BA-0305: Why It's So Important (and Troubling) to Keep Ducts and Equipment in Conditioned Space

Perhaps the single most challenging BSC performance for Building America production homebuilders is that all ducts and HVAC equipment must be within the conditioned space (this means no ducts in outside walls and no ducts or air handlers in garages, vented attics or vented crawlspaces). It’s an important and even driving element for each of the four case studies in this report. So, why all the heartache over this single design element? There are really two sources of the heartache—how important it is to locate all ducts and equipment in conditioned space and how hard it can be for homebuilders to achieve this.
Perhaps the single most challenging BSC performance requirement for our Building America production homebuilders is that all ducts and HVAC equipment must be within the conditioned space (This means no ducts in outside walls and no ducts or air handlers in garages, vented attics and vented crawlspaces). It's an important and even driving element for each of the four case studies in this report. So, why all the heartache over this single design element?
There are really two sources of the heartache—how important it is to locate all ducts and equipment in conditioned space and how hard it can be for homebuilders to achieve this.
Figure SC-7: Conditioned crawlspace
It is clear that we need an air distribution system that is as high performance as the rest of a Building America home. We don’t design a home for wiring and plumbing runs because they are small enough to go in most any interior interstitial space, except for maybe one, by design, two-by-six plumb wall (and the sales and marketing folks seem to be OK with this loss of floor space). If we can get the efficiency of the air distribution system high enough to match the efficiency of our high performance building envelope, we just might be able to get the ducts down to a size to fit in available interior walls and presto, keeping the ducts inside is no longer a design issue. And that is just what BSC Building America project #3 is all about (see 2.C.1 Final Report: Results on Advanced Residential Systems). In the meantime, builders and architects need to design space for the ducts and air handlers into their homes or move the conditioned line down into or up into the crawlspace or attic respectively, and educate their home buyers (and building inspectors) on the importance and appropriateness of this approach. . .
Download complete report here.