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As a Midwesterner, it is absolutely bonkers to me how common it appears to be for HVAC systems to get installed in attics.
Don't do that. Stop doing that! WTF?
That's bad enough, but then you go and run the ducts up there, too?
Y'all.
If you keep the system and the ducts within the space you're trying to heat and cool, you don't have to account for any losses, now, do ya?
Put the air handler in a utility closet. Run ducts /below/ the ceiling. Enclose with soffiting if you must.
The end.
@TechConnectify although not super common yet (getting more so), my attic is inside the insulation envelope and is part of the heated/cooled area of the house.
@kurtisfranklin I take it that the attic is useful space, then?
That's my main hang-up, honestly. Some of it is semantic - an "attic" to me implies it's not a habitable space, and if you make it one, it's no longer an attic. And unless you're actually going to be up there regularly, why bother doing it at all?
If the only answer is to make HVAC more efficient, you could just... not run it through the attic.
@TechConnectify nope not useful at all. Think typical new construction attic, but instead of blown-in insulation sitting on top of the ceiling, the insulation is held against the roof line. Still many many inches of blown-in insulation, but instead of sitting on the “floor” of the attic, it’s held over your head.
@kurtisfranklin Then you're /really/ not selling it, gotta tell ya.
We (usually) just let the attic breathe and the blown-in insulation is something like R-50 these days.
If you're not doing anything with the space, it can get as hot (or cold) as you like without affecting indoor comfort or energy use. And if you're insulating roof rafters /in place of/ the ceiling, that honestly seems like it would be less effective.
@TechConnectify I think the piece you're missing here for southern climates is R value only defines resistance against heat flow assuming there's no air flow. The issue with blown insulation is that it doesn't provide an air barrier (nor vapor). And that's where the whole summer humidity problem comes from. The dew point in an attic can approach 80 degrees and that blown insulation is going to do nothing to stop condensation from forming on the 70-ish degree ceiling drywall, light fixtures, etc.
@jeremy_akers I'll admit I don't know enough about those mechanisms. We have stretches every summer where nights are just as hot and humid as in the South, but perhaps they're brief enough moisture problems aren't an issue.
However, I question whether insulating the roof rafters makes more sense than having a proper vapor barrier above the ceiling.
@jeremy_akers The other thing I'm really trying to pick apart is how much worse are moisture problems in attics when the equipment is up there?
Simply putting the cold-making thing and the ducts that distribute the air it makes cold inside the attic seems like asking for trouble. How much less would you have to battle moisture if you just removed it?
@TechConnectify I don't know. But in my Austin Texas home, I spray foamed the attic (The roof side) and it made a huge difference in our ability to control humidity. It seems silly to me to try and create a vapor barrier at the ceiling below the attic... which is damn near impossible given all the holes in a ceiling (Lights, fans, vents, etc) vs just putting it at the roof. It's not really more costly to do the vapor barrier at the roof either.