Over its lifespan, a conventional building constructed to basic energy standards in the US consumes about 84% of its lifecycle energy during the buildings use phase (heating, cooling, and lighting).Only 16% of a typical buildings lifecycle energy is consumed during the construction phase (as in the manufacture of building materials).In most cases, to make an overall reduction in the lifecycle environmental impact of this type of building, it is far more important to focus on the buildings energy use during its lifetime than it is to focus on what materials are used in construction, except when those materials impact the overall operations energy.There are two exceptions: energy efficient buildings and buildings that use lower-impact sources of operations energy like solar, wind, or hydroelectric.In these cases, the embodied environmental impacts in the buildings materials may exceed those associated with the buildings operations over its lifetime and so it is critical to consider the environmental impacts of the manufacture and eventual end-of-life of construction materials as a way to reduce the buildings overall lifecycle environmental impact.In this paper, I show how life cycle assessment (LCA) data and methodologies can be used during a buildings design phase to estimate the relative environmental impacts of building materials and building operations energy to make design decisions.In particular, I will show how LCA can be used to determine whether improved building construction (such as increased insulation) can be "paid back" environmentally in increased building efficiency and how to use LCA to avoid inadvertently shifting the environmental burden from the use phase to the construction phase in green buildings with no overall lifecycle environmental savings.