ASHRAE Journal - August 2019 - 62

COLUMN BUILDING SCIENCES

for EIFS? Yup, thousands of EIFS buildings with failures
Figure 1 Two-Stage Joint. Precast panel assemblies, as well as concrete tilt-up
(Photo 3).
systems, employ two-stage joints that facilitate drainage. We have the precast
industry for this innovation from the 1950s and from a legend by the name of
So we actually have to thank failure. Lots and lots of
Kirby Garden of the Division of Building Research of the National Research Council
failure. Now, according to "industry observers" the failof Canada.
Drainage Space
ure "appears to be limited" to stucco and previously to
Backer Rod
"face-sealed" EIFS, and "particularly to stucco over wood
Sealant
Sealant
based sheathings."‡ My comment is: "yes, up to now".
Backer Rod
Danger! Danger! Will Robinson!§
Because of the current large number of failures to
stucco over wood based sheathings the model codes are
going to be limiting the drainage requirement to stucco
assemblies over wood based sheathing, and to regions
where it rains more than 20 inches of rain per year
(IECC Climate Zones Moist [A] and Marine [C]). This is
great as far as it goes.
I respectfully disagree with this limitation. I think
drainage should be required in all assemblies except mass
Exterior Face of
walls, curtain walls, storefront glazing systems, precast
Cladding
and concrete tilt up. And with all of the exceptions noted
Drainage Opening
(except mass walls) there needs to be drainage at joints.
A modern curtain wall is a marvel of internal drainage. The precast industry pioneered two stage drained
joints (Figure 1). And just because stucco
is installed over gypsum sheathing it
Figure 2 Inward Vapor Drive. With reservoir claddings such as brick veneer, stucco and unpainted wood, rain
absorbed by claddings can be driven inwards by incident solar radiation. Reservoir claddings behave much
should not be exempted.
like "moisture capacitors." When the sun hits the "moisture capacitor," it discharges it. The heating of the
The argument for a drainage exempstored water raises its vapor pressure. The warm water in the cladding is driven both inward and outward.
tion for stucco over gypsum sheathing
Exterior Conditions
Conditions Within Cavity
Interior Conditions
is that the gypsum sheathing is more
Temperature: 80°F
Temperature: 100°F
Temperature: 75°F
Relative Humidity: 75%
Relative Humidity: 100%
Relative Humidity: 60%
vapor permeable (vapor "open") than
Vapor Pressure: 2.49 kPa
Vapor Pressure: 6.45 kPa
Vapor Pressure: 1.82 kPa
wood based sheathings and therefore
is able to dry easily and dissipate and
Solar Radiation Strikes Wall
redistribute moisture more effectively.
Brick Veneer is Saturated
Both of these points are correct but-and
With Rainwater
Vapor is Driven Both Inward and
here is the big but-hydrostatic pres1 in. Air Space
Outward by a High Vapor Pressure
sure rules; the wall still will leak and
Differential Between the Brick and the
Felt Paper
Interior and the Brick and the Exterior
therefore the risk is high, especially in
Fiberboard Sheathing
well-insulated assemblies that have less
Fiberglass Insulation
Polyethylene
available energy for drying.
Interior Gypsum Board
There is more. There always is. Inward
vapor drive and the need for hygric
redistribution. And yes, we have been here before
2010). You do not need much of a space to provide drain("Inward Drive-Outward Drying, ASHRAE Journal, June
age, but you might need a bigger space to deal with
2017 and "Mind the Gap, Eh?", ASHRAE Journal, January
reducing the effects of inward vapor drive (Figure 2) if
‡Overheard

during testimony at the "Committee Hearings" in Albuquerque for proposed building code changes for the 2021 IBC and
the 2021 IRC. Weasel words at their best. Sounds like 1995 to 2000 all over again during the "face-sealed EIFS denial stage."

§Quote

from Robot B9 and the science-fiction television series Lost in Space. It was not from Robby the Robot of "Forbidden Planet"
fame although Robby the Robot did actually make guest appearances in two "Lost in Space" episodes.

ASHRAE JOURNAL

ashrae.org

A U G U S T 2 0 19

https://www.ashrae.org/

ASHRAE Journal - August 2019

Table of Contents for the Digital Edition of ASHRAE Journal - August 2019