PDT U - Exterior Wall System Review

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Bob Curtis’s Long-Awaited Talk on the Evolution of PDT Wall Systems

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Bob Curtis, PDT’s guru of codes and wall systems, started with the typical commercial wall system of twenty years ago, which resembled the “house system” still used in single family construction, and advanced to today’s rainsceen technology.  He paused along the way to discuss the evolution of our choice of wall insulation, from itch-scratchy fiberglass batts installed between steel studs, to supposedly continuous foam plastic insulation boards interrupted by metal zee furring, to our present system of continuous mineral fiber board insulation, penetrated only by fastener or rainscreen support brackets.

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Describing the constant battle to keep moisture out of walls, Bob described the old walls as ineffective because of their potential to generate condensation within the wall assembly due to thermal bridging across steel studs.  Bob repeated the PDT mantra of favoring simplicity over complexity in wall assemblies by reducing the number of wall assembly components whenever possible.  Multiple layers of building materials, insulation materials, and membranes with a variety of permeabilities and water retention characteristics can trap moisture within a wall, which can damage the walls and lead to indoor air quality problems. 


Bob contrasted this with the current PDT walls, constructed with a single plane combined air/water/vapor barrier, and a single plane of continuous insulation, that are able to self-regulate their moisture content by drying to the interior or to the exterior as required by changing ambient conditions of temperature and humidity.

Bob reviewed the pros and cons of several of the newer rainscreen “clip and rail” systems designed for use with mineral fiber board insulation and discussed the challenges of maintaining the integrity of a single plane air/water/vapor barrier that is subject to numerous penetrations from fasteners, conduits, and structural members. 

PDT is currently designing two major schools to the 2015 edition of the International Energy Conservation Code, above the standards required by the State of Maine.  Our office standard building enclosures for all project types typically exceed the current State standard.  In the evolving world of building enclosure technology, we always try to anticipate, monitor, and adopt improvements in our choice of systems and materials.  This effort makes our buildings tighter, dryer, more comfortable and more energy efficient than ever, with higher R-values and lower utility bills.

–Robert R. Curtis, LEED AP BD+C, Maine Licensed Architect, holds a Maine Code Enforcement Certificate and is an associate of PDT Architects.