Moisture in buildings has become increasingly worrisome for construction professionals in recent years. American buildings are subject to a wide variety of defects related to water in its different forms – liquid, vapor and sometimes snow and ice. These range from thermal discomfort, poor indoor air quality and health hazards (mold), all the way to permanent structural damages (when the building literally rots or gets corroded).

Image 1: Biological decay of wooden roof due to moisture buildup – Credit: Anonymous, posted on GBA by Peter Yost

The situation has become significantly more critical since recent building code updates have introduced mandatory air tightness requirements. Air sealing itself is actually very important for the durability of a building, i.e. to minimize air carrying moisture into the assemblies of the thermal envelope and thus avoiding interstitial moisture buildup. An air tight building is also much more comfortable for its occupants (no cold drafts!), outside noise is blocked from coming in and energy consumption is greatly reduced too. With proper training, detailing and care, even first time builders can easily meet this air leakage requirement.

Image 2: Beartooth Passive House under construction in Red Lodge, Montana. On the first try, the building exceeded the Passive House air tightness requirement of 0.6 ACH50 (air changes per hour at 50 Pascal testing pressure), much more stringent than Building Code requirements (3.0 ACH50 in that climate zone, and 4.0 ACH50 as locally adopted by the State of Montana).

However, air sealing brings up the question: what happens with the moisture? 

Whatever your climate is, most moisture comes from within the building. According to ASHRAE, a family of 4 produces in average 2.5 gallons of water vapor every single day – by breathing, cooking, taking showers etc. Without a proper moisture management in place, that amount of vapor becomes problematic even in a dry climate, exposing to defect litigation those construction professionals that are not keeping up with the evolution of an ever more demanding market.

Many American construction professionals still latch onto the Dew Point method – an oversimplified, obsolete method of looking at moisture transport in building assemblies. This method was developed decades ago when computers where scarcely around, and professionals had to crunch numbers by hand (the author of this article reminisces upon his college time spent calculating dew points with a basic calculator).

Among other things, the Dew Point Method fails to account for:

  • any material properties besides perm rating and R/in value,
  • assembly orientation,
  • color and slope,
  • construction moisture,
  • air exfiltration,
  • occupancy,
  • ventilation strategies,
  • wind, rain, sun…

Advanced moisture modeling results in virtual prototyping of building assemblies, a process that allows to evaluate potential risks from different sources. Among others, this includes air leakages and rain infiltration – conditions likely to occur beyond a designer’s ability to control the construction process, as in case of less than perfect product installation. The goal is to simulate actual operational conditions in local climate, and to allow for the assembly to dry out without damage in case it were ever to get wet – a design principle called “moisture resiliency”.

Image 3: Virtual prototyping of an Emu Passive House wall. The moisture content of the external  OSB sheathing is simulated over 10 years after building completion, in order to evaluate the risk for biological decay (i.e. rotting). According to current international standards, moisture-driven wood damages can occur even in complete absence of condensation.

Over the years, manufacturers have developed advanced membranes to intelligently manage moisture in building assemblies. These are called “smart vapor retarders” (SVR) because they can effectively regulate the amount of moisture going through them (i.e. their perm rating) depending on the ever-changing humidity conditions. 

Image 4: Perm ratings of different smart vapor retarders, depending on moisture conditions inside the building assembly. The horizontal dashed lines show the range limits of the different vapor retarder classes as per Building Code. Note: some SVRs have different perm rating depending on the direction of the moisture flow.

Unfortunately, even the smartest products cannot compensate for poor practices including insufficient design and faulty installation. The more performing the buildings, the better thought out the detailing need to be, and the more thorough installation, supervision and quality assurance need to be. For example, higher R-values equals lower heat losses in the building, resulting in longer time needed for assemblies to dry out if they are wet. 

Image 5: Readings of moisture content on installed materials during construction, as part of Emu’s North American Pilot Program. The results will be combined with data collected after building occupancy, with the goal is to verify the built performance of the standardized Passive House construction systems developed by Emu Systems for different climate zones of North America.

Best practices lead to drier, longer lasting buildings. This also includes substantially lower moisture content in the materials once the building is complete compared to industry standards.

Image 6: Continuous monitoring of moisture content in key assembly layers in Beartooth Passive House – Emu’s first Pilot Project – as compared with ASHRAE’s assumed moisture conditions after building completion. Data collected over two months shows significantly lower moisture than expected, resulting in very better performing building, as well in substantially reduced liability for the builder.

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