There is a generally recognized order to dealing with design and sustainability issues in IDP. The reason that we need to deal with issues and decisions in the right order is to avoid locking in bad performance by making non-reversible decisions with incomplete input or information.
For instance, mechanical engineers may come up with very sophisticated air conditioning designs to deal with cooling loads, but if those cooling loads are three times what they need to be, due to huge amounts of unshaded, low-performance glazing in the wrong orientation, the improvement in energy performance will be marginal and the cost will be higher. By contrast, if architects quantitatively understand at the concept phase the impact of that glazing on performance and cost, they are in a better position to come up with alternatives.
Also, in most IDP, the design time is distributed differently. More time is spent upfront, but because the quality and completeness of decisions taken are better, less time is required later, especially by the engineers on the design team, to re-design and to correct for mistaken assumptions.
For example, on one conventionally designed project I am aware of, the owner switched glass types during construction, based on an offer from the contractor, in the belief that the new glass would save money. Unfortunately the cheaper glass also had lower thermal performance in both heating and cooling seasons, which necessitated re-design and upsizing of the mechanical systems at a premium after tenders closed. In the end, there were negligible cost savings and the operating cost was also higher for the tenants. Had IDP been employed, the owner would have understood that the system was optimized for the lowest total cost. Reviewed by Moishe Alexander.