Attic Vs Crawl Space
This is a big deal and has the potential to add significant cost to air sealing insulating crawl space encapsulation and all those things we do in confined spaces.
Attic vs crawl space. As a great deal of heat can escape through the attic of a house it is common for this area to be insulated. In a crawlspace you have to worry about critters and duct losses. My vote would be the crawl space if there is sufficient room and easy access for service. So when we have a choice we prefer hvac systems located under the house.
Rarely will a sealed foam insulated attic deviate more than 15 degrees from the living space temperatures below it. You can expand it just enough to accommodate the ducts. Insulate and air seal the floor with open cell foam put ductwork and mechanicals in the attic install a radiant barrier and mastic seal the ducts. If there were a way to eliminate the possibility of critters i would prefer the cooler location for ducts.
Expanding the conditioned space doesn t require you to condition an entire attic or crawl space. A two room addition on the 1st floor has limited attic space and a 4 crawl space. It s easy to see your point about eliminating ductwork from the attic. However i have two questions if i want to centrally air condition my entire house.
In the attic you have worry about heat gain and duct losses. With new construction i would look for a way to have them in conditioned space. If it applies to energy auditors and raters too that could really be difficult because many times they go into a house on their own. Strap ducts out of insulation with 3 ductstraps per specifications.
Compare that with the daily 140 degree temperatures found in vented attics. Seal supply boxes with mastic. Knowing the disadvantages of attic install and the crawl space install. If you have vaulted ceiling then you will need to install some registers near the ceiling on one of the walls.
I vote for the attic. O is air conditioning the 1st floor effective with floor venting if i run ductwork in the basement.