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Why unconditioned space duct runs matter
Ductwork routed through an attic, crawlspace, garage, or unconditioned basement is constantly exchanging heat with a space that's much hotter or colder than the conditioned air inside it. In summer, a supply duct carrying 55°F air through a 130°F attic can gain enough heat to noticeably raise the temperature of the air actually reaching the register. In winter, the same duct carrying 120°F heating air through a 30°F attic loses heat the same way. This calculator quantifies that loss or gain directly.
The conduction heat loss formula
Duct wall heat transfer follows Q = (A x DT) / R, where A is the duct's exposed surface area in square feet, DT is the temperature difference between the duct air and the surrounding unconditioned space, and R is the insulation's thermal resistance. This is the same basic conduction formula used for building envelope heat loss, applied to the thin sheet metal or duct board wall separating conditioned air from an unconditioned space. Use the duct surface area calculator first if you only know duct dimensions and length.
Why the space temperature isn't the outdoor design temperature
A common design mistake is using the outdoor design temperature for this calculation instead of the actual unconditioned space temperature. Attics routinely run 20-40°F hotter than outdoor air on a sunny summer afternoon due to solar radiant heat gain through the roof, while an unheated crawlspace or garage tracks closer to outdoor conditions but rarely matches it exactly. Standard HVAC design references provide typical temperature offsets for common unconditioned spaces; use the actual expected space temperature, not outdoor design temperature, for an accurate result.
Rule of thumb vs. calculated loss
Industry practice often estimates duct heat loss as a flat 1-5% of total system capacity for well-insulated ducts in semi-conditioned spaces, but uninsulated or poorly insulated ducts in a hot attic or cold crawlspace commonly lose or gain 10-30% of system capacity, sometimes more. This is why most energy codes set a minimum R-6 or R-8 insulation requirement for ducts in unconditioned locations. If system load is entered, this calculator shows the calculated loss as a percentage of that total for direct comparison against these rules of thumb.
Frequently Asked Questions
Duct heat loss through conduction is calculated as Q = (Surface Area x Temperature Difference) / R-value, where Q is in BTU/hr, surface area is in square feet, temperature difference is the difference between the air inside the duct and the surrounding unconditioned space in degrees F, and R-value is the duct insulation's thermal resistance. A lower R-value or larger temperature difference increases heat loss proportionally.
Uninsulated or poorly insulated ductwork running through a hot attic or cold crawlspace commonly loses or gains 10-30% of the total system capacity, well above the 1-5% industry rule of thumb used for properly insulated ducts run through conditioned or semi-conditioned space. This is why most energy codes require a minimum R-6 or R-8 insulation value on ducts located in unconditioned attics, crawlspaces, and garages.