Currently, the home building industry enjoys virtually no consensus regarding building paper, the stuff that wraps the house.
Despite all the technological advances in building technology and in the fields of building science and home performance, a “unified field theory” of the building envelope eludes us. Instead, we’re faced with a number of differing approaches.
Those who advocate for the old methodology of 15lb. felt and wood siding ignore the rising cost of energy. More airtight building wraps require a breathable layer and/or drainage mat that allows the backside of the siding to drain and dry (a feature that is now part of the building code for new homes) as well as additional care for air quality. Breathable building wraps used in conjunction with “rainscreen” systems combine features of these methods but leave unanswered the question of air permeability as it relates to energy efficiency. Passive House standards require that a virtually airtight, vapor-permeable envelope be created, with indoor air quality maintained by an HRV system.
The point is that every one of the four paths mentioned above represents a fundamental disagreement about the important aspects of the building envelope. It’s a classic example of “do you walk to school or carry your lunch?” These different methodologies offer divergent answers to questions about the building envelope. Actually, they don’t even ask the same questions about the functionality of the building envelope.
This blog post uses the analysis of different types of building paper/house wrap to define the real question: How do we want our buildings to perform?
The building envelope should provide shelter. The building envelope should be durable. The building envelope should be made of materials that are efficient in their assembly and energy consumption. But that’s about the end of common ground for the building envelope. Anything more specific requires a choice that weights the following values relative to one another: cost, insulative capacity, air-tightness, vapor permeability, sustainability, and aesthetics.
Whatever choice is made needs to fall in line with larger home performance requirements of efficiency, comfort, safety and durability. The relative mix of these requirements differ from person to person, and the industry offers building papers and envelope systems that fit better with one set performance criteria than another.
But the most important thing to emphasize is that the house performs as a system. The envelope is just the skin. More on the “system” concept later…
Only when a type of paper conflicts with the design and performance characteristics of the house can there be real trouble. An example: when an air tight, vapor semi-permeable housewrap is used with no equipment installed to address air quality, both in terms of pollutants and humidity differentials. Not only can the air get stinky and dangerous, but also the dew point can form inside the wall cavity. Another example: siding systems that leak (all siding systems) without a drainage layer behind them to avoid water build-up inside the envelope.
So decide what’s important to you about how your house performs, understand the performance characteristics of different house wraps and make a good match to avoid disaster.
– Sam
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