The Passiv Report

Assessing Durability of High Performance Building Enclosures

“All Materials are durable. It is the environment we place a material that determines how long it will perform as intended.”-A Wise Building Scientist

As we design our building enclosures, our understanding of the initial performance of our envelope assemblies is well understood. Whether we are considering thermal resistance, air-tightness or even moisture management, we are confident in our analytical means to quantify these performance metrics.

However, one of the most important aspects of good envelope design is to maintain these initial performance metrics throughout the expected service life of the assembly – essentially, we must understand the durability of the system we have designed. It is of no worth if a building enclosure assembly supports a Passivhaus building on day one and fails miserably on day 365. As designers of these assemblies, it is our responsibility to understand and maintain long-term performance.

As we move towards the design and construction of high performance building enclosures necessary to support Passivhaus buildings, we are often finding ourselves in uncharted waters; many of us are operating well beyond the comfortable range of typical enclosure assemblies that have been proven to work in their respective climates for over 20 years. As we deploy super-insulated enclosure assemblies, our envelopes typically become thicker. The understanding of moisture transport within the assembly, across many years becomes paramount. Poorly designed and/or understood assemblies will accumulate moisture over time, essentially decaying from the inside out. Our experience has shown us that as we move into the super-insulated extremely air-tight enclosure realm, we are shaving down the traditional factor(s) of safety governing wetting and drying.

One tool to support our understanding of the combined heat and moisture (hygrothermal) transport mechanisms within our enclosure is WUFI®

“WUFI® (Wärme und Feuchte instationär) is a software family which allows realistic calculation of the transient coupled one- and two dimensional heat and moisture transport in multi-layer building components exposed to natural weather. It is based on the newest findings regarding vapour diffusion and liquid transport in building materials and has been validated by detailed comparison with measurements obtained in the laboratory and on IBP’s outdoor testing field.”1

WUFI® is a powerful software package, however it is easy for the untrained to create false and misleading simulations. We have been using this software to analyze long term durability of building envelope assemblies for over 15 years. We understand the underlying fundamental physics driving hygrothermal performance of enclosure assemblies.

We have in-house expertise in building physics fundamentals as they apply to assessing the long-term durability of Passivhaus building enclosures. If you would like more information on this, please do hesitate to reach out to us.

1. Retrieved from April 7, 2014

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