Selecting the Right Drainage Mat

The Problem with Furring Strips

In his article Mind the Gap, Eh! Dr. Joseph Lstiburek established the need for a “drainage space” (a gap or void) behind the veneer and in front of the weather resistant material on the backup wall to alleviate moisture-related failure of the building envelope.  This gap or void can be created with furring strips, but there are inherent problems with this method.

furring strips as rainscreen drainage matIn most furring strip applications, additional materials are required to complete and support the construction of this rainscreen drainage plane detail.  These supporting materials include some type of semi-rigid moisture-resistant panel or roll stock material used in combination with construction paper as a slip-sheet.  A rainscreen drainage system created with furring strips (Example A) commonly consists of the following:

  1. Sheathing
  2. Building Paper
  3. Main Furring to Coincide with Studs
  4. Intermediate Furring
  5. 3-Ply Semi Rigid Asphalt Board
  6. Building Paper
  7. Stucco Lath
  8. Metal Drip Flashing
  9. Ventilation Openings Covered with Screen
  10. Metal Stucco Stop
  11. Taped Vertical Joint of Asphalt Board
  12. Three-Course Stucco

 

There are several inherent problems with this type of rainscreen drainage system.  The three that are probably the most negative are:

  • Too many components (cost issue)
  • Too labor intensive (cost, time, and complexity issues)
  • Consistency and quality of final system cannot be guaranteed

Another factor that negatively impacts this type of system is its violation of the most basic principle of moisture management; it fails to provide a means for the moisture to effectively exit the building envelope.  (Example B)  The International Building Code, Section 1403.2 says, “The exterior wall envelope shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing…a means for draining water that enters the assembly to the exterior.”  Section 703.1.1 of the International Residential Building Code also affirms this point.

Creating the “gap or void” isn’t enough if the water that reaches the bottom of that drainage space has no way out of the wall when it gets to the bottom!

But I Have a Weep Screedtraditional weep screed product examples

The common component used at the bottom of a stucco veneer is referred to as a “weep screed.”  This terminology infers that weeping will take place; therefore, the assumption is that the bottom of the wall will be able to drain.  However, the manufacturer’s literature for these commonly used components states that the holes in the bottom of the weep screed are for “attachment” purposes and that the weeping will actually occur out of the shrinkage crack that forms between the metal and cementitious material (stucco).  If there is any weeping ability with a “shrinkage crack,” it is minimal at best.  The metal termination at the bottom of the drainage mat (drainage plane) must provide numerous, large openings for the moisture that is drained from the wall to adequately exit the wall. 

Learn More about Weep Screeds

Why Use a Manufactured Drainage Mat

The new technology that is attempting to improve the rainscreen drainage detail is the use of “mat” materials.  Most of these mats come in the form of roll stock.  These mats are created with various types of manufacturing procedures and materials.  The primary manufacturing procedures and materials are as follows:

  • Continuous vacuum molding of various types of plastic sheet roll stock material creating a variety of shapes or raised patterns and dimples in the material.  A second manufacturing process involves adhering a synthetic fabric to the tops of the raised pattern. (See Example B)
  • The extruding of various types of plastics into filaments of varying dimensions and compressing and organizing them into continuous mats of various thickness and widths.  A second manufacturing process involves adhering a synthetic fabric to one or both sides of this entangled filament mat. (See Example C)
  • The extruding of various types of plastics into filaments of variety dimensions and compressing them into a sheet.  This sheet of random filaments is then formed into various corrugated patterns. (See Examples D)
  • The forming and perforation of a plastic roll stock material into a variety of corrugated patterns and thicknesses.  A second manufacturing process involves adhering a synthetic fabric to one side. (See Example E)

Examples of Drainage Mat Products

These four manufacturing procedures are used to create six of the major product lines that represent the new mat rainscreen drainage plane technology in North America.  They differ from one to another in drainage efficiency, compressive strength, and ease of installation.  What they all bring to the construction industry and specifically the exterior building envelope segment of the construction industry is ease of installation and in most cases a more predictable cost effective rainscreen drainage plane.  When choosing which one of these new rainscreen drainage mat (drainage plane) technologies to use in your next project, select the one that best suits your veneer/rainscreen type, and a one that comes from a manufacturer who will work with you to design the right moisture management system for your project. Download the complete Furring Strips Vs. Drainage Planes article below.

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