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2 or 3 Coat Traditional Stucco Application with Drain Cavity

DarrenKC | Posted in General Questions on

Mineral wool outside insulation is becoming more common in residential in my location. I favour this over rigid insulation by far for many reasons, but I’m trying to figure out a best approach for traditional stucco applications.

Typically with these applications include a 1×3 vertical strapping board over the insulation, screwed through the insulation and then a horizontal siding cement board attached to the strapping. When it comes to stucco, one could default to a EIFS product with either EPS or mineral backing, but EIFS systems are primarily not used in residential construction. The traditional 2 layers building paper metal lath, base and top coats still rules.

It highly recommended these days that the stucco have an actual drain space behind them rather than just two layers of building paper, so the outside insulation standard install method with the strapping works in favour of this.

So to the question: What is the best backing for the traditional stucco applications with outside insulation and vertical strapping/air space? The standard metal lath would likely not be rigid enough to span between the vertical strapping at 406mm oc. (16″ o.c.) or would it? Could a heavier guage metal lath be used such as a security mesh type product which is commonly put into walls of retail stores?

I bring this up because I witnessed a house being build with just this case and saw the contractor putting in excessive amounts of vertical strapping. I was scratching my head why so much vertical strapping until I saw a cement board go on top. They were using a cement board as the stucco backing to which the metal lath went on. This seems like a very heavy (hence the reason for excessive vertical strapping) and costly assembly. See image 2.

If anyone has some experience with this, I would appreciate your thoughts.

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  1. GBA Editor
    Martin Holladay | | #1

    This article discusses stucco over furring strips: To Install Stucco Right, Include an Air Gap.

    Further discussion (along with evidence that paper-backed metal lath can indeed span the distance between vertical furring strips) can be found in the comment I posted (quoting Kohta Ueno) here: Stucco over exterior rigid foam - yea or nay?

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