About the worst place you can install a duct is up against the roof deck. Dave Roberts, a senior engineer at the National Renewable Energy Lab (NREL), wrote about a paper he co-authored a few years ago about the penalty for putting ducts anywhere in an unconditioned attic. Up against the deck may be the worst place in the attic to install ducts, but Roberts shows that putting them in the attic at all is the worst place in the house you can install ducts.
Ducts in the Attic? What Were They Thinking?
The report, Ducts in the Attic? What Were They Thinking?, summarizes the research that's been done about putting ductwork in unconditioned attics and basically says it's about the stupidest thing we do in homes that do a lot of air conditioning. I encourage you to download and read this report. If you're working on homes and have the option of getting ducts out of unconditioned attics, this paper may help make the case, either for you or someone else who needs convincing.
I love the analogy they use to introduce one of the main problems with this location. “Heat exchangers,” they write, “are designed to transfer as much heat as possible from one fluid to another.” Comparing this configuration to a solar water heater, they make the case that putting air conditioning ducts in a hot attic is an effective way to heat up the conditioned air as it travels from the air handler to the conditioned space inside the home.
Since you’re in the HVAC business, you no doubt know that the rate at which heat moves from a warmer to a cooler body depends on the temperature difference, which we abbreviate as ΔT. An attic can get up to about 130° F in the summer, and the conditioned air entering the ducts is about 55° F or so. With hundreds of square feet of ductwork surface area in the attic and a ΔT of 75° F, the air coming out of the vents in the home will be significantly higher than the temperature it was when it left the air handler (~55° F). Throw duct leakage into the mix, and the problems are even worse.
How much can it save to move ducts inside conditioned space?
What Roberts and his co-author Jon Winkler did, in addition to reviewing the literature about this topic, was to model the savings possible when you relocate the ducts from an unconditioned attic to the conditioned space inside the building envelope. They chose Houston, Phoenix, and Las Vegas as the locations for their modeled houses. The table below summarizes the main results.
In addition to comparing ducts in the attic to ducts inside the building enclosure, Roberts and Winkler also looked at electricity savings of other measures, such as adding insulation, installing better windows, and using higher efficiency air conditioners. The graph below (Fig. 5 from their paper) shows that moving the ducts inside is the first thing you should do to save more electricity than all other measures.
In addition to saving on air conditioning operating costs, the cooling capacity is lower in efficient homes. Roberts and Winkler looked at the effect on cooling capacity from moving the ducts inside and compared that to other building enclosure improvements, Again, moving the ducts inside beats all the other methods for achieving this objective. As you can see in Fig. 4 from their paper, it’s not even close.
This report, which the authors delivered at the ACEEE (American Council for an Energy Efficient Economy) Summer Study in August 2010, shows clearly that putting ducts in attics in cooling dominated climates is a practice that needs to end.
Download the paper here: Ducts in the Attic? What Were They Thinking?