ASHRAE Journal - April 2020 - 76

BUILDING SCIENCES
COLUMN

Joseph W. Lstiburek

Rain Control: Drained,
Barrier and Mass
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

To state the obvious: controlling rainwater entry is the single most important factor in
the design and construction of durable buildings. Let me repeat: controlling rainwater entry is the single most important factor in the design and construction of durable
buildings. In terms of overall importance it follows ﬁre and structure. You don't die
from rain; you can die from ﬁre and structural failure. So a bit of perspective is necessary. But after ﬁre and structure it is a really big deal.
Rain control can be broken into two broad strategies:
* Reduce the amount of rainwater deposited on
building surfaces and assemblies
* Control the rainwater deposited on building surfaces and assemblies
Things work best when both strategies are used. The
ﬁrst deals with the amount of rainwater reaching a
building surface and involves siting, exposures and
overhangs. The second deals with the building assembly
details themselves.
Reducing the amount of rainwater deposited on building surfaces and assemblies has traditionally been a
function of siting and architectural design. Old-timers
view all of this as common sense:
* Site buildings so that they are sheltered from prevailing winds, to reduce exposure to wind-driven rain;
* Provide roof overhangs to shelter exterior walls
from rain deposition;
* Provide architectural detailing that sheds rainwater
from building faces; and
* Slope sites away from buildings.
The next bit requires an understanding of the physics
of rain entry. Again, old-timers view this as obvious. Five
76

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APRI L 2020

typical driving forces are responsible for rain entry:
* Gravity (hydrostatic pressure);
* Surface tension;
* Capillarity;
* Momentum; and
* Air pressure differences.
Each of these driving forces can act independently
or in combination. And again, old-timers ﬁgured out
pretty simple approaches to dealing with them:
* Drain rainwater from building surfaces;
* Prevent rainwater from entering building assemblies;
* Store, absorb, redistribute and dry rainwater that
enters building assemblies;
* Drain any rainwater that enters building assemblies by employing a water control layer and drainage
space to control gravity (hydrostatic pressure);
* Employ kerfs, drip edges and ﬂashings to control
surface tension;
* Provide an air gap or non-porous material to control capillarity;
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 https://www.ashrae.org/

ASHRAE Journal - April 2020

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

Contents
ASHRAE Journal - April 2020 - Intro
ASHRAE Journal - April 2020 - Cover1
ASHRAE Journal - April 2020 - Cover2
ASHRAE Journal - April 2020 - 1
ASHRAE Journal - April 2020 - Contents
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ASHRAE Journal - April 2020 - Cover3
ASHRAE Journal - April 2020 - Cover4
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