Duct Insulation Calculator

Two outputs in this calculator split cleanly along different sets of inputs, and knowing which is which changes what you should adjust to hit a target. Insulation material needed depends only on duct geometry -- Duct diameter (or width) and Total duct length both raise it monotonically across their full declared ranges -- and is completely inert to Insulation R-value, Supply air temperature, and Ambient temperature: you need the same square footage of wrap or board to cover a given duct run regardless of how much R-value that wrap provides or how hot the air running through it is. Heat loss reduction runs the opposite way: across their full declared ranges, Duct diameter (or width) and Total duct length are both completely inert on it, because the percentage improvement insulation buys over bare duct cancels the duct's surface area and temperature difference out of the ratio, leaving Insulation R-value as the dominant, near-sole driver of how much of a percentage cut you get.

That cancellation isn't perfect at the extremes, though: this calculator floors the bare-duct heat-loss denominator at 1 BTU/hr to avoid dividing by zero, so very close to the point where Supply air temperature equals Ambient temperature -- where there's almost no heat to lose in the first place -- the reported percentage reduction can swing away from the R-value-only relationship that holds everywhere else. Supply air temperature's effect on Heat loss (insulated) is not one-directional: the formula uses the absolute difference between Supply air temperature and Ambient temperature, so raising Supply air temperature only raises Heat loss (insulated) when Supply air temperature starts above Ambient temperature (a heating-duct run). In a cooling-duct scenario -- Supply air temperature below Ambient temperature -- raising Supply air temperature toward Ambient temperature shrinks that difference instead and LOWERS Heat loss (insulated), reaching zero right where the two temperatures match before rising again on the other side.