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Climate Considerations and Walls

Finally we can come to you with some news regarding the 2012 International Energy Conservation Code. For the longest time we have been doing some guess work on just how to bring stucco into compliance with the 2012 IECC requirements. Clear data have not been available to make the determinations necessary so that we could move forward with constructability testing. While some data from industry organizations have been published, for the most part, when it came to stucco as an exterior cladding, most of the data were empirical with the only supporting data being the weight was 12 pounds per square foot.

We at AWCI realized some time back that we would have to perform testing to resolve the perceived constructability issues involved in placing insulation under the lath. What we have not known up until now was the thickness required to achieve the u-factor of the wall. The u-values are determined by climate zone established by the Department of Energy. There are eight climate zones for the United States. Climate zone 1 covers a very small portion of South Florida, and climate zone 8 covers Alaska, so we in reality only have to concern ourselves with 6 climate zones. By the way, California is totally different as the state has its own energy code with climate zones that are different from the rest of the Unite States. The climate zones are established by county lines and generally ignore state lines, so use caution when determining the boundaries of climate zones.

Early in the process the available information indicated a maximum foam thickness of 4 inches in climate zone 7.This thickness definitely presents problems with the fasteners by introducing a moment arm that would be difficult to resist. A review of ASHRAE 90.1-2010 reveals the following u-values for a wall above grade using cold-formed steel framing by climate zone:

Climate Zone   U-Value   Min R-Value

1   0.124   R-13

2 thru 6    0.093   R-13 + 7.5c.i. (continuous insulation)

7    0.042   R-13 + R-15.6

8    0.037   R-13 + R-15.6 c.i.

Note: The values are condensed from Tables 5.5-1 through 8 and are taken from the Residential category. The category’s title is confusing in that it includes what we would normally call commercial construction.

Using the ASHRAE values we can determine the thickness of insulation required—expanded polystyrene has an R value of 4 per inch of thickness; extruded polystyrene has an R-value of 5 per inch and polyisocyanurate has an R-value of 6.5 per inch. Therefore, the results for the worst case scenario climate zone 7 are EPS=4 inches; XPS=3 1/2 inches and polyisocyanurate=3 inches. Please understand that some of these values are currently under appeal and could change.

So where do we go from here? With these values we can proceed to build a test wall panel. We have three volunteers who will provide the real estate upon which the panels will be constructed. The purpose of these test panels is solely to determine if the stucco will in fact stay on the wall without deforming the fasteners. Other issues involve the accessories, casing beads, control joints and weep screeds. Under current conditions casing beads and weep screeds are equal to the thickness of the stucco. This will have to change to now enclose the insulation as well as the stucco.

The test panels will be 12 feet long by 8 feet high and divided into 3 equal panels. We will use insulation thicknesses for climate zone 7, which presents the worst-case scenario because it presents the greatest challenges and also because we have indications that in future years the requirements for climate zone 7 may well be applied to the other climate zones—or at the least the u-values for the other climate zones will increase. Once the walls are completed and allowed to age for a period of time, we will then remove portions of each panel for forensic investigations to determine if unseen failures occurred. A couple of universities with construction departments have expressed interest in assisting in this portion of the testing program, and this involvement is sure to add to the validity of the program.

There are two more steps involved in the overall testing program. The second set of tests will be hot box tests to determine the thermal values, and potentially other environmental tests for each selected test panel. These tests will be documented for future reference and proof that the designs do in fact work. The final test will be a fire test of at least one system. A fire-rated design for a stucco system using insulation under the lath currently does not exist. The fire-rated design is important due to the wildfire requirements in some of the Western states as well as building code life safety requirements.

When the testing programs are completed and ASTM C1063 is revised to bring the standard into compliance with the IECC, AWCI will publish a white paper describing the changes needed to the design and installation of stucco cladding.

Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.

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