ASHRAE Journal - July 2020 - 52

BUILDING SCIENCES
COLUMN

Joseph W. Lstiburek

Understanding Walls*
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

Wall assembly design and construction needs to consider rain; temperature;
humidity, as defined by the hygrothermal regions; annual rainfall; and the interior
climate classes as environmental loads that affect mold, decay and corrosion, as
well as other degradation mechanisms. Design and construction needs to consider
the exterior and interior environmental loads, the nature of the materials that
comprise the environmental separation, and the energy flow across the environmental separation.
Wall assemblies should be designed and constructed
for specific hygrothermal regions (Figure 1), rain exposure zones (Figure 2) and interior climate classes (Table 1).
To this end the wall assembly should have four principal
control layers or control approaches as overlays to the
structure. They are presented in order of importance:
* A water control layer or water control approach for
rainwater;
* An air control layer or air control approach for airtransported moisture;
* A vapor control layer or vapor control approach for
vapor-transported moisture; and
* A thermal control layer or thermal control approach for thermal transfer.
If a control layer approach is selected, the best place
for the control layers is to locate them on the outside
of the structure to protect the structure. The optimum
configuration is presented in Figure 3. However, many
configurations are possible. In many assemblies a combination of control layers and control approaches are
typical.
*In August 2004, ASHRAE Journal ran a paper I wrote titled "Understanding Vapor Barriers." After almost two decades, I thought I
would update it and focus it on rain and air control as well as vapor
control-all things that "walls" need to do, hence, the revised title,
"Understanding Walls."
52

ASHRAE JOURNAL

ashrae.org

J U LY 2 0 2 0

All claddings leak. Once you accept that, you can begin to understand walls.

The most common examples of configurations of
control layers and control approaches follow for wood
frame and concrete masonry unit (CMU) assemblies.
The configurations cite various classes of vapor control
layers (Table 2) and the thermal resistance of exterior
insulation to avoid condensation (Table 3).
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.



http://www.buildingscience.com http://ashrae.org

ASHRAE Journal - July 2020

Table of Contents for the Digital Edition of ASHRAE Journal - July 2020

Contents
ASHRAE Journal - July 2020 - Intro
ASHRAE Journal - July 2020 - Cover1
ASHRAE Journal - July 2020 - Cover2
ASHRAE Journal - July 2020 - 1
ASHRAE Journal - July 2020 - Contents
ASHRAE Journal - July 2020 - 3
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ASHRAE Journal - July 2020 - Cover3
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