View source for Talk:Heavy Duty Armour/@comment-41.50.83.214-20181011142706/@comment-5821715-20190301125132

This is true.  The mass penalty is a % increase, and since lightweight alloy bulkheads have a mass of 0, they do not get any heavier.

Other modules have similar quirks, such as the Detailed Surface Scanner upgrade.  Increases the scan radius at the cost of % mass increase.  Since it also has 0 mass, there's no mass to multiply.