I would question the time frame you are placing on buildings? 15 to 20 years seems shortsighted for the construction of a new building. A new structure should have aspirations of 50, 75+ years. Building systems can be updated, removed, recycled over that life, but to assume that a building payback and is disposable at 15 to 20 years seems ridiculous. Designing a "shell" to withstand time and can be easily upgraded for changes in use or upgrading for energy efficiency seems more reasonable and better for the environment.