Yeah, so this is why I wish these men would just be willing to ask for help because I have no idea what you are saying. :)I'm not sure what a top plate is. Is that the top of the wall. So if the top of the wall is not load bearing (it isn't) it doesn't need to be attached to the ceiling somehow?

Here's what happened to me: Last Sunday morning, it got really cold, so, for the first time this year, I turned my heat on. Not fifteen minutes had passed before I smelled this AWFUL smell, like burning hair, and my smoke alarm went off. :-( Now, to explain a bit, I live in a fairly new trailer, which I got in June 2014, and, all last winter, I didn't have any trouble with rats or mice. But, for a few weeks prior to turning the heat on, I had been hearing sounds like mice or rats scratching around in the ductwork. Still, I didn't really worry about it, since I had had mice in the ductwork of my old trailer, which I had from 1993-2014 every single winter, and none of them had ever caught fire. They would just run around like crazy, when it would first kick on, and then, I would assume, they simply got the hell out of Dodge, and waited for it to kick off again, before they would get back up in there. Whenever I would actually see them INSIDE the trailer, I would set traps for them, and dispose of them, of course. But, as for them being up in the ductwork, I just figured "Out of sight, out of mind", and didn't really worry about it. I had heard quite a bit about mice and rats chewing on the electrical wires inside the walls of trailers, and causing them to catch fire in the winter, when the heat would kick on. So, needless to say, when that one caught fire, it scared the hell out of me, and I immediately turned the heat off, not wanting to run the risk of my trailer burning down, particularly with me in it. ;-( So, yesterday, I put some poison traps down under the trailer, but, the whole time, I kept thinking: "What if they eat this and crawl off back into the ductwork and die?" "How will I ever get them out then?" :-/ I realize now that it was the wrong thing to do, and will take them out, and use snap traps in the vents instead. But I guess I was just wondering whether anybody else had had the same experience, with one actually catching fire like that, and the smoke alarm going off? You'd think mice would have better sense than to just hang around and let themselves get singed like that, unless it was actually STUCK in there and COULDN'T get out? That's the possibility that worries the hell out of me. :-( Do they ever go up there and then get stuck?

Thank all for the advise. I may have confused a few people by not explaining that the heat pump will be in a conditioned mechanical room. I got some feedback on how it is usually done in this area. It is recommended that you get an HVAC expert to sec out your system and the location of the ducts. You would then order your trusses with "duct chases" cut out of them. The contractor then builds open boxes out of plywood for the chases and the ducts are installed. The attic is then insulated, the HVAC system is tested for leaks etc and then the ceiling material is installed. If there are tray ceilings or rooms with lower ceilings in those areas the duct chases would not be required. What are your thoughts on this solution? It sounds similar to what several of you have suggested. Sorry, my other discussion on "Three Foot Crawl Space" has ended up as a duplicate posting.

Okay, I think I'm starting to see the what's going on. On the other side of this wall is a peaked roof that only goes part way up (see pink lines in picture below)? The duct enters from the tip of the peak into the middle of the wall (blue circle) and then travels up the wall into the attic (blue rectangle). If that's the case, then as someone suggested above, the only option might be to come into the room in the bottom right corner and go up from there. Are you in a pretty mild climate? Because that attic space looks like its open to the outside and the duct isn't insulated. That seems weird to me.