A reader comments on fiber cement feature

In our September 2015 issue, we published the article, “Fiber Cement Panels as Rainscreens,” by Carolina Albano. One reader, Richard Keleher (RJKeleher Architect), wrote to offer his thoughts about the piece and the subject matter in general. We shared with the author, who thanked him for his comments, and present it here:

On page 96, the author writes that one of the required components of a rainscreen wall assembly is “a few inches of air chamber or cavity separating the cladding from the support wall, reducing splashing and capillary moisture transfer.” However, she should make it clear the 3/8 or 7/16 in. is enough. In this section, Albano also refers to “large, protected openings (e.g. vents or weep holes) positioned at the top … of the wall promote convective airflow,” but this is not necessarily the case, especially for pressure-equalized rainscreens (PERs). Nowhere in the article does she note the importance and difficulty of protecting those openings from wind-driven rain.

Later in the article, the author says, “The key to an efficient PER lies in the ability to control the airflow,” to which I would add, “and, more importantly, to baffle the water while letting air in.” Along these same lines, Albano  suggests the structural designer or engineer would determine the number and size of these breaks, but there is no mention of the importance of baffling these breaks to prevent entry of water. If that is not done, one might as well have no exterior cladding. Also, it may not be a structural engineer that does this—her limitation to the structural discipline is incorrect; although this work is most often performed by structural engineers, it is also done by members of other disciplines.

Albano mentions “Mechanical seals…increase cost and complications,” and I would add, “and they often have undesired gaps, unlike wet sealants.” She goes on to describe AAMA 508-7 and AAMA 509-9. I was on the AAMA committee that developed 508, and therefore know something about this. AAMA 509 is not a pass/fail test as is 508. Therefore, the specifier needs to specify the water and ventilation performance required. For example, I would only recommend W1 maximum and V4 minimum for performance.

A few other edits for page 99:

1. When the author says, “The specifier must ensure…according to the test’s standards,” I would add, “for both types of systems,” and change “test’s” to “tests’.”
2. To “The system is a form of double-wall construction that uses an outer layer to keep out…,” I would add, “most of” before the words “the rain.”
3. Immediately after, Albano gives a list of functions, to which I would add, “prevent penetration of water that gets past the cladding.”
4. In the next paragraph, she writes, “The structural frame of the building is kept absolutely dry, as water never reaches it or the thermal insulation.” However, “never” is a gross oversimplification.
5. The author adds, “Evaporation and drainage in the cavity removes water that penetrates between panel joints,” to which I would suggest adding, “if it hasn’t gotten through the weather barrier and into the building through imperfections in the water barrier.”
6. To the statement, “There is no significant pressure differential to drive rain through joints,” I would add, “except during gust winds.”

On page 100, the author writes, “A factory-applied sealant is applied…” I would note that factory-applied sealants do not bond to the second substrate in the field, and are therefore an incomplete seal. Other jointing methods offered by fiber-cement panel manufacturers do not create a complete seal, either (except for a traditional sealed joint with a backer rod).

A final point—in the concluding section, as in the rest of the article, it says nothing about the critical importance of the weather water barrier, as an air and water barrier meeting Air Barrier Association of America (ABAA) standards for air barriers and performing as a water barrier. To determine the adequacy of the latter, a field mockup test is recommended, but is not mentioned in the article.