Overview

The most important building serviceability and durability consideration over the life of a wood-frame building is related to how wind-driven rain is addressed to protect the structure and its contents from the immediate and long-term damaging effects of water intrusion.  A robust water-resistive barrier system is the key to long term water resistance because most claddings are not barriers to water and, even if intended to be so, water will eventually get behind the cladding. If it is not stopped there and then directed back out, the structure will become damaged sooner or later.

A good water-resistive barrier strategy involves both simple detailing (to better ensure installation quality) and effectiveness.  FPIS-ci provides a means to address or enhance multiple code-required building envelope functions (insulation, WRB, water vapor control, and air barrier) which can simplify overall wall construction when properly integrated through coordinated design decisions and material specifications.  

The first step is to determine the water-resistive barrier strategy by selecting one of the options shown in Table 3 based on the pros and cons and informed designer judgment or preference (all are code-compliant options):

Table 3
WRB Strategies with Representative Pors and Cons

WRB StrategyProsCons
FPIS-ci used as the WRB
  1. System resistance to water penetration (highest of all WRB criteria)
  2. Simple flashing detailing when used with flanged fenestration units (see STEP 3B).
  3. Cost-effective (eliminates a separate WRB material layer)
  4. Can be used to effectively control water vapor in coordination with vapor retarder selection (STEP 3C)
  1. Only applies to FPIS-ci products that have been tested and approved for WRB application (refer to manufacturer code compliance data ?DrJ FSC report).
  2. Must use manufacturer specified joint sealing treatments (joint tapes and adhered flashing)
  3. Requires proper conditions for application of joint sealing treatments.
Separate WRB layer applied behind FPIS on wall substrate
  1. Separate WRB layer is protected by FPIS-ci.
  2. With proper R-value of FPIS-ci, the permeance of the WRB and sheathing is less of a concern when coordinated with interior vapor retarder selection (STEP 3C).
  3. With reservoir claddings like stucco or adhered veneers the FPIS-ci layer prevents inward vapor drives through higher perm WRBs.
  1. Additional cost of WRB separate from FPIS-ci
  2. Potential for more complex and less reliable flashing details at windows if flanges are flush with FPIS-ci and back of siding as usually detailed while WRB is recessed from the flange (STEP 3B).
  3. May need to use more expensive wrap to create small gap for drainage behind FPIS-ci.
Separate WRB layer applied over FPIS and directly behind cladding
  1. Easier to flash windows to WRB if window flanges are placed flush with outside of wall (on plane with FPIS-ci outer surface).
  2. Not preferred location for WRB behind stucco and adhered veneers which will prevent drainage unless an additional drainage layer or gap is provided between the WRB and stucco.
  1. Additional cost of WRB separate from FPIS-ci
  2. Difficulty attaching WRB layer through FPIS-ci layer.
  3. WRB should be higher perm than FPIS-ci layer in cold climates or mixed climates.
  4. WRB layer is not protected by FPIS-ci.
  5. Added metal penetrations/thermal bridges through FPIS-ci layer.
Dual WRB: FPIS-ci is used as the primary WRB layer with a separate WRB layer applied behind FPIS
  1. Perhaps the most ?fail-safe? installation.
  2. Otherwise Pros are similar to Strategy #2.
  1. The most costly WRB installation.
  2. Otherwise Cons are similar to Strategy #2.

Resources

Return to STEP 3 or continue to STEP 3B.

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