Best place for duct: under floor or over ceiling?
All other things being equal, where is the best location to run duct? With duct under the floor, to floor and baseboard registers, or with duct in the attic, with ceiling registers? By best location, I mean a combination of comfort and efficiency.
Assume semi conditioned crawl for floor runs, and insulated rafters with no venting for ceiling runs. Assume climate zone 3A.
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I'm not sure what you mean by a "semi-conditioned crawl." If you are thinking about installing ducts in a crawl space, the crawl space needs to be sealed (unvented) and it needs to have insulated walls. Here is a link to an article with more information on this topic: Building an Unvented Crawl Space.
Similarly, it you are planning to build a house with "insulated rafters," it's important to get the details right. Here is a link to an article about unvented attics: Creating a Conditioned Attic.
So, the answer to your question -- "Where is the best place for ducts?" -- is "Inside your home's conditioned envelope." As long as your ducts are indoors, not partially outdoors, your ducts can be under your floor or over your ceiling. Either way works just fine.
Other issues -- proper duct design, proper duct sizing, minimizing of duct elbows, and sealing of duct joints -- matters much more than whether the ducts are under your floor or over your ceiling.
Yes, unvented, insulated & sealed crawls are typically referred to as semi conditioned crawls here. Planning for all the details typical for that. Similar with the attic. You are correct, it would have been more correct for me to write that with either configuration, the ducts will be in the conditioned envelope. Also, agree, the system must be properly designed and installed, and tested/commissioned.
Also, I'm aware that supply (or return) registers at floor level can sometimes be challenging if they are blocked by furniture.
However, all other things being equal, which is better for climate zone 3A - forced air supply at floor level, or forced air supply at ceiling level?
Based on the question I THINK you are asking, if I were in your shoes and had no other option- I would opt to put the ducts in the attic and insulate the hell out of them and bury them in cellulose after they are sealed with spray foam. Then the penetrations should also be spray foamed well too before the cellulose as well. I mean, you could effectively bring that into the envelope with closed cell spray foam. I only say this because you should be able to insulate and seal them better than below the crawl space. But I guess it would depend on what each space looked like and how much room you had for insulation and to work in, in each comparative space. If you have a 4 foot tall crawl and a 2/12 pitch on your roof with no room to crawl around up there in the attic, I might say the reverse. Also, your AC might be more effective coming down from overhead. Can you do ductless mini splits instead?
I'll repeat the opinion I expressed in Comment #1: either way can work just fine, as long as the duct design is competent and the installation quality is high.
These factors -- competent duct system design and competent insulation -- are the crucial factors. These factors matter much more than whether ducts are located in a crawl space or an attic (as long as the crawl space and attic are inside the home's thermal envelope).
IMO it entirely depends on the design of the house (GLA, # of stories, ceiling height).
Two story then registers in the floor on the first floor and ceiling on the second floor. I just believe that with a multi-story home floor registers on the ground floor allow better mixing during the heating season. One story home it won't matter
Option three: Ductless.
Thanks for the replies. Ductless is not an option due to it's aesthetic. It will be a ducted system. House is about 2/3 one level and 1/3 two level.
So ALL other factors being equal (ducts within conditioned envelope, competent design & installation, etc. etc.) the responses thus far are:
1. It does not matter if the supplies are at the floor or the ceiling (Martin Holladay)
2. Supply at the ground floor to allow for better mixing during the heating season (Chris M)
3. Supply at the ceiling because AC might be more effective coming from overhead (Joe S)
Since there is no consensus on that, what about where the returns should be? Any agreement on that?