Yes, but the ducts themselves are normally insulated too, so either you're getting heat through the insulated walls of the ducts, or you're getting heat through the roof and walls (on 2 sides) into a much, much larger volume of air, which you're now using more energy to keep cool. I'm not sure which one is going to be more wasteful. This does make the ductless ("mini-split") HVAC systems look more attractive, however those lose the advantage of economy of scale (having a bunch of small ACs instead of one big one), but might gain some back by making it really easy to not waste energy cooling all the rooms if you only want some of them kept cooler.
> Yes, but the ducts themselves are normally insulated too, so either you're getting heat through the insulated walls of the ducts, or you're getting heat through the roof and walls (on 2 sides) into a much, much larger volume of air, which you're now using more energy to keep cool.
It's not an either-or though. If you have a vented attic, you're still getting heat through the roof and walls into the attic, and you're getting much more of it because it's not insulated (because it isn't a conditioned space). And then that heat is going to try and go through the relatively poor insulation of your HVAC ducts. At least in the south, it's common for a vented attic to get even hotter than the ambient outside temperature, going well above 120 degrees or more. Conversely, when you're going with a conditioned attic, you would insulate the ceiling of the attic, blocking as much of the heat from getting in as you can. And then your HVAC ducts only need to insulate against a gradient of maybe 74 to 50 degrees rather than a gradient of 120 to 50 degrees or even more on an especially hot day.
Yeah, I understand all that, but now you're having to waste energy to condition all that volume, instead of just wasting energy overcoming the heat coming through the duct insulation. So I'm questioning which approach actually wastes more energy. Also, there are other options: you could install a fan to ventilate the attic, so that the temperature isn't higher than ambient outside temperature.
I think you’re underestimating the efficiency you get from having a well insulated conditioned space. You also mentioned mini-split systems as possibly being more efficient because they don’t “waste energy cooling all the rooms”, but that doesn’t really work because if you only cool some of the rooms in your house you just end up with those AC’s fighting a constant temperature gradient as the heat from the rest of the house rushes into the conditioned room. And if you stop fighting that temperature gradient it’s only ever because you’ve cooled the whole house anyway. Likewise, if you have an insulated and conditioned attic, keeping it at whatever temperature you’re cooling it to doesn’t have to take a ton of energy.
Ultimately what this basically comes down to is that you can insulate the ceiling of your attic to a much higher R value than your ducts are going to be insulated.
> Also, there are other options: you could install a fan to ventilate the attic, so that the temperature isn't higher than ambient outside temperature.
I guess you could do that, but now you’re spending energy running that fan to cool the attic from maybe 120 degrees to 100 degrees, and hoping that your central AC saves enough energy from that to make up for running the fan? I dunno, it sounds pretty marginal to me, especially in terms of whether or not the fan is even going to pay for itself.
