ASHRAE Journal - May 2013 - 68

BUILDING SCIENCES
of the 2 × 4s. Rigid insulation is installed between the 2 × 4
furring and then a continuous layer of rock wool is installed.
The exterior ﬁber cement cladding is installed as before.
In Figure 5 can I use rock wool between the wood structural
transition members? Yes. Do these 2 × 4s need to be ﬁreretardant treated? No, but check with the “authority having
jurisdiction.” Can I install more layers of continuous rigid
insulation if I want to go “super-insulated?” Yes. What about
the structural implications of all this?
I love structural engineering and structural engineers. Especially the older ones. They build stuff and break it to see what
happens. Then once they know the answer, they make sure the
calculations work. Check out Photo 1. How do you ﬁgure out
how to attach cladding through various types of rigid insulation? You build a whole bunch of assemblies and see. There
is no other way. Really. How do you stress the assemblies?
Check out Photo 2. Fill 5 gallon plastic pails with rocks and
hang them from the assemblies. And wait. And wait. And still
wait. You have to wait a long time. This “creep” thing might
creep up on you and you don’t want to be surprised. Surprises
are bad if you are an engineer. The simple tests get you enough
information to do more detailed tests (Photo 3) and then you
get to write papers and get graphs and stuff (Figure 6). The
deﬂection testing shown here is from a nice piece of work done
by Mr. Baker and Mr. Smegal in the Skunk Works at our lab in
Waterloo, Ontario.†† The adult supervision for this work came
from Professor John Straube of the University of Waterloo.
††The engineering basis for attachment of claddings through various rigid insulations including rock wool can be found at the following link. http://tinyurl.com/
ccveqfy. A “hat tip” to Peter Baker of our firm for this work and to the U.S.
Department of Energy’s Building America Program for supporting it.

Photo 1 (left): Build Stuff and Break It. The only way to know is
to test it to failure and then you know. How do you figure out how
to attach cladding through various types of rigid insulation? You
build a whole bunch of assemblies and see. There is no other way.
Photo 2 (right): Stress Test. How do you stress the assemblies? Fill
5 gallon (19 L) plastic pails with rocks and hang them from the
assemblies. And wait. And wait. And still wait. You have to wait a
long time. This “creep” thing might creep up on you, and you don’t
want to be surprised. Surprises are bad if you are an engineer.
Waterloo has a pretty reasonable engineering school, but not
as great as the engineering school at the University of Toronto.
We can do a lot with ﬂuffy rocks, sheet rocks, metals, ﬂuffy
treated paper, metal and salt treated wood and good old fashioned trial and error. Knowing the physics helps too. But build
it and see ﬁrst, just in case you don’t know all of the physics.
Note to self, no one ever knows all of the physics.
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building
Science Corporation in Somerville, Mass. Visit www.buildingscience.com.

1,000
900

Vertical Load Applied to Strapping (lbs)

800
700
600
500
Adhered Stone Veneer

400

Stucco
300
3 Cavity Rock DD - B1

200

3 Cavity Rock DD - B2

3 Cavity Rock DD - B3
Fiber Cement Siding

100

Photo 3: Detailed Test. The
simple tests get you enough
information to do more detailed tests. The deflection
testing shown here is at our
Skunk Works.
68

Vinyl Siding
0

0.05

0.10

0.15

0.20

0.25

0.30

Average Deflection of Strapping (in.) ([Right + Left] – Framing)

Figure 6: Deflection Testing. A nice piece of work from Mr. Baker and the Skunk Works at our lab
in Waterloo, Ontario. Check out the footnote to get the entire report.

ASHRAE Journal

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May 2013

ASHRAE Journal - May 2013

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