What is the outside sheet metal size of an insulated duct with an airside dimension of 18"x10" and 1" insulation?

To determine the outside sheet metal size of an insulated duct with given airside dimensions, it's important to account for the insulation added to the duct dimensions.

The problem provides an airside dimension of 18 inches by 10 inches, with 1 inch of insulation applied around the duct. This means that the insulation will add to both the width and height of the duct.

For the width, starting with an airside measurement of 18 inches, you add 1 inch of insulation on each side, resulting in an increase of 2 inches overall. Thus:

18 inches (airside width) + 2 inches (1 inch on each side) = 20 inches (total width).

For the height, starting with the airside measurement of 10 inches, again adding 1 inch of insulation on each side results in:

10 inches (airside height) + 2 inches (1 inch on each side) = 12 inches (total height).

Putting it all together, the outside sheet metal size of the insulated duct ends up being 20 inches by 12 inches, making this option the correct choice.

The other options do not align with the calculations based on the insulation added to the specified airside dimensions.