New HVAC question - duct heat/colling loss in attic

[MC]

I have a question about proper running ductwork in attic.

I have had someone tell me best to have ductwork in an attic next to and even run between ceiling joist when possible and have the installation blown in on top instead of having the ductwork hanging from several feet above the joists. Said that even with insulation wrapped around the ductwork, the heat/cold in the attic will eventually seep into the duct and will get a blast of unheated/hot air out when first comes on. By having the duct as close to the living space when possible and insulating as much on top of the ductwork keeps the heat/cold loss to a minimum.

I can see where this may make sense, But is this done in any real installations or would be praticle. I have seen somewhat both ways of being done, not so sure if that was intended when I have seen the ductwork laying between or across ceiling joists instead of being hung a few feet above ?

Any ideas as to do/not do it that way ?

Thanks, MC

[Dr. Hardcrab]

I'm not saying it's not done, but i have never seen duct "hanging" in an attic. Unless you were going to gain a little space, I don't see why anyone would go to the trouble......

[Joseph Meehan]

I have a couple of ducts in my attic and they sure are not hanging.

[Art]

My ductwork pretty much is on hangers in the attic but my builder was a moron.

[JimL]

I agree with him.

About 1700 homes in my neighborhood have all the ductwork hanging from the rafters.

[ds549]

one of my rentals was like that. when i put central ac in, i had to stack 2 feet of insulation around the intake up there to get it to cool the house.also 2 power vents . ill say one thing for the setup, its quieter than the intake in the wall.. lucas

[stretch]

MC,

If you ever have to work on or modify the ducts, it is nice to have it hanging. (But WELL supported.)

With a heat pump and R-6 ducts in the attic, here in South Ccarolina, the heat loss in the heating cycle is 5%. The heat gain in the summer (cooling) is 10%. Source: ACCA Manual J, 7th edition.

Hang it up so it can be serviced. Also, if you bury it, depending on the vapor barrier, sometimes the ducts can sweat, which can cause problems as well.

(Paul, I know there is a snide remark coming, so let 'er rip.)

Stretch

[MC]

Well, The house was one of the ones several years ago before Georgia got stricter on builders that was the run of the mill redneck built, anyone with a hammer was building houses, only needed to have a business permit and some may not have had that. Everything done about as cheap as it could have been and they called it custom built ?

About half of the ductwork was a flex type with a plastic covering that completely disintegrated in most places and the insulation is starting to come undone despite my attempt to rewrap and tape into place.

I am adding another room on, The Furnace/AC needs replacing so I am just getting advice on how the new unit should be installed, I have done some HVAC work in the past but not much on the duct work part.

Thanks for the info, any additional comments welcomed.

MC

[stretch]

Use the foil /mylar covered flex to replace the old flex. Use at least R-6 flex, R-8 If available. Flex duct should be sized at least 1 inch bigger in diameter than sheet metal to pass the same amount of air.

Seal ALL connections with mastic and fiberglass mesh tape, NOT duct tape.

Stretch

[TURTLE]

This is Turtle

Stretch , is that 1" Bigger flex duct than sheet metal duct sizes. Is this a ruff estimate or close to facts ? Not a loaded Question here !

TURTLE

Shut Up Paul !

[stretch]

Turtle,

The resistance to air flow is higher with flex due to corrugated inside surface in flex VS smooth surface with sheet metal.

ACCA sells a ductulator (duct calculator circular slide rule with scales for: Sheet metal, flex, duct board, lined sheet metal). Check out the same air flow with the same static; the flex will be one size bigger than sheet metal. The above ductulator is $45.00 to non members like me.

Just bought one because I lost mine. Bought it at their online store.

Stretch