The Continuous Interest
March 1, 2011
On January 26, hundreds of interested parties from around the nation called in to hear the presenters of the Webinar “Continuous Insulation: Plaster & the Energy Code” speak about the increased use of continuous insulation and how to make cement plaster (stucco) work with the upcoming energy code requirements. Listeners could submit questions live to the speakers but several may have been disappointed that their questions were not asked. The event generated more than 150 questions in less than 20 minutes and with only 15 minutes to answer on air; it was not possible to answer everything. We have selected the most commonly asked questions and tried to answer the most relevant ones.
Since 16-inch on-center framing is preferred for stucco, with new advanced framing techniques, 24-inch on-center will likely be the norm. How do you intend to handle this?
The reason the plaster industry has always shied away from recommending 24-inch on-center framing (which is code legal) for cement plaster cladding is because of the cracking. Experience has taught most plasterers that 16-inch on-center framing provided fewer cracking problems.
However, there is some good news, the lamina option (which is a polymer enriched cement with a fiberglass mesh over the brown coat) greatly reduces the likelihood of cracks. This lamina should be used over all cement basecoats when framing is engineered to be 24-inch on-center.
Isn’t this going to make stucco systems too costly to install?
When the energy code required the window industry to use low-e coatings on all windows a few years back, the uproar could be heard across the nation from the window industry.
Today, low-e coatings provide superior energy efficient windows, so much so that the retrofit market went nuts. Shouldn’t we look at it the same way?
Imagine the tens of thousands of homes that could benefit from exterior continuous insulation, like EIFS. Plasterers will be busy for years to come, all to help lower energy costs (for customers). If you’re a commercial plastering contractor and think it won’t help you, it might. All those residential guys taking your work now will be too busy with the retrofits. After all, low-e coatings and window replacements cost money but people are doing it and most are happy about it.
Has anyone thought about the weight of stucco and the cantilevering of this heavy cladding with 2 inches of foam?
Actually, there has been much discussion, testing and empirical proof that three coat stucco over rigid foam is not a problem. The Portland Cement Association said cement plaster bonds “tenuously” to most rigid foam. This bond helps mitigate the cantilever issue. In addition, the Foam Sheathing Coalition (www.foamsheathing.org) has an engineering technical paper with data to verify that claddings weighing 11 psf (three coat stucco) can be attached successfully over rigid foam up to four inches thick.
The Western Conference prefers designers and contractors to limit the foam to 2 inches thick maximum for cement stucco cladding. When thicker foam applications are desired, EIFS is strongly recommended as a better choice.
Shouldn’t the water resistant barrier be over the foam for extra protection?
Cement stucco over rigid foam is not new. In fact, the one coat stucco market has been doing it for more than 30 years. The plaster industry has learned a thing or two about the WRB and rigid foam. While we do not prohibit WRB over the foam, the bond of the cement to the foam improves performance. Flashing, windows and other penetrations are simpler and in a more direct plane to help incidental moisture prevent ponding and facilitate drainage.
Most rigid foams (EPS and XPS) are not affected by water and need no protection from incidental contact with moisture. Moisture-sensitive foam sheathing or foam with facings which come into direct contact with plaster may need to have the WRB placed over the foam. Placing the WRB over rigid foam is difficult to install without tearing larger holes in the WRB. Generally, the plaster bureaus prefer the rigid foam placed over the WRB that is integrated with flashings and a weep screed.
How do you attach corner aid or cornerbead with 2-inch foam?
Your question probably implies that putting up a straight corner trim and attaching it through thick foam is difficult at best and you are correct. Corner trims (the Western Conference prefers corner aid) can be wire tied to the lath. This is common practice with lath attached to masonry substrates, so lath over the rigid foam would be no different.
Don’t the fasteners used to attach the lath negate the thermal break of the continuous insulation?
The Western Conference would agree with you but all claddings have to be attached. The energy code has deleted the fasteners as part of the equation.
Remember, the CI requirement is cladding neutral and the energy code folks had to make some concessions. For example, brick ties would technically defeat the thermal beak, but they received a pass as masonry veneer cannot be done without them. Adhesively applied EIFS has no fasteners and would provide the ultimate in thermal break.
Your PWA 106 assembly uses metal Z furring, wouldn’t that negate the thermal break?
The Western Conference would agree with this too but we have had designers and contractors who really want the extra stoutness afforded with the Z furring. This is why we recommend two layers of gypsum sheathing on each side of the Z furring and a foam break tape. For those who are wondering what PWA 106 is, please go to www.tsib.org and click on the “The Energy Code and Plaster Assemblies” link-the brochure is a free download. The California Energy Commission has approved a very large project with this exact system. A committee has toured the building for a year after completion and was very pleased with the results. So, we do not expect the state to alter their current position of acceptance.
I thought the U factor is just the reciprocal of the R value. I am not sure I agree with your definition.
The Western Conference is an expert at lath and plaster-not the energy code or defining what the R value or U factor is. We simply used established definitions by ASHRAE and the state energy code. Questions like this are probably better answered by those experts and we ask they leave the stucco questions to us.
There were many more questions that were specific to the energy code, a particular state or how the national code is implemented. The Western Conference feels it is more appropriate to find those answers at the state level. Almost all states have an energy code, an energy department which will typically provide these answers on their respective websites. What they do not have is an explanation of how to make cement plaster work over rigid foam. That is where the Western Conference of Wall and Ceiling Institutes feels a duty to provide answers.
The various plaster bureaus on the west coast have years of experience with cement plaster over foam and feel we can help make this revolutionary change to stucco a more palatable and successful option for all parties. Please note that EIFS and one coat stucco systems are proprietary systems and provide code-approved CI solutions that are time tested. They should not be overlooked as an option.
The Webinar “Continuous Insulation: Plaster & the Energy Code” is archived at www.wconline.com/Articles/Webinars.