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indc08
duct - fiberglass vs metal?

Hi,

My first time here and a beginner home renovator. I was told there are ducts made out of fiberglass instead of metal, and they seem to have advantages. I am in DC metro area, and I was told homes up here don't use fiberglass duct, but homes in FL use them. Does anyone know the reasons why?

Thanks!

A. Spruce
Re: duct - fiberglass vs metal?

There are advantages to both.

Fiberglass is faster and easier to install. It's actually a flexible plastic duct with a fiberglass insulation wrap. The insulation means that less thermal loss/gain occurs to the air inside the duct in relation to the space the duct is located. Being that it's flexible, it is easier to route around obstacles. The major disadvantage is that it is susceptible to rodent damage.

Metal ducting is resistant to rodent damage, but suffers more thermal loss/gain, even when insulated. It's more expensive and time consuming to install.

There are probably more pro's/con's to each, these are the biggies that I can think of.

rdesigns
Re: duct - fiberglass vs metal?

Besides what Spruce says, flexible fiberglass ducts create more friction loss due to their corrugated surface. You will get about 20% less airflow thru a flex duct, compared to smooth metal duct of the same inside diameter.

OTOH, flex duct systems are generally more airtight because they have no seams, and the only joints are at the beginning and end of each run. When these joints are sealed tightly, there is virtually no leakage of air. To achieve the same air-tightness with metal requires tedious sealing with mastic or metal tape.

Like any product, there is a right way to install flex duct, and there's a sloppy way. The instructions that come with it tell the right way.

For a DIY'er, flex duct can be the better, easier way to go.

Sten
Re: duct - fiberglass vs metal?

One of the places that I work at got a notice from the
Manufacturer (Hart and Cooley) telling what the outcome was on Air Flow Studies with Flex Duct. They told the friction loss because of dips between ceiling joists, length, and if they weren't pulled tight enough. There was more but the paperwork I had is gone. As rdesigns says all this may be in the box that the Flex Duct comes in even though I have never seen it there. Sorry, my knowledge is limited but I can tell you about duct leakage.

Sten
Re: duct - fiberglass vs metal?
rdesigns wrote:

Besides what Spruce says, flexible fiberglass ducts create more friction loss due to their corrugated surface. You will get about 20% less airflow thru a flex duct, compared to smooth metal duct of the same inside diameter.

OTOH, flex duct systems are generally more airtight because they have no seams, and the only joints are at the beginning and end of each run. When these joints are sealed tightly, there is virtually no leakage of air. To achieve the same air-tightness with metal requires tedious sealing with mastic or metal tape.

Like any product, there is a right way to install flex duct, and there's a sloppy way. The instructions that come with it tell the right way.

For a DIY'er, flex duct can be the better, easier way to go.

You forgot to tell them that while it's easier to put in it really only last about 3-5 years before the Ductboard starts to sag and fail.

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