Building Science (known as Building Physics in Europe) is really a very old blend of science, craftsmanship and art. The science goes beyond the mathematical use of material parameters to calculate thermal or energy performance of buildings. It includes combining materials to take advantage of their natural thermal, moisture movement (hygric) and weathering properties to create building assemblies that will expand / contract and perform durably in the variations of climate that occur. This is the very beginning of creating sustainable buildings in any environment.
Proactive: Building science used in conjunction with Heating, Ventilating, and Air Conditioning (HVAC) system operation dynamics can predict locations within the building and the building envelope assemblies that will be problematic by design for moisture damage or mold development. Identifying these locations allows for changes to occur in the design stages to minimize these risks, creating structures that are less vulnerable. Taking this concept one step further incorporates knowledge of "build-ability" into the analysis; further adjusting the design to deal with realities of real materials and construction limitations that 'weaken' the intended function of water and air barriers, meant to protect assemblies from moisture damage. The effects of these realities are often the difference between success and failure of building assemblies.
Reactive: After water damage, moisture and / or mold is discovered - the WHAT, WHERE and WHY must be determined in order to mitigate the situation and return the building to the fully operational, non-issue facility it was intended to be. A multitude of calculations, Diagnostic Testing procedures and observations / measurements can be made depending on the situation. These diagnostic tests and calculations require significant equipment and software expenditures, building access equipment (scaffolds, lifts, etc.) and high experience and knowledge levels to zero in quickly on the actual problems without overlooking or ignoring other contributors. Bottomline: can be very costly to the building owner. But finding the issues is only part of this step. Communicating the problems well to both technical personnel and non-technical building owners and decision makers is the next issue. Then, creating solutions that are financially in line with the issues and risks, as well as the expected life of the building -- AND that will truly resolve the deficiencies may have to be done by a whole different group of people who really understand they dynamics involved. Finally, once the solutions are implemented, testing needs to confirm their effectiveness to complete the resolution of the situation.
Codes and Guidelines: There are a multitude of codes that apply to the properties and performance of every type of building material used in building assemblies, from nails and caulk to the actual materials being held together. There are testing procedures that are accepted and required by these codes and guidelines to determine the properties of each of the materials, then there are performance tests that can be done and are often required to determine the actual performance of the materials in particular situations. Additionally, there are guidelines and codes governing the actual installation details for various materials, creating 'systems' that have been tested and approved for use in various situations. In field performance tests have also been developed and approved by various codes and guidelines for specific installations, to verify the systems are functioning as planned and expected after the installation details are complete and other trades have done their work on the specific project. These requirements are always in addition to the manufacturer's installation guidelines! Additional testing methods are sometimes required to evaluate the performance of the overall installation in a particular building, to verify the building and systems are performing as expected and to adjust / modify to get to that performance level. These multiple levels of code application / testing / verifying are meant to get to an end product that provides not only a level of safety for occupants that is acceptable, but also a level of performance that is predictable and acceptable. Compliance with the codes and guidelines can be a complicated process!