" 1/2" wide, wood plug in the end.
[image]

A cut out for a ceiling duct.
[image]

I had thought about possibly creating this out of metal as was suggested, but then decided this is not good in this RV attic situation. The unvented RV attic can get way over 100 deg F in the summer. The metal conducts heat well and cold too. When the AC runs, the metal hot duct is an added heat load due to the stored heat energy in the metal. And when it starts cooling, the cooling then can convect off into the attic. This appears to add high inefficiency to the system. The RV industry used foam board with foil on the outside. The outside foil I believe can help reflect heat off the duct and the foam board at least does not conduct heat or cooling as fast as metal would. Plus with the RV, foam board is mostly likely cheaper, lighter and easier to work with = $$$ saved.

I found 1/4" residing board at Home Depot, but you need to by 50 ft of it. It's an option, just there will be a ton of waste doing 1 or 2 campers. https://www.homedepot.com/p/Owens-Cornin........oard-Insulation-Sheathing-21UM/100320301

If anyone else has something to add on where to get the 1/4" foam board, ideally with foil on one side, it will be greatly appreciated.

Thanks

John

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