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475 Does WUFI®: A Series Introduction

July 18, 2013
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Generic WUFI Dynamic Modeling Example

Traditional buildings leak air and are poorly insulated, allowing the enclosure assembly to essentially bake- or air-dry.  In doing so it maintains a resilient and robust construction but is inefficient and uncomfortable. Today, as we evolve to achieve greater levels of comfort and efficiency the enclosures are more insulated and airtight, become colder, and are less inclined to dry. Drying becomes dependent on vapor diffusion and requires a robust vapor profile. Consequently, the vapor permeability of various materials and how they perform together in a complete assembly become ever more critical. And then there are particular climactic, exposure, and orientation issues that provide further potential complications in the assembly’s drying potential. Often assemblies are “self-evident” in their drying power or weakness, and safety or danger. For example, two vapor barriers enclosing an assembly will have no drying potential and pose a great danger. Yet as we seek to optimize the drying potential, often it is not clear which assembly provides the best protection or even sufficient protection. In such cases it can be very useful and necessary to run a WUFI dynamic hygrothermal calculation to evaluate and support informed decisions.

To assess whether the highly insulated assemblies will manage moisture well, we use WUFI Pro 5.2 (available from Fraunhofer IBP): a computer program that has been validated with a large number of assemblies for its predictive accuracy. A limited-use version is available from ORNL , but the 2-year time frame for calculation runs is, in most cases, too short to actually evaluate assembly performance.

Drying potential is larger than (un)anticipated moisture stress Floris Keverling Buisman, 475’s technical director, is a certfied WUFI®-ORNL instructor. With support from Pro Clima’s technical team, he evaluates assemblies that have challenging build-ups (walls with vapor retarding exterior sheathing (OSB), flatroof and greenroofs). We can analyze and assess if the drying capacity of these walls is larger than the (un)anticipated moisture load from rain, diffusion, and air convection (see John Straub’s 2001© image on the left).

Pro Clima’s safety formula:

drying capacity > moisture stress => freedom from structural damage

“The greater a structural system’s reserves for drying out, the greater the unanticipated moisture stress it can balance and [keep] the structure free from structural damage.”

To be sure that the assembly will perform as designed, we have to be sure that the un-anticipated moisture stress is minimized. Proper detailing, execution (construction), and verification (blowerdoor tests with results < 1.0ACH50) are essential for the structure to perform as intended (prevent damage and mold).

WUFI with ZIP, cellulose and INTELLO -475

Assembly Diagram with Sensor Locations

The modeled example shown here is a double stud wall located in Boston, MA with ZIP/OSB vapor retarding sheathing, vapor open WRB, and no exterior insulation board. It has a hygroscopic active dense-packed cellulose material used as insulation (10″ thick) and INTELLO Plus as a smart vapor retarder/dense pack netting on the interior. The graph below shows the construction moisture that was inserted in the model (start date 1/10/2013) has left the assembly within a few months – a first indication of a good drying reserve and a safe design.

WUFI- toatl water content of doubel stud wall Zip, cellulose and INTELLO -475

Overall Moisture Content

The graph below shows the water content in mass percentage (M%). The safe maximum for OSB is 18 M%, but preferably it stays below 15 M%. Drying out the construction moisture in the first winter causes the material to go above that limit briefly. We will analyze in the next graph whether this will cause issues. The assembly then dries out sufficiently in the summer, so the next winter the OSB stays below 12.5 M% – a safe level with plenty of drying reserves, offered by INTELLO Plus.

WUFI water content of ZIP-OSB sheathing in double stud wall -475

Moisture at OSB

The  graph below is taken at the condensing surface – the interior face of the OSB sheathing.  The vapor retardancy of the sheathing (perm 1-ish) is slowing the outward vapor drive in winter. The first winter, the humidity builds up to 90% relative humidity (RH) –  it only occurs when the temperature of the OSB is below freezing so rot/mold won’t occur.  The following winters, as expected, the RH only breaks 80% on the very coldest days – again not a concern. The basic rule is that if RH is over 80% on average for 30 days with and temperature average >50F, there is potential for damage.

WUFI at condesning surface of ZIP, cellulose and INTELLO -475

Moisture at “Condensing Surface”

In future blog posts we will look at various assemblies 475 has run in WUFI and the results of their data outputs.   If you have an assembly you’d like us to take a look at – let us know.

Additional 475 Does WUFI blog posts now include:

Keeping Sheathing Dry in High-R Double Stud Walls – a WUFI study

Related Blog Posts:

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