Drainage Mat Gap


POSTED: January 30, 2010

In a 2010 ASHRAE Journal article entitled Mind the Gap, Eh?*, recognized entrapped moisture expert Dr. Joseph Lstiburek stressed the importance of a drainage mat (drainage plane) in the  exterior building envelope. The article looked at the trends in building envelopes over the last 100 years and showed how the adoption of OSB, XPS and certain WRBs has increased the entrapped moisture problem.

Lstiburek states that when the wall is sheathed with OSB and housewrap and the veneer is an airtight cladding, “we have to help out the drying part of the equation. To help out with drying, we need an air gap between the cladding and housewrap/OSB interface, especially with airtight claddings, such as stucco, manufactured stone veneer, and especially where cavities are insulated with those new fancy spray insulations (such as cellulose, or spray foam or blown-in-batt). The air gap is simple, elegant and unbelievably effective in helping out drying. Back vent your cladding and be happy.” (Lstiburek, p. 58). He goes on to say that a 3/8″ or 10mm gap is a safe amount of space. An effective rainscreen drainage mat (drainage plane) creates this permanent, predictable void/air gap.

Masonry Technology, Inc. has been promoting this idea for years. That’s what we are all about – creating a predictable void behind the veneer to promote drying and pressure equalization. That’s what our Sure Cavity™ and Gravity Cavity™ exterior drainage mat (drainage plane) products do! Canada seems to understand this issue better than the U.S. and has made it code.

There are many important points for including a predictable air gap between the veneer and OSB/housewrap made by Lstiburek in this article, but here are a few that need to be stressed:

  • “The air gap’s primary function with stucco is to prevent the buildup of hydrostatic pressure…”
  • “There is no way that an exterior OSB skin that gets wet on a SIP can dry inwards.”
  • “Most housewraps don’t work.”
  • If foam sheathing is used instead of OSB/housewrap, there are some things that need to be addressed. “The water vapor impermeability of foam sheathing must be dealt with,” and, “we have to control hydrostatic pressure.”
  • “If you have wood or wood-based siding, the wood rots, and the paint falls off because the wood or wood-based siding cannot dry into the foam sheathing. Guess what fixes this? Yes, you guessed it, a small air gap…” (Lstiburek, pp. 59-61)

We can continue to be in denial that a permanent, predictable void (a rainscreen drainage mat or drainage plane) behind the exterior veneer in our rainscreen building envelope isn’t necessary, but the mold and sustainability issues won’t disappear. I encourage you to read Lstiburek’s Mind the Gap, Eh? article.

* Lstiburek, Joseph W. Ph. D.  “Mind the Gap, Eh?”  ASHRAE Journal January (2010): 57-63. http://www.ashrae.org/.  American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Jan. 2010. Web. 29 Jan. 2010.

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