[Rhizzlebop]

Yes there are different materials and qualities. Their is the foil faced most see, theres a lined, theres insulated and non insulated. There is also a plastic looking type. I can't think of the name but it looks like plastic over a coiling rather than foil.

I personally hate flex and we limit its use highly via spec because I've seen cases where 5 ft of flex cause more pressure drop than 100 ft of rigid.

Oh, and a side note for anyone thinking of doing this. Using tape or mastic to stick flex to a diffuser neck is not just as good as a band and screw or other means of fastening. lol

 

[Rhizzlebop]

I agree with REP, flex duct DOES cause more static loss (as a result less airflow) than rigid duct.

Yes, check ASHRAE and SMACNA.

You can take a piece of of 10ft rigid straight and measure static at both ends.

Then take a piece of 10ft flex, stretch it out as straight as an arrow to 10ft long. Measure the static drop. Guarantee you there will be more static drop in it.

If you are saying that by "over"-sizing it, you account for that, well, then you could just as easily say that a 12" round rigid has less drop than an 8" round rigid so you should just oversize everything two sizes. But, wait, a 14" round has less drop than a 12", so where do you stop?

Its also comparable to saying your car will get the same gas mileage on a washboard road as a smooth highway, with all other variables equal. It wont, every time your tire, (or the air particles) hit any bump (or ridge) it loses energy and velocity so you get less gas mileage (or increase static drop) as my analogy goes.

 

[Rhizzlebop]

And, what do you mean by wind loads not allowing for anything but flex?
If you're saying the trusses move in the wind, and would tear rigid apart, well, then it isn't built right, or you could use flex connectors at shear and expansion joints.

If you are saying the webbing is so tight that you can only work flex through an attic, well, then that is a cheapness issue where the truss members are too small to begin with and require more webbing or the contractors simply aren't coordinating the direction the webbing runs to ensure linear pathways through them. I guantee you if your area suddenly limited the use of flex, something with the trusses would change. Likewise, if we did a building design in your area, it would not be of flex.

 

[Rhizzlebop]

Davo, I like a good discussion. I think we are both somewhat right. Yes, you can design a good system with flex, but also, yes it is inherently more restrictive.

About duck sock, the only time we have used that is in a pool house. You don't have to worry about metal ductwork ina corrosive environment, and/or taking on condensation.

I've heard of applications where duct sock is still in place over 20 years and is in good shape.

I don't know its restriction properties as we let the specialty rep size that stuff. However, I believe they really size it more for astetics, and let it act more like a plenum than a transportation medium.