In this article I will attempt to explain why some old buildings still perform after a hundred years while some newer buildings built in the 70's 80's, 90's and today are failing.
First performance is relative, I rate performance on the existence of water on moisture sensitive materials that given time will cause those materials to deteriorate. This definition is debatable because some may believe that performance is that the structural system is protected as the exterior layer is destroyed as a sacrificial layer for 5-20 years. I disagree but for this illustration we will characterize performance as a wall that manages the water in such a way that eliminates quick deterioration of the building.
We will explain the factors for differences in building performance by addressing the following reasons:
Quantity of insulation
Wall assemblies more tolerant of moisture
Quality of detailing
Exposure conditions (macro and Micro)
Complexity of building form (more details)
In older buildings insulation was not required by the building code. Developers builders, and contractors just left out the insulation and relied on the mechanical system to make the dwelling comfortable. In newer buildings insulation is installed in every cavity.
In the drawing on the left the old wall section shows the lack of insulation the existence of air leakage from the interior to the exterior (drafty) and the lack of an air barrier or weather resistive barrier. In the image at the top you see similar conditions except a layer of building paper is used and stucco is used as the water shedding surface.
Lets look at the science of the insulation as part of the wall assembly. Buildings fail because water reaches sensitive materials (wood, steel), the temperature is sufficiently high enough for mold to grow or rust to form and the material doesn't dry out.
When you have a drafty assembly with no thermal insulation the assembly can effectively manage the water that gets behind the siding. This factor is not the only one that contributes to envelope failure.
The detail at right shows the weakness of the newer assembly. This wall assembly is not tolerant to moisture because any water that makes its way past the siding sits and accumulates against the building paper. This water will cause the building paper to disintegrate after continued exposure and water will run sideways or find back laps in the materials. once the water has found it's way to the sheathing that is made of relatively weak materials like wood chips and glue mold grows and is sustained as long as it remains moist and warm.
Air leakage although reduced is not eliminated and can cause localized damage as moist warm air leaks through small imperfections in the wall assembly.
To have this assembly perform perfection must be achieved in the construction and maintenance of the assembly.
I have believe that our architectural knowledge and quality of design has both deteriorated and advanced at a very high level. First the deterioration is caused by reduced design fees and quickened construction schedules and the ability to transfer the problems off to the future owner. The advancements have occurred by consultants that have filled a niche left as building problems have increased and designers have contracted the risk out to professionals who work only with those specific issues.
In older buildings window system contained
elements seen in today's more advances building designs and most definitely contributed to better performance of the older wall assembly. The image at right illustrates an older window detail re-drawn for illustration purposes. Note the sloped sill and hidden flashing that redirects any water that enters the window opening back to the exterior.
Exposure conditions have changed since buildings began to fail on a large scale. Buildings are being built taller and with fewer overhangs and cornices. Buildings are built near the water and include many more windows and penetrations.
Buildings are also more complex than they used to be, a typical 1920' s office building will be a box with over hangs and cornices inset windows and one main roof. The building illustrated below is just as described a box with punched windows and lots of cornices and overhangs over critical junctures.
The picture at right shows a modern new construction that includes a multi -roofed, multi faceted facade with lots of penetrations including stair bays that allow water into the building and need to be waterproofed at horizontal surfaces. This building is much more complicated than the older building and is going to take great care to keep from becoming damaged.
In conclusion older buildings performed because of many reasons and those reasons have caused a greater need to be careful when designing and constructing a building. These reasons sometimes require a professional firm that understands the science behind the failures and can develop solutions.
The above images were created to illustrate how to integrate metal flashing at window to metal and fiber cement panel siding (Hardie). We create the envelope of the building to integrate as much of the architectural design and integrity as possible. In some instances design of the building envelope is compromised by the aesthetic design. In these instances we are required to add more expensive fixes and elements to get it to perform or change the design. It is critical in the early design process that an exterior envelope professional is hired to help drive the energy efficiency and durability of the project without compromising the design intent. 
