Overall earth shelter construction is often viewed by architects, engineers, and the public as an unconventional method of building. Techniques of earth sheltering have not become common knowledge, and much of society is unaware of this type of building construction. Generally speaking, the cost of excavation, increased need for damp-proofing and the requirement for the structure to withstand greater weight relative to above grade houses means that earth sheltering remains relatively rare. In this respect, the Passive House (PassivHaus) energy performance standard applied to above grade airtight, superinsulated low carbon or zero carbon buildings has had much wider uptake in modern times. Over 20,000 buildings certified to PassivHaus standards have been constructed across Northern Europe. Some postulate that over time the reducing availability of building space, and the increasing need and interest for environmentally friendly housing will make earth shelters more common.