ASHRAE Journal - April 2020 - 79

COLUMN BUILDING SCIENCES

Furring

FIGURE 9 Drained Assemblies. Drained assemblies have multiple layers dedicated
to water management. An example of a drained assembly is a wood frame wall
with oriented strand board sheathing (OSB) covered with building paper and clad
with wood siding on furring.

FIGURE 10 Mass/Storage Assemblies. Mass/storage assemblies store, absorb,

redistribute and dry rainwater and groundwater without being damaged such as a
masonry wall rendered with a Portland cement based stucco.

Sheathing

Concrete Slab
Stucco
(Cladding)

Wood Siding
(Cladding)

Masonry Wall

Building Paper
(Water Control Layer)
Flashing

Drained assemblies are typically used with water
sensitive building materials such as wood framing,
steel studs, wood-based sheathings and gypsum-based
sheathings. An example of a drained assembly is a wood
frame wall with oriented strand board sheathing (OSB)
covered with building paper and clad with wood siding
on furring (Figure 9).
Barrier assemblies typically rely on the exterior cladding to act as the water control layer and are not commonly used in residential construction. Examples of
barrier assemblies are precast panel walls, glass curtain
walls and insulated metal panel systems.
Mass/storage assemblies are used with water-resistant
materials such as masonry, concrete, multi-wythe brick
and stone. An example of a mass/storages assembly
that can store, absorb, redistribute and dry rainwater
and groundwater without being damaged is a masonry
wall rendered with a Portland cement based stucco
(Figure 10).
Drained assemblies and mass/storage assemblies
anticipate water entry past the exterior cladding-barrier assemblies do not. In drained assemblies the penetrating rainwater is drained back to the exterior in liquid
form along the water control layer behind the cladding.
Some water also penetrates the water control layer and
is stored and redistributed and subsequently released
in the vapor form. However, the tolerance for such
stored and redistributed water is significantly less than
for mass/storage assemblies due to the water-sensitive

Weep Screed Flashing
(Provides Drainage at StuccoMasonry Connection; Also
Termination for the Stucco So That it
Does Not Extend Into the Ground)

"Seat" in Slab Acting as a Flashing
for Masonry-Slab Connection

nature of the building materials commonly used with
drained assemblies.
In mass/storage assemblies the penetrating rainwater
is typically stored and redistributed in the water resistant building materials until it can be released to either
the exterior or interior in the vapor form in a controlled
manner that does not damage interior or exterior
finishes.
Drained systems assume some rainwater will enter
and provide a mechanism to remove it. Barrier systems
typically rely on the exterior cladding to shed water
effectively for the life of the building. Mass/storage systems assume that rainwater that enters dries both outward and inward without damaging the materials that
comprise the system.
A hybrid approach to controlling rainwater entry recognizes that most rainwater enters building assemblies
at joints in cladding systems. Rather than applying a
drained approach for the entire assembly a drained
approach is applied only at the joints. An example of this
is the two-stage joint common with precast panel claddings (Figure 11).
The approach involves (from the interior to the
exterior) a continuous inner seal and a drained and
vented cavity between the continuous inner seal and
a weeped outer seal. The outer seal is exposed to the
elements (principal damage functions of water, heat
APRI L 2020

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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
ASHRAE Journal - April 2020 - 3
ASHRAE Journal - April 2020 - 4
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ASHRAE Journal - April 2020 - Cover4
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