OK, so I am a bit out of my comfort zone here

[Nukedaddy]

I was a steam and hot water guy for more years than I care to remember. But I bought this house in South Mississippi and the furnace AC system is creeping past 20 years old. Big brick house, 12/12 pitch simple truss, plenty of room in attic. The existing ductwork looks like a knuckle-dragging mouth-breather put it together.
It lies directly on the bottom chord (joists) of the truss.
If I replace the system, with the air handler supported up around waist high and run new duct, disconnecting existing flex and connecting to the new when near complete, I want to know..... Would I be crazy to make the trunk runs out of round pipe? I would leave the inside smooth, and use wraparound insulation on outside. The branches would be mostly 8" a couple of 6". I am guessing the main trunk would start with 16" round and taper to 12 after the first 2 or 3 branched take off. The trunk would come off the airhandler and go in opposite directions with the number of branches on each side about equal. What do you think? Am I nuts? I really hate seeing duct and pipe laying on the floor in the insulation for the lack of proper hangers and a more workmanlike design.

 

[WorthFlorida]

Here is a you tube with a powerpoint presentation. It give a lot of information on rigid vs flex duct.

 

[Sponsor]

 

[Dana]

Good video- covers a lot of territory, but tells it like it is!

When replacing a system it's THE opportunity moment for getting the sizing right, which can make all the difference! It's pathetic how many HVAC contractors (knuckle dragging mouth breathers?) are slapping in AC using an old school "ton per 400 feet of floor area" rules of thumb, which is about right for air-leaky tarpaper shacks with clear glass single pane windows all facing west, and furnaces at 30-40 BTU per square foot of space. Real houses don't have loads anywhere near that size, and in Mississippi oversizing the AC by 2-3x usually results in houses that are either sticky-warm or clammy-cold inside during the cooling season due to the poor latent load handling that comes with operating at a low duty cycle.

Most existing homes that have been tightened up a bit will have a ratio of a ton per thousand feet, often higher in newer houses with better windows or decent shading factors. An HVAC consultant in Atlanta pulled together the load and size data on a wide variety of houses his company had done the load calculations on and came up with this graphic, plotting square feet per ton against the size of the house:

The only reliable way to know where YOUR house is going to fit on that plot is to run the Manual-J, but you'll note that smaller houses are usually under 1500' per ton, but many 3000' and up houses are way over that. In this sample set NO houses of 4000' or larger came in below 1000' per ton.

So... have a qualified third party ( not an HVAC contractor) run the Manual-J using aggressive assumptions before soliciting quotes on equipment & new ducts. You'll save more than the cost of the engineering on smaller equipment & higher as-used efficiency, and end up at higher comfort levels to boot.

 

[WorthFlorida]

I would recommend get the AC guys to do all the work if you want to change out your duct work. The attic will be hot, breathing in insulation fibers floating around and the contractor will have all the parts on hand and I gather you're not 25 anymore. Neither am I.

In the southern states just about everyone uses flex duct in residential homes. The old duct work may not work well on a new system. It could be filled with dust, the previous owner may have had pets, etc. and the diameter may be too small. A lot of efficiency comes from having large volume of air moving throught the air handler and rooms at a slower rate than years past, therefore the main trunk needs to be sized correctly. It's a common practice that I've seen that the flex duct is laid down on the attic floor. When hung from straps you can get too many crimp as shown in the video.

Do replace your AC. This year my son replaced his AC unit (3.5 ton seer 16, Altamonte Springs, FL) that was installed in 1994. His peak summer electric bill went from $350/month to $225 with a pool pump running 10 hours a day and an electric water heater. About ten years ago the ductwork was replaced by the previous owner.

 

[Nukedaddy]

Great! The guy in the video seems to agree with me even if he did not mention round metal mains.
I think I will get an engineered system design from a third party and specify round steel duct mains with outside insulation. No kinks hanging that. I would use flex only for the last 10' or less before the registers.
I would love to make the attic "conditioned", but a 12/12 pitch over 26oo SqFt???? Mother, save me! Dat's a lotta foam dere, Boss!!
Maybe one of the best ideas is the added in dehumidification system. and the variable flow compressors? I'll have to check on pricing to see if there is a return on investment.
But the main thing is to find if my existing duct sytem is as bad as I think.

 

[Dana]

Matt Risinger calls out hard-piped trunks, and limiting the flex runs to registers to under 10'.

Even though it's quicker when you have the budget for it, you don't need to go with an all-foam solution for a conditioned attic. In US climate zone 2A the IRC precriptives would allow you to get away with 1" of closed cell foam on the under side of the roof deck, with another ~R32 (batts or blown) to bring it up to the code minimum R38 directly under and in contact with the closed cell foam. If it's a simple roof line that can be vented soffit-to-ridge everywhere, it's also possible to do it with fiber insulation, no foam, as long as there is an unobstructed channel at least 1" deep between the roof deck and insulation from soffit to ridge.

There are other solutions too. Although it'll never be as cheap as R38 cellulose on the attic floor, code minimum performance on a U-factor basis can still be pretty cheap (compared to a spray foamed roof) doing it as a DIY (or with low-cost outside labor) in a vented design using high density batts in the rafters (thin enough to guarantee the vent space) and USED rigid roofing foam in a continuous layer mounted to the under side of the rafters.

How deep are the rafters?

Are you planning to re-roof any time soon?

Some 2600' houses can be reasonably heated & cooled with ductless mini-splits, others are more awkward. But with ductless mini-splits the load is lower, due to much reduced air infiltration and no parasitic duct gains/losses in the hot-in-summer-cold-in-winter attic. With no ducts or air handlers in the attic its cheap & easy to air-seal and insulate at the attic floor.

If you look at Bailes' chart in my prior response, a middle-of-the-road 2600' house would have a likely total cooling load of at per ~1300', or ~2 tons. A 2-3 ton 3-4 zone ductless multi-split heat pump & air sealing and insulating the attic floor is probably going to come in cheaper than a 2-3 ton ducted system (with complete duct replacement), and going with the conditioned attic.

Converting to a conditioned attic would make that space more useful for other purposes (for storage, crazy-aunt's quarters, bootleg distillation or clandestine grow-room, etc.) but for lowest overall cost higher efficiency leaving the attic unconditioned to keep the insulation costs cheap and going for the more expensive ductless cooling & heating may have a lower bottom line.

 

[Reach4]

Converting to a conditioned attic would make that space more useful for other purposes

One bonus of the conditioned attic is that stuff you store in the attic does not get the dust that comes in vents.

 

[WorthFlorida]

If you do not have a conditioned attic space flex duct may be better. MISS is the same as Florida with heavy humidity. You may have a better chance of condensation on hard metal ducts where as flex duct has almost none. Something to consider. There is rigid insulated board (made of Fiberglass and foil) A home I had built in 1990 used this to make ducts. It's readily available and any AC guy know how to use it. There are prefab boxes made of the same material for plenums and boxes to branch out to individual rooms. I know at one time and were sold at Home Depot prefab ducts that were folded and all you did was open it up and tape one corner. Available in several rectangular dimensions. Not sure if they are still available since every one used flex duct.
http://www.homedepot.com/p/Master-Flow-48-in-x-120-in-Duct-Board-R-6-DBOARD1/202974245
https://www.gaf.com/Other_Building_...ible/Master_Flow_Ductwork_Product_Catalog.pdf

 

[Jadnashua]

A smooth duct will almost always have less turbulence than a flexible one, and be both quieter and more efficient (assuming you insulate it properly). By its nature, though, a flexible one can't be bent into a sharp turn (and still work!), but it will not be as smooth inside. It is the turns that must be managed to have a rigid duct work well. Sizing is critical on any ductwork you design.

 

[Dana]

I've recently been looking at "diffusion vent" schemes for using fiber insulation in unvented roof assemblies as a cheap but still fairly moisture safe way to insulate at the roof deck. With a diffusion vent the soffit vents are made air tight. The ridge vent is made wide by removing ~3" of roof decking on both sides of any ridge (including hips of hipped roofs), but made air tight with an overlay of highly vapor permeable shingle underlayment, and covered over with standard ridge venting mesh & shingles.

It's a bit outside of code prescriptives- the code inspectors may throw a fit, but there's plenty of reason to believe that it will work in US climate zone 2A, Mississippi.

Read this.

And this.

On a 12:12 roof installing a diffusion vent isn't likely to be a DIY job for most people, but air-sealing and insulating the attic and underside of the roof deck from the interior might be.