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Commentary on Stucco Building Envelope Failures


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Editor's note: I received the following e-mail commentary from a Canadian reader.

It is coming to my attention there are an escalating number of issues concerning exterior envelope failures in terms of water permanence in exterior wall stucco finish assemblies. I may be blowing a horn too late, but the problem persists: In Canada, there appears to be a discrepancy between manufacturer spec assembly and the national building code. Every building paper and air barrier manufacturer I researched on the internet specified two layers of product, not one.

The 1995 NBC, still in force in several provinces north of the USA border, does not specifically prescribe how building paper is to be applied for stucco applications beyond it:

  • being required
  • being of a particular type
  • orientation and min. overlap at seams.
Part of the problem exists because building paper falls under 9.23.17. 'wall sheathing membrane' and makes no specific exceptions or requirements for stucco. The prescription application makes no reference to 2 layers being required in order to prevent stucco and stucco surfactants defeating the water repellence of the membrane. In other words, if only one layer or paper or olefin building wrap is used, the membrane is prone to capillary action allowing moisture and water to seep through to the wall structure beneath, manifesting in water leakage at wall penetrations and interior surfaces in general.

The problem is two fold:

  • 1. builders are using a min. national standard as a justifiable building spec, ignoring manufacturers recommendations

2. the national building code is ignoring a vernacular standard of practice that once incorporated 2 layers of membrane application as a given, while not enforcing the caveat of manufacturers recommendations because they are too numerous to track.

The building official is covered in terms of liability because manufacturers specs typically rule unless there is a specific safety alert. The other problem is that when it comes down to the wire, the building official can only enforce the min. standard prescribed in the code and not the manufacturer spec. In either case the consumer, who is our client, is left at the mercy of the builder. Since most inspections take place after a wall is finished, how can an inspector properly protect the client?

The codes, on either side of the border, should adjust the wording to include 2 layers of wall sheathing membrane unless the manufacturer specifies otherwise. Inspectors should be aware of this.


David Stewart

B.A., B.Arch., RHI

National Certificate Holder


213 Clearwater Court

Saskatoon, SK

S7K 3Y9

tel: 306-931-9995

fax: 306-931-3290




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