most expensive feature to add to a building. Energy efficiency for green buildings can be evaluated from either numerical or non-numerical methods. These include use of simulation modelling, analytical or statistical tools. Water efficiency Reducing water consumption and protecting water quality are key objectives in sustainable building. One critical issue of water consumption is that in many areas, the demands on the supplying aquifer exceed its ability to replenish itself. To the maximum extent feasible, facilities should increase their dependence on water that is collected, used, purified, and reused on-site. The protection and conservation of water throughout the life of a building may be accomplished by designing for dual plumbing that recycles water in toilet flushing or by using water for washing of the cars. Waste-water may be minimized by utilizing water conserving fixtures such as ultra-low flush toilets and low-flow shower heads. Bidets help eliminate the use of toilet paper, reducing sewer traffic and increasing possibilities of re-using water on-site. Point of use water treatment and heating improves both water quality and energy efficiency while reducing the amount of water in circulation. The use of non-sewage and greywater for on-site use such as site-irrigation will minimize demands on the local aquifer. Large commercial buildings with water and energy efficiency can qualify for an LEED Certification. Philadelphia's Comcast Center is the tallest building in Philadelphia. It's also one of the tallest buildings in the USA that is LEED Certified. Their environmental engineering consists of a hybrid central chilled water system which cools floor-by-floor with steam instead of water. Burn's Mechanical set-up the entire renovation of the 58 story, 1.4 million square foot sky scraper. Materials efficiency Building materials typically considered 'green' include lumber( that has been certified to a third-party standard), rapidly renewable plant materials (like bamboo and straw), dimension stone, recycled stone, hempcrete, recycled metal (see: copper sustainability and recyclability), and other non-toxic, reusable, renewable, and/or recyclable products. Materials with lower embodied energy can be used in substitution to common building materials with high degrees of energy consumption and carbon/harmful emissions. For concrete a high performance self-healing version is available, however options with lower yields of pollutive waste entertain ideas of upcycling and congregate supplementing; replacing traditional concrete mixes with slag, production waste, and aggregates. Insulation also sees multiple angles for substitution. Commonly used fiberglass has competition from other eco-friendly, low energy embodying insulators with similar or higher R-values